MEDICINE AND SURGERY "F"
Course of LABORATORY MEDICINE
Respiratory gases and the blood pH; the hemogas analysis
HEMOGAS ANALYSIS
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Notice: this lecture requires previous attendance to the lecture on water and electrolyte balance. and renal excretion.
RESPIRATORY GASES AND BLOOD BUFFERS
The function of the lung is gas exchange; respiratory gases are O
2
and CO
2
. N
2
is inhaled and expired with air, and is dissolved in all body fluids, but it does not take part in cellular respiration and is completely inhert in our body. H
2
O in the vapour phase is inhaled and expired, and respiration causes water loss (approx. 200-300 mL/day).
The study of respiratory gases in the expired air and dissolved in the blood is important to assess respiratory function
. Moreover, CO
2
, together with bicarbonate, is one among the most important blood buffer and participates to determine the blood pH; thus,
the study of respiratory gases is essential for the diagnosis of acid-base imbalances
. The concentration of bicarbonate in our blood is controlled by: (i) conversion to and into CO
2
and lung excretion of the latter; and (ii) direct urinary excretion. As a consequence a complete study of the acid-base balance requires also the assessment of kidney excretion of bicarbonate.
ASSESSMENT OF LUNG FUNCTION: THE RESPIRATORY GASES
The essential laboratory test for respiratory gases is the hemogas analysis, carried out on a sample of
arterial blood
taken in a gas tight syringe to avoid contamination with air. In babies venous blood may be used becauze of the difficulty of puncturing an artery. The hemogas analysis evaluates
the respiratory function of lungs and blood
and
the blood's pH
. The respiratory and buffer activities of blood and lung are strictly correlated, but it is important to discuss them separately, to improve clarity.
Hemogas anaysis: reference values for arterial blood
PO
2
75-100 mmHg (age dependent)
Hemoglobin O
2
saturation
96-100%
Total O
2
content
20 mL/dL or 8-8.5 mmol/L (depends on Hb content; reduced under anemia)
PCO
2
35-45 mmHg (1.1-1.3 mM)
HCO
3
-
24-27 mM (plasma)
total CO
2
(bicarbonate+CO
2
+carbamates)
26-30 mM (plasma); 20-22 mM (blood)
pH
7.4-7.44
The main function of the lungs is the
exchange of respiratory gases O
2
and CO
2
. Since CO
2
is also a blood buffer,
the lung is the principal organ controlling the blood pH
(but kidney is also extremely important because of the excretion of bicarbonate). An accessory function of the lung is the participation to
the regulation of blood pressure
via the renin-angiotensin system. Lung diseases are often revealed by altered blood gases in the hemogas analysis (reduced PO
2
and increased PCO
2
, with respiratory acidosis). Diseases of the lung may compromise its mechanical ventilatory capacities (e.g. emphysema) or its ability to exchange gases (e.g. interstitial pneumonia), or both.
During respiration O
2
and CO
2
are exchanged in comparable amounts; the ratio CO
2
excreted : O
2
absorbed is called the
respiratory quotient
and varies between 0.7 and 1.0 depending on the nutrient we are utilizing, 0.7 being the respiratory quotient for triglycerides and 1.0 for glucose.
The two respiratory gases have very different solubility in water and this has profound effects on the mechanisms of their transport; some relevant differences are reported in the table below.
O
2
CO
2
total content in arterial blood
20 mL (gas)/dL or 8.5 mMoles/L
20.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration)
Partial pressure in arterial blood
95-100 mmHg
40 mmHg
total content in venous blood
15 mL (gas)/dL or 6.5 mMoles/L
22.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration)
Partial pressure in venous blood
40 mmHg
45 mmHg
physical status
97% bound to hemoglobin in the erythrocytes; 3% physically dissolved in erythrocytes and plasma
90% as bicarbonate; 5% as CO
2
; 5% as protein carbamates
artero-venous difference
2 mMoles/L ; 60 mmHg
1.7 mMoles/L ; 5 mmHg
Three important differences between the two gases are:
(i) O
2
is mostly Hb-bound, CO
2
is mostly present as dissolved bicarbonate. Anemias may cause a very significant reduction of O
2
content, while affecting CO
2
content to a negligible extent (the amount of Hb-carbamates accounting for only 5% of the total CO
2
). Severe anemias may cause hyperpnea and hypocapnic respiratory alkalosis.
(ii) The O
2
capacity of blood is saturable and follows the O
2
binding curve of Hb; the capacity of CO
2
is non-saturable (except that at very high levels the patients enters a comatous state).
(iii) O
2
has higher artero-venous differential pressures than CO
2
and higher diffusion coefficient. The arterovenous gradient of PCO
2
(normal values under rest for arterial and venous PCO
2
, respectively: 45 mmHg - 40 mmHg) is lower than that of PO
2
(normal values under rest: 100 mmHg - 40 mmHg); by contrast the O
2
content of arterial blood (8-8.5 mMoles/L) is much lower than the total CO
2
content (28 mMoles/L in the plasma, 22 moles/L in whole blood, due to the lower bicarbonate content in the erythrocyte cytoplasm).
Alterations of blood gases and other parameters of the hemogas analysis
may be observed under conditions where the airways present partial obstruction (e.g.
severe asthma; bronchitis; Chronic Obstructive Pulmonary Disease, COPD
) or the gas exchange in the lung alveoli is compromised (e.g.
pneumonia, interstitial pneumonia, emphysema
); however this effect may not occur to the same extent for the two respiratory gases, and
dissociations in the alterations of PO
2
and PCO
2
are quite common. Indeed, while one may expect that pulmonary lesions should impair O
2
and CO
2
to a similar extent, and thus the decrease of arterial PO
2
should parallel the increase of PCO
2
, this is often not the case and either gas is more compromised than the other. Two opposite cases should be considered:
(i) PCO
2
is moderately increased, while PO
2
is severely reduced. This may be observed in acute interstitial pneumonia and is due to several causes, the most important being hyperventilation. Hyperventilation (dispnoea; air hunger) causes the values of partial pressure of arterial gases to move in the direction of their partial pressures in air: PO
2, air
= 150 mmHg; PCO
2, air
< 1 mmHg. Since the normal value of arterial PO
2
is 90-100 mmHg there is little room for it to increase due to hyperventilation, whereas there is large room for PCO
2
to decrease. Thus
hyperventilation has greater effect in reducing PCO
2
than in increasing PO
2
. If O
2
is administered, it may occur that hyperventilation is diminished and the PCO
2
increases.
(ii) PCO
2
is severely increased, while PO
2
is moderately reduced. This may be observed in chronic conditions (e.g. COPD, emphysema), if hyperventilation is scarce, and is due to the combined effect of lower diffusion coefficient of CO
2
, and to the renal retention of bicarbonate; thus pulmonary lesions that cause impaired gas diffusion may affect the CO
2
content to a greater extent than the O
2
content. Remember that, according to Fick's law of diffusion, the gas flow is proportional to the pressure gradient, which is greater for O
2
than for CO
2
.
Reduced PO
2
may cause reduction of Hb O
2
saturation (in arterial blood Hb is almost completely O
2
saturated and close to the upper asymptote of the O
2
binding isotherm; thus a significant decrease of PO
2
is required to observe desaturation). Low Hb O
2
saturation causes cyanosis, i.e. a purplish o bluish discoloration of the skin. Cyanosis in the absence of reduced PO
2
is suggestive of diseases that do not involve directly the lung (e.g. hemoglobinopathies, heart failure). Signficant alterations of both PO
2
and Hb O
2
saturation in the absence of increased PCO
2
may be observed in congenital artero-venous shunts (e.g. atrial septum defects, Fallot tetralogy). Indeed an important consideration is that a cause of the incomplete O
2
saturation of Hb is the (minor) physiological mixing of arterial and venous blood occurring in the anastomoses between pulmonary and bronchial veins.
Hypoxemia, hypoxia and the relationships between O
2
and hemoglobin
The condition in which the O
2
content of arterial blood is reduced is called
hypoxemia
. It may have several causes and the laboratory analyses are essential to establish a correct diagnosis. The principal symptoms of hypoxemia are hyperpnea (increased respiration rate), and tachycardia; cyanosis, a purplish discoloration of the extremities due to the purplish colour of deoxygenated Hb, may be present. Myocardial infarction, arrhythmias and ischemic damage of tissues are possible complications.
The normal value of the O
2
content of blood is 20 mL O
2
measured in the gas phase under standard conditions per dL of blood; corresponds to 8-8.5 mmoles/L. The plasma content of O
2
is practically negligible (approx. 2-3% of the total blood O
2
) and 97-98% of the blood O
2
is bound to Hb with a maximum stoichiometry of 4 O
2
per tetrameric molecule of Hb.
Some definitions are in order:
hypoxemia
indicates a reduced oxygen "level" in the blood. Level is a generic term that some authors interpret as "reduced PO
2
", other authors as "reduced O
2
content". As shown in the table below the former definition would exclude anemias and hemoglobinopathies, the latter definition would include these conditions.
ischemia
is the condition in which obstruction of the artery that carries blood to an organ reduces or abolishes the oxygen supply to that organ (this condition may cause an infarction, i.e. the ischemic death of the tissue)
hypoxia
refers to the condition of insufficient oxygenation of the tissues. Hypoxia occurs when the O
2
content of the blood is reduced (as in the second definition of hypoxemia), in which case all organs are affected, or when an arterial obstruction causes ischemia, in which case the condition is limited to an organ.
The O
2
content of blood depends on the following factors:
(i)
partial pressure of O
2
(PO
2
; normal range for arterial blood 90-100 mmHg); depends on the partial pressure of O
2
in inspired air (reduced at high altitude) and on the gas exchange function of the lung (reduced in several lung diseases: pneumonia, interstitial pneumonia, emphysema, fibrosis, etc.);
(ii)
blood hemoglobin content
(normal value 14 g/dL, corresponding to 2.2 mmoles Hb tetramers/L; reduced in anemias);
(iii)
hemoglobin saturation
(SaO
2
; normal range 94-98% for arterial blood); reduced if PO
2
is reduced, or in the case of low O
2
affinity Hb mutants (see the lecture on hemoglobinopathies), or in the presence of (usually congenital) artero-venous shunts (e.g. defects of the atrial septum). Reduced O
2
saturation is associated to
cyanosis
.
laboratory findings associated to reduced blood O
2
content
disease
PO
2
SaO
2
PCO
2
other laboratory findings
lung diseases
decreased
decreased
increased
acute or chronic respiratory acidosis
high altitude; mountain sickness
decreased
decreased
decreased
respiratory alkalosis
anemias
normal
normal
normal or decreased
reduced Hb content
low O
2
affinity hemoglobinopathies and methemoglobinemias
normal
decreased
normal
abnormal electrophoresis of Hb; mutations on Hb genes
right-to-left (congenital) arterovenous shunts
decreased
decreased
increased
ecographic evidence of shunt
Clinical example
: severe viral pneumonia in a elderly subject:
pH = 7.44
PO
2
= 60 mmHg
SaO
2
= 92%
PCO
2
= 43 mmHg
HCO
3
-
= 29 mM
Comment: the patient has impaired gas exchange due to interstitial pneumonia, and significant compensatory hyperventilation (air hunger); this helps keeping PCO
2
at nearly normal levels, but is not sufficient to restore the PO
2
. Bicarbonate and pH are unremarkable.
Complementary analyses
An important complement to hemogas analysis is the determination of respiratory volumes by
spirometry
.
Pulse oximetry
can be carried out at the patient's side and may be very useful. A pulse oximeter uses red and infrared light to measure the arterial Hb O
2
saturation, thus it provides only one of the different parameters of the hemogas analysis; however the simplicity and affordability of pulse oximeters allows the physician to carry out this single measurement very efficiently.
Measurement of pulmonary blood flow and ventilation perfusion ratio
The measurement of of pulmonary blood flow (PBF) and ventilation perfusion ratio provides important information on lung function and disease. It is conceptually similar to the measurement of glomerular filtration rate (GFR) in that it uses the partition of a tracer (oxygen for PBF, creatinine for GFR) among two fluids (air and blood for PBF, urine and blood for GFR). The analysis is carried out as follows:
- the patient breathes in a spirometer capable of measuring volume, pressure and O
2
content of inspired and expired air;
- samples of arterial and venous blood are taken and their O
2
content is determined.
Typical results are as in the following table:
inspired air
5 L/min at 1 atm, T=25
o
C O
2
content = 20%
(12 respiratory acts/min of 400 mL each)
total O
2
in inspired air
n = 5 x 1 x 0,2 / 0,082 x 298 K
0,041 moles/min
expired air
5 L/min at 1 atm, T=25
o
C O
2
content = 15%
total O
2
in expired air
n = 5 x 1 x 0.15 / 0,082 x 298 K
0,031 moles/min
differential O
2
content, air
10 mmoles/min
O
2
content in arterial blood
Hb=14 g/dL and saturation 97%
8.2 mmoles/L
O
2
content in venous blood
Hb=14 g/dL and saturation 75%
6.4 mmoles/L
differential O
2
content, blood
1.8 mmoles/L
pulmonary blood flow
O
2
absorbed from air, per min / differential O
2
content per liter of blood
10 moles/min / 1.8 mmoles/L = 5.5 L/min
ventilation/perfusion ratio
5 L/min / 5.5 L/min = 0.91
Normal values of ventilation/perfusionn ratio are 0.9-1.1 variations may occur as a result of heart insufficiency, impaired ventilation, etc.
ACIDOSIS AND ALKALOSIS
The alterations of the blood pH are called acidoses (if arterial pH < 7.35) and alkaloses (if arterial pH > 7.45). They are due to abnormal production or excretion of acidic or basic solutes in the serum, and of course are counteracted by the blood buffers. An interesting and well-written tutorial may be found at
this link
; a very useful article on the subject may be read at
this link
.
BLOOD BUFFERS
Buffer
Concentration (ionizable group equivalents)
pK
Production/excretion
Hemoglobin
circa 45 mEq/L
(see text)
produced during the red blood cell differentiation; degraded by macrophages
CO
2
/ bicarbonate
CO
2
:1.2 mM - 40 mmHg
bicarbonate: 26-28 mM
6.1
Krebs cycle; excreted by the lung (CO
2
) and the kidney (bicarbonate)
Phosphate
1.2 mM
7.0
in equilibrium with calcium phosphate in the bone matrix; excreted in the urine
Audio: Blood buffers
The blood buffers behave in quite a complicated manner. The most concentrated buffer system is provided by the titratable aminoacid residues located on the surface of proteins, most notably
hemoglobin
. The concentration of proteins in the blood presents interindividual variability; morover each exposes several titratable aminoacid residues on its surface, each with its own pKa. Thus it is not convenient to describe their action analytically and the functional concept of
buffer capacity
is used instead. This concept was introduced into clinical use by D.D. Van Slyke in 1922; it is defined as the amount of strong acid (or strong base) required to change the pH of 1 L of blood by one unit; thus it is measured in mEq/L.
Buffer capacity of blood
whole blood
38 mEq/L
plasma
16 mEq/L
As shown in the Table the buffer capacity of plasma, due to plasma proteins, phosphate and bicarbonate/CO
2
, is less than half that of whole blood. One should take into account that the concentration of bicarbonate and CO
2
in the erythrocyte is only slightly lower than in the plasma; thus one may assume that in the whole blood over 22 mEq/L of buffer capacity are provided by hemoglobin.
In order to better understand the concept of buffer capacity
, we may consider the case of a simple buffer made up by a monoprotic weak acid and its sodium salt, e.g. acetic acid and sodium acetate. We call the total concentration of this buffer the
sum
of the concentrations of its components, namely: C
tot
=C
a
+C
s
. The pH of this buffer is given by Henderson and Hasselbalch law:
pH = pKa + log (C
s
/ C
a
)
We now select the two pH values pH
1
=pKa-0.5 and pH
2
=pKa+0.5, such that the two are equally spaced on the pH scale on the left and on the right of pKa, and their difference is 1 pH unit. We can easily calculated that to raise the buffer pH form pH
1
to pH
2
we need an amount of strong base (e.g. NaOH) corresponding approximately to 0.52xC
tot
(the actual value for the maximum buffer capacity measured on a smaller pH interval is 0.57xC
tot
). For example, if the total buffer concentration were 0.1 M we would need to add 0.052 Eq/L of a strong base to raise the pH from pH
1
to pH
2
.
Unfortunately, the buffer capacity depends not only on the total buffer concentration, but also on the buffer pH and reaches its maximum when pH=pKa. The buffer capacity measured at pH=pKa+1 (or pH=pKa-1) is only 0.2xC
tot
. Thus, the buffer capacity of 22 mM bicarbonate/CO
2
, with pKa=6.1, at the physiological pH=7.4 accounts at most for one tenth of the total buffer capacity of blood.
The buffer capacity of a mixture of different buffers is additive: each buffer contribute its own, and the total buffer capacity is the sum of the buffer capacities of each buffer in the system.
Which is the most important blood buffer? Answering this simple question is complex and not unequivocal. From a
static
view point, i.e. if we take a sample of arterial blood and measure its buffer capacity, we observe that over 80% is due to proteins, predominantly hemoglobin, and the rest to inorganic components (mainly bicarbonate and phosphate). If we add a
dynamic
dimension to our analysis we observe that in one hour the amount of CO
2
/bicarbonate produced by our metabolism exceeds that of all other buffers contained in the whole extracellular fluid and in the erythrocytes. Moreover, although hemoglobin is the most important buffer component of blood,
it is not the most informative
; indeed, except in cases of severe anemia, hemoglobin is scarcely influenced by dynamic conditions (e.g. diseases).
Much more relevant from the diagnostic view point is bicarbonate
, because it is continuosly produced by our metabolism (as CO
2
), and continuously excreted by the lung and the kidney, the principal organs involved in the regulation of buffer concentration and blood pH, given that they excrete CO
2
and bicarbonate (and the kidney excretes phosphate as well). The lung alone eliminates some 20 moles CO
2
/ day, a massive amount. Thus from the view point of the clinical laboratory, bicarbonate is , if not the most important, certainly the most informative buffer in our blood. A simplified scheme of the gas exchanges in relation to blood pH is depicted in the following figure.
Audio: Bicarbonate buffer
The reactions and Henderson Hasselbalch equation for bicarbonate are as follows:
CO
2
+ 2 H
2
O <=> [ H
2
CO
3
+ H
2
O ] <=> HCO
3
-
+ H
3
O
+
pH = pK
a
+ log ([HCO
3
-
]/0.031 PCO
2
)
Since carbonic acid is scarcely populated (its concentration being 700-times lower than that of CO
2
), we can ignore its contribution, and assume CO
2
as the acidic component of the buffer. Under this assumption the apparent pK
a
of CO
2
at 37
o
C is 6.1. [HCO
3
-
] is measured in mMol/L and PCO
2
is measured in mmHg; 0.031 mM/mmHg is the solubility coefficient of CO
2
.
Audio: The Henderson and Hasselbalch equation of bicarbonate buffer
Production and excretion of CO
2
and bicarbonate
: nutrition provides us an apport of energy, in the form of reduced carbon in food. Oxidation of nutrients with oxygen obtained from respiration develops this energy. In the case of glucose, the overall reaction is as follows:
C
6
H
12
O
6
+ 6 O
2
--> 6 CO
2
+ 6 H
2
O + 720 kcal /mole of glucose
As an example of the oxidation of fats, we can take the case of palmitic acid, the principla fatty acid produced by our own biosynthesis:
C
16
H
32
O
2
+ 23 O
2
--> 16 CO
2
+ 16 H
2
O + 2,380 kcal/mol
The caloric equivalent of O
2
is very similar in the two cases (fat produces more energy but requires more oxygen to be oxidized); for glucose we calculate 720/6=120 kcal/mol O
2
; for palmitic acid 2380/23=104 kcal/mol. An average value of 117 kcal /mole is usually accepted, and one can easily calculate that a basal metabolism of 1600 kcal/die requires approx. 14 moles of O
2
and produces almost as much CO
2
. The amount of O
2
consumed and CO
2
produced are approx. 9.7 and 8.4 mMoles/min. respectively. With a pulmonary blood flow of 5 L/min. this implies that the amount of O
2
uploaded and CO
2
excreted by the lung are approx. 2 mMol/L and 1.7 mMol/L. In the case of O
2
this amount is accounted for by the increase of hemoglobin saturation from 75% in venous blood to 98% in arterial blood,
but for CO
2
the amount of gas excreted exceeds the blood content (1.2 mMol/L)
. This is made possible by conversion of bicarbonate to CO
2
(catalyzed by carbonic anhydrase), but the reaction requires hydrogen ions, provided by the other blood buffers (essentially hemoglobin); in the absence of this contribution respiration would cause a massive pH increase (see the figure above).
total O
2
content of arterial blood
circa 8.5 mMoles/L
total O
2
content of venous blood
circa 6.5 mMoles/L
artero-venous difference in O
2
content
circa 2 mMoles/L
total CO
2
content of arterial blood
circa 22 mMoles/L
total CO
2
content of venous blood
circa 23.7 mMoles/L
artero-venous difference in CO
2
content
circa 1.7 mMoles/L
Bicarbonate can be excreted or retained by the kidney in an exchange with chloride; however the kidney has no direct effect on CO
2
. In practice, because of the other buffers, the blood pH varies relatively little (at most between 7.0 and 7.6, below and above these values the patient entering in a comatous state that, if untreated, leads to death). Thus, excretion of CO
2
is always associated to its reformation at the expense of bicarbonate and
vice versa
; yet the respiratory excretion of CO
2
consumes hydrogen ions and raises (slightly) the blood pH, while renal excretion of bicarbonate acts on the blood pH mainly via the alteration of its ratio to CO
2
. It is important that, being pH almost constant and bicarbonate concentration much higher than CO
2
, changes in CO
2
are easily (if not completely) compensated by bicarbonate, whereas the opposite is not true.
Regulation of bicarbonate/CO
2
and pH
organ
action / defect
effect
lung
hyperventilation
increased excretion of CO
2
- pH increases (alkalosis); PCO
2
decreases
lung
reduced ventilation, emphysema, reduced gas exchange
decreased excretion of CO
2
- pH decreases (acidosis); PCO
2
increases
kidney
increased excretion of bicarbonate
pH decreases; bicarbonate and total CO
2
decrease; Cl
-
increases
kidney
increased reabsorption of bicarbonate
pH increases; bicarbonate and total CO
2
increase; Cl
-
decreases
Disturbances of the blood pH and buffer concentrations
are called respiratory if caused by altered functioning of the lungs, metabolic otherwise. It is important to stress that disfunction of the lung can be to some extent compensated by the kidney and vice versa; thus each organ tends to oppose the disfunction of the other.
Since the concentration of protein buffers is relatively constant, the only buffer which can adapt to pH changes is bicarbonate
, whose concentration can be rapidly varied; this is why plasma bicarbonate from the diagnostic view point is the most important blood buffer. The key features of the different forms of acidosis and alkalosis are as follows:
TYPES OF ACIDOSIS AND ALKALOSIS
Abnormality
possible cause(s)
Blood pH (normal values: venous 7.36; arterial 7.4)
Pressure of blood CO
2
(normal values: venous 45 mmHg; arterial 40 mmHg)
total CO
2
(i.e. CO
2
+ plasma bicarbonate; normal values 27-30 mM)
Metabolic alkalosis
acid loss (renal; vomiting; etc.)
increased
normal to moderately increased
increased
Metabolic acidosis
production of abnormal acidic substances (e.g. ketoacid, lactic acid); renal loss of bicarbonate
decreased
decreased (compensatory hyerventilation)
decreased
Respiratory alkalosis
hyperpnea (high altitude; lesions of the respiratory centers in the brain)
increased
decreased (primary)
decreased
Acute (uncompensated) respiratory acidosis
reduced ventilation; impaired gas exchange(e.g. interstitial pneumonia); emphisema
strongly decreased
increased (primary)
slightly increased
Chronic (compensated) respiratory acidosis
same as acute respiratory acidosis
slightly decreased
increased (primary)
strongly increased (compensatory renal retention of bicarbonate)
Audio: Acidoses and alkaloses
The following remarks will help explaining the above table:
(i) pressure of CO
2
in the blood indicates the concentration of pure CO
2
, i.e. it does not include bicarbonate; total CO
2
is dominated essentially by bicarbonate ion concentration (the ratio [HCO
3
-
] / [CO
2
] being approximately 20).
(ii) The metabolism produces mainly acids (CO
2
and lactic acid, acetoacetic acid, etc.); however our organism is also better equipped to eliminate acids (CO
2
by respiration; organic acids by liver conversion to other products, e.g. lactic acid to glucose) than bases. Acidosis is frequent, varied and may be severe. Respiratory alkalosis is rare, whereas metabolic alkalosis due excess loss of acidic fluids (e.g. severe vomiting) may be frequent especially in the elderly.
(iii) Acids can be volatile (CO
2
), excreted by the lung at a very fast rate, and non-volatile (lactic acid, acetoacetic acid), metabolized by the liver or excreted by the kidney at a slower rate.
(iv) Metabolic alkalosis is most often due to loss of acids (e.g. vomiting forces the gastric mucosa to replace the gastric juice, whose HCl content is obtained by a mnechanism that increases the serum bicarbonate and causes alkalosis), excess intake of alkaline substances (e.g. gastric antiacids, bicarbonate), and diuretics; it is corrected mainly by the kidney that excretes the excess bicarbonate.
(v) Metabolic acidosis, caused by overproduction of non-volatile acids (e.g. diabetic ketoacidosis) or by their impaired renal excretion, stimulates respiration that excretes the volatile acid (CO
2
): hence compensatory hyperpnea and hypocapnia (reduced P CO
2
; Kussmaul breathing). It is interesting to remark that pulmonary correction of metabolic acidosis is more effective than of metabolic alkalosis because respiration frequency can be increased to a more significant extent than it can be decreased.
(vi) Respiratory alkalosis is a consequence of hyperventilation (loss of CO
2
); this occurs when the oxygen content of blood is reduced (e.g. because of anemias), or under unusual environmental conditions (e.g. muscular effort at high altitude, where atmospheric P O
2
is decreased - air hunger); moreover it may be due to (usually ishemic) lesions of the respiratory centers in the central nervous system.
(vii) Respiratory acidosis is a common consequence of impaired gas exchanges (e.g. depression of respiratory centers in the CNS, insufficient mechanical ventilation in polyomyelitis or tuberculosis, or ventilation perfusion imbalance in chronic obstructive pulmonary disease, emphysema, etc.).
Audio: The hemogas analysis
The
hemogas analysis
is the measurement of the pH and the concentrations and partial pressures of O
2
and CO
2
in a sample of the patient's blood drawn in a gas tight syringe. The measure is usually effected by means of potentiometric methods, used gas-specific electrodes.
As a general rule, a hemogas analysis will indicate if an abnormality is present and will give some indication of its possible cause; the fundamental indications are as follows:
Diminished pH and diminished total CO
2
= acute or chronic metabolic acidosis with respiratory compensation (e.g. diabetic ketoacidosis).
Strongly diminished pH, strongly increased PCO
2
and slightly increased total CO
2
= acute respiratory acidosis. The exchange of CO
2
in the lung is impaired (e.g. because of acute viral pneumonia); this leads to increased arterial PCO
2
and decreased arterial pH. Metabolic compensation is scarce or absent because it requires several days to become operative; this causes bicarbonate and total CO
2
to increase only slightly.
Normal to diminished pH and strongly increased total CO
2
= chronic respiratory acidosis. If the impaired gas exchange in the lung lasts long enough for the kidney to retain bicarbonate, metabolic compensation occurs (e.g. chronic obstructive pulmonary disease). Retention of bicarbonate (partially) restores the arterial pH but causes a strong increase in bicarbonate concentration and total CO
2
.
Increased pH and increased total CO
2
= metabolic alkalosis with respiratory compensation (e.g. vomiting).
Increased pH and decreased PCO
2
with moderately increased bicarbonate
= acute respiratory alkalosis with minimal metabolic compensation (uncommon; e.g. neurological hyperpnea).
Normal to increased pH and decreased total CO
2
= chronic respiratory alkalosis with metabolic compensation (e.g. life at high altitude).
The above set of rules allows one to interpret simple deviations from the healthy conditions, i.e. those conditions where one disease (either respiratory or metabolic) is present and compensation is respiratory. These conditions are typical of young patients suffering of acute acid-base imbalance (notice that respiratory compensation of metabolic conditions is almost immediate, whereas metabolic compensation requires time). Notice that two parameters (pH and total CO
2
) are necessary even in the least complicated cases.
Audio: Classification of blood pH disturbances
Audio: Acute respiratory acidosis
Audio: Chronic respiratory acidosis
Audio: Metabolic acidosis
Audio: Alkaloses
The
anion gap
, is the difference between the concentrations of (sodium + potassium) and (chloride + bicarbonate). The blood, as any other mixture, has zero net charge; thus the anion gap estimates the amount of non-measured negative charges (e.g. proteins). The normal value for plasma is around 15 mEq/L. An increased anion gap may provide a gross indication of the presence of excess unmeasured negatively charged ions usually derived from carboxylic acids (e.g. lactate or acetoacetate). Accordingly there are two types of metabolic acidosis, either with normal anion gap (NAGMA: Normal Anion Gap Metabolic Acidosis) or with increased anion gap (HAGMA: High Anion Gap Metabolic Acidosis). The
expected anion gap
is calculated if albumin concentration is lower than normal. This correction is important in hypoalbuminemia because albumin provides an important contribution to the non-measured anions. The correction is as follows:
expected anion gap = 15 - 2.5 x (normal albumin - actual albumin)
where albumin is expressed in g/dL and normal albumin = 4 g/dL. If the patient has hypoalbuminemia his/her anion gap is to be interpreted with reference to the expected value, rather than the usual normal value.
HAGMA
is usually characterized by normal chloride and reduced bicarbonate, and indicates the presence of abnormally high non-measured anions (e.g. acetoacetate in diabetic ketoacidosis). An important component of bicarbonate loss is its conversion to CO
2
and increased respiratory elimination.
NAGMA
occurs because increased renal loss of bicarbonate is associated to chloride retention (thus chloride is increased and the sum Cl
-
+ HCO
3
-
is constant).
common causes of HAGMA
common causes of NAGMA
Renal failure
Ketoacidosis
Lactic acidosis
Several types of poisoning
Some inherited defect of metabolism
Gastrointestinal loss of bicarbonate
Renal tubular acidosis
Carbonic anhidrase inhibitors
A very useful representation of the results of a hemogas analysis is a plot of bicarbonate concentration versus PCO
2
. Because of the Henderson-Hasselbalch equation each couple of these parameters identifies a pH value, and couples corresponding to the same pH appears as lines in this graph.
The graph identifies regions of the (PCO
2
,[HCO
3
-
]) space corresponding to the main categories listed above: acute respiratory acidosis and alkalosis, chronic respiratory acidosis and alkalosis, metabolic acidosis and alkalosis. The main reason why respiratory conditions require the distinction between acute and chronic is that renal compensation requires several days, thus acute respiratory conditions are poorly compensated or uncompensated, whereas chronic respiratory conditions are usually well compensated. Metabolic conditions require respiratory compensation, which is established rapidly: thus they are usually well compensated irrespective of their onset being acute or chronic.
We remark that chronic, compensated respiratory conditions may be compatible with a normal blood pH of 7.4, but are revealed by grossly altered values of PCO
2
and bicarbonate.
Basic 4-parameter diagnostic interpretation of the arterial hemogas analysis
.
1) The first parameter to analyze is the
blood's pH
. This defines the conditions of acidosis and alkalosis, even though in the presence of effective compensation the alteration of the blood's pH may be minimal or even absent.
2)
PCO
2
is the second most important parameter, which distinguishes hypocapnic from hypercapnic conditions. The general rule for interpreting this parameter in conjunction with the blood's pH is reported in the above Table "Types of acidosis and alkalosis". Since the PCO
2
is primarily controlled by respiration, important changes of this parameter reflect either pulmonary disease or respiratory compensation. Even though PCO
2
and bicarbonate are correlated by the Henderson and Hasselbalch equation, changes in PCO
2
due to pulmonary disease affect the pH more strongly than the bicarbonate concentration, unless metabolic compensation intervenes.
3) Total
CO
2
is essentially determined by
bicarbonate
concentration. Bicarbonate is controlled by both the kidney and the lung, and important changes of its concentration reflect metabolic disease or compensation. It is important to consider that while the kidney has great freedom in increasing or decreasing the excretion of bicarbonate, the lung has this freedom in one direction only, i.e. it can only increase the elimination of CO
2
/bicarbonate via hyperventilation; it cannot significantly decrease elimination via hypoventilation because this would also entail reduced uptake of O
2
, and hypoxemia is a strong stimulus to increase ventilation.
4) The
anion gap
is required to distinguish HAGMA from NAGMA.
Audio: Chronic respiratory acidosis
Some typical examples
.
1) Chronic obstructive pulmonary disease (COPD), diffuse interstitial pneumonia, etc. reduce the efficiency of gas exchanges. A typical hemogas analysis may be as follows:
oxygen saturation 78% on room air (normal value > 90%)
arterial pH 7.25 (normal value 7.44)
PCO
2
70 mmHg (normal value 40-44 mmHg)
PO
2
50 mmHg (normal value > 80 mmHg)
plasma bicarbonate concentration 35 mM (normal value 26 mM)
Anion gap 12 mEq/L
4-parameters analysis of this case is as follows:
1) pH is decreased, thus this condition is an acidosis.
2) PCO
2
is significantly increased, thus this condition is a respiratory acidosis.
3) Bicarbonate and total CO
2
are increased, thus metabolic compensation is present. Since metabolic (renal) compensation requires several days, this condition is a chronic respiratory acidosis.
4) The anion gap is normal; this occurs because the increase in bicarbonate is associated to renal excretion of chloride.
Description: chronic hypercapnic acidosis, associated (in this case) to reduced oxygen content and oxygen saturation.
2) Adaptation to high altitude (4.000 m above sea level or higher). A typical hemogas analysis may be as follows:
oxygen saturation 75% on room air
arterial pH 7.48
PCO
2
20 mmHg
PO
2
60 mmHg
plasma bicarbonate concentration 16 mM
Anion gap 13 mEq/L
4-parameters analysis of this case is as follows:
1) pH is increased, thus this condition is an alkalosis.
2) PCO
2
is significantly decreased, thus this condition is a respiratory alkalosis.
3 and 4) Bicarbonate and total CO
2
are decreased, thus metabolic compensation is present, suggesting that the condition is chronic. The normal anion gap is normal because increased urinary excretion of bicarbonate is associated to chloride retention.
Diagnosis: chronic respiratory hypocapnic alkalosis due to hyperpnea (this is an attempt to compensate for the reduced atmospheric PO
2
)
3) Type I diabetes mellitus:
plasma glucose > 250 mg/dL
arterial pH < 7.25
PCO
2
20 mmHg
PO
2
normal
serum bicarbonate 10-20 mM
anion gap 25 mM
4-parameters analysis of this case is as follows:
1) pH is decreased, thus this condition is an acidosis.
2 and 3) PCO
2
and bicarbonate are both significantly decreased, thus this condition is a metabolic acidosis with respiratory compensation.
4) The anion gap is increased, i.e. the plasma contains an excess of non-measured anions: HAGMA.
Diagnosis: the most important causes of HAGMA are:renal failure, ketoacidosis, lactic acidosis, inherited defects of metabolism (e.g. methylmalonic aciduria), and several types of poisoning. In the present case the increased glycemia suggests diabetic metabolic acidosis due to ketone bodies (acetoacetic acid and 3-hydroxy butanoic acid). The anion gap is increased because of the presence of the non-measured anions acetoacetate and 3-hydroxy butanoate.
MORE COMPLEX CASES REQUIRE ADDITIONAL PARAMETERS AND CONCEPTS
In the presence of an acid-base imbalance whose diagnosis is not obvious, more refined measurements are indicated in order to separate the respiratory and metabolic cotributions. Special attention is required in the elderly given that metabolic and respiratory conditions of equal or opposite sign may coexist (e.g. pulmonary emphysema, causing chronic respiratory acidosis, may be present together with vomiting, causing acute metabolic alkalosis, or with diabetes, causing chronic metabolic acidosis). Several clinical concepts (and measurements) have been developed to discriminate the metabolic and respiratory components of blood buffers inbalance, as listed below.
Measurement of the "standard" parameters
Standard pH
, historically the first concept introduced to rationalize complex deviations from the healthy conditions of blood buffers balance was introduced into the clinical practice by Hasselbalch in 1916. It is the pH of the patient's arterial blood measured under standard conditions (P CO
2
=40 mmHg, hemoglobin fully saturated with O
2
, T = 37 C). Essentially, the use of standard conditions has the effect of reversing the compensatory effect of respiration and thus to make more evident the eventual presence of a metabolic component in the pH unbalance.
In order to gain an understanding of the concept of standard pH and those that derived from it (standard bicarbonate and base excess) one should consider that
CO
2
behaves as an acid because of the reaction CO
2
+ 2 H
2
O <==> HCO
3
-
+ H
3
O
+
. Thus,
if P CO
2
< 40 mmHg, CO
2
is absorbed during equilibration, and standard pH < pH
. On the contrary,
if P CO
2
> 40 mmHg, CO
2
is released during equilibration, and standard pH > pH
.
With the use of three parameters (total CO
2
, pH, and standard pH), one obtains a better description of the underlying clinical condition, according to the following table:
Typical laboratory data for acidoses and alkaloses
pH
P CO
2
total CO
2
(or bicarbonate)
standard pH
(Base Excess)
diagnosis
<< 7.4 (e.g.: 7.2)
>> 40 mmHg (e.g.: 60)
> 26 mM (e.g.: 30)
7.4
(BE = 0)
uncompensated respiratory acidosis
(e.g. acute interstitial pneumonia)
< 7.4 (e.g.: 7.34)
> 40 mmHg (e.g.: 50)
>> 26 mM (e.g.: 50)
> 7.4
(BE > 0)
respiratory acidosis with metabolic compensation
(e.g. chronic obstructive pulmonary disease, COPD)
< 7.4
> 40 mmHg
< 7.4, > pH
(BE < 0)
combined respiratory acidosis and metabolic acidosis
(e.g. COPD and diabets mellitus)
<< 7.4 (e.g.: 7.2)
<< 40 mmHg (e.g.: 20)
<< 26 mM (e.g.: 10)
< pH
(BE < 0)
metabolic acidosis with respiratory compensation
(e.g. type I diabetes mellitus)
> 7.4
<< 40 mmHg (e.g.: 20)
< 26 mM (e.g.: 20)
7.4
(BE = 0)
uncompensated respiratory alkalosis
(e.g. acute adaptation to high altitude)
> 7.4
< 40 mmHg
<< 26 mM (e.g.: 15 mM)
< 7.4
(BE < 0)
respiratory alkalosis with metabolic compensation
(e.g. chronic adaptation to high altitude)
> 7.4
> 40 mmHg
>> 26 mM (e.g.40 mM)
> 7.4, < pH
(BE > 0)
metabolic alkalosis with respiratory compensation
(e.g. severe vomiting)
Standard bicarbonate
, a concept developed by Astrup and Siggaard Andersen in 1957, is the concentration of bicarbonate one measures when a sample of arterial blood is equilibrated under standard conditions. The rationale of this procedure is that of setting the concentration of one of the components of the major blood buffer (CO
2
) and determinining that of the other. The number one measures is not the same one would obtain in a standard hemogas analysis, given that part of the bicarbonate originally present in the blood may be lost as CO
2
(if the P CO
2
was higher than 40 mmHg) or part of the gas may be absorbed and converted to bicarbonate. The standard bicarbonate measures the metabolic component of the acid-base balance of the blood and corrects for respiratory compensation. It can substitute for standard pH.
Base excess
, again by Astrup and Siggaard Andersen, is the amount of strong acid (or base, in which case the resulting value is negative) required to restore the normal pH of 1 L of blood sample equilibrated under standard conditions.
Standard base excess
is the same concept except that erythrocytes are partially removed, to a final Hb content of 5 g/dL. BE (or SBE) is another indicator of the metabolic component of the disturbance: acidosis in the presence of positive base excess is an indication of respiratory acidosis.
Notice that if acidosis is present (i.e. pH<7.4) and the base excess is positive (i.e. acid is required to restore the pH to 7.4), this implies that the process of equilibrating blood under standard conditions causes the pH to raise above 7.4 (in respiratory acidosis P CO
2
> 40 mmHg; thus equilibration with standard P CO
2
removes CO
2
and bicarbonate)
. By contrast, acidosis in the presence of a negative base excess (base deficit) indicates an important metabolic component, with respiratory compensation. Base excess provides information analogous to, but more quantitative than, standard pH and standard bicarbonate (see the above Table "Typical laboratory data for acidoses and alkaloses").
Calculated parameters and nomograms
An alternative to the measurement of standard parameters makes use of calculated (approximated) parameters. The principle is as follows: given the usual parameters measured in a hemogas analysis one can infer which would be the "expected" parameter in the presence of optimal compensation.
Winter's formulas for PCO
2
. Winter's empirical formulas are used to calculate the PCO
2
one would record under metabolic acidosis or alkalosis, in the presence of optimal pulmonary compensation. There are two formula, one for metabolica acidosis, the other for metabolic alkalosis
Winter's formula for
metabolic acidosis
: PCO
2, theor. in mmHg
= 1.5 x [HCO
3
-
] + 8
Winter's formula for
metabolic alkalosis
: PCO
2, theor. in mmHg
= 0.7 x [HCO
3
-
] + 20
(allow an uncertainty margin of + or - 2 mmHg). If PCO
2
>> PCO
2, theor
then respiratory acidosis is present, in addition to the metabolic disturbance. If PCO
2
<< PCO
2, theor
then respiratory alkalosis is present, in addition to the metabolic disturbance.
Examples:
- Compensated metabolic acidosis
: pH=7.31; [HCO
3
-
] = 18 mM; PCO
2
=36 mmHg
Winter formula: PCO
2, theor. in mmHg
= 1.5 x 18 + 8 = 35 mmHg
Measured PCO
2
is within + or - 2 mmHg from calculated one: optimal respiratory compensation is present.
- Insufficiently compensated metabolic acidosis
: pH=7.22; [HCO
3
-
] = 18 mM; PCO
2
= 44 mmHg
Winter formula: PCO
2, theor. in mmHg
= 1.5 x 18 + 8 = 35 mmHg
Measured PCO
2
is significantly higher than calculated one: insufficient respiratory compensation. Most probably in this patient we have the contemporaneous presence of a metabolic acidosis (bicarbonate is low) and a respiratory acidosis (PCO
2
is higher than expected). Notice that in this example
the PCO
2
is normal!
; but, given the acidosis (pH = 7.22) we do not expect it to be normal, it should be lower if the lung were correctly compensating the disturbance.
Nomograms
are graphs in which the measured parameters are reported, together with the indication of statistically determined areas or regions corresponding to single disease conditions. The use of nomograms is equivalent and alternative to the use of empirical formulas such as Winter's; as a general rule, however, nomograms are preferable because they provide a visualization of usual ranges, rather than single numbers, from which the estimation of a range is not obvious. Two such nomograms are reported below, the topmost by Siggaard-Andersen, the bottom one by Arbus.
To use the nomogram we report the hemogas analysis parameters of our patient on the nomogram: if the patient falls in a shadowed area we can assume that he/she has a single acid-base defect, with the appropriate compensation (The areas usually include 95% of the "pure" cases); if the patient falls between two areas, then we assume that two concomitant diseases are present.
Advanced, 5-parameters interpretation of the arterial hemogas analysis
.
1) The usual 4-parameters evaluation is carried out as above.
2) The standard pH, or standard bicarbonate, or base excess is added to the picture and interpreted as in the Table above.
3) The above parameters are located on specifically designed nomograms to help differential diagnosis.
Examples
4) Comatous patient. Hemogas parameters are as follows: pH= 7.0; bicarbonate = 10 mM; PCO
2
= 40 mmHg; anion gap = 25 mEq/L. Our standard four parameters interpretation is metabolic acidosis, becase the pH is low and bicarbonate is low. The anion gap is high, thus this is a case of HAGMA. PCO
2
is "normal", but this contradicts our expectation that it should be reduced because of respiratory compensation. We locate the data of this patient in the Arbus' nomogram and we find that he falls in a white region, located between the metabolic acidosis and the acute respiratory acidosis. Thus the nomogram suggests that the patient has actually an acute respiratory acidosis superimposed over a (presumably chronic) metabolic acidosis.
Diagnosis: the patient had diabetic ketoacidosis. Initially his condition was compensated (low PCO
2
); when the pH fell at 7.0 he entered a comatous state and the respiratory centers in the brain were depressed. Hypoventilation reduced the compensation and added the acute respiratory component. Glycemia was 300 mg/dL. Insulin and potassium therapy were promptly instituted. Note: the patient condition was a medical emergency, and was rapidly progressive, because the increase of PCO
2
further depressed the respiratory centers, creating a vicious circle.
5) Chronic obstructive pulmonary disease (COPD)
Severe dispnoea; patients breathes a mixture of 75% air and 25% pure oxygen (O
2
partial pressure in inspired air 270 mmHg; O
2
content 36%). 60 years old male, heavy smoker.
Hemogas analysis:
PO
2
= 87 mmHg
PCO
2
= 79 mmHg
HCO
3
-
= 37 mM
standard HCO
3
-
= 31.5 mM
Base Excess = 10.4 mEq/L
pH = 7.28
Comment: chronic respiratory acidosis. Standard pH, calculated from the above parameters is 7.50, indicative of effective renal compensation; this is confirmed by the large Base Excess. A possible metabolic alkalosis component should be inquired (did the patient suffer of vomiting?). The decrease of PO
2
is less critical, but this parameter is supported by an O
2
enriched gas mixture.
6) Ketoacidosis in diabetes mellitus (see slide below for complete information). 15 years old patient suffering of unstable type I diabetes mellitus. Poor compliance to therapy and dietary prescriptions. Mental confusion.
Laboratory data:
arterial blood pH = 7.06
PO
2
= 71 mmHg
PCO
2
= 23 mmHg
[HCO
3
-
] = 6.5 mMoles/L
Anion Gap = 25 mEq/L
standard [HCO
3
-
] = 7.4 mMoles/L
glucose = 430 mg/dL
Comment: the primary disturbance in this case is a high anion gap metabolic acidosis (HAGMA). The history of the patient and the very high glycemia suggest diabetic ketoacidosis. Very low PCO
2
and bicarbonate are mainly due to respiratory (and renal) compensation.
The low PO
2
is puzzling: since in this case the pulmonary compensation is achieved via hyperventilation, we expect normal or even slightly increased PO
2
.
Application of
Winter's formula for metabolic acidosis
tells us that the expected PCO
2
under these conditions is: 1.5x6.5+8 = 18 mmHg, lower that the measured value.
Interpretation: the patient has a severe diabetic ketoacidosis; because of the low pH he is entering a condition of coma (mental confusion preludes to this evolution). The brain overstistimulation of the respiratory centers is reduced, and as a consequence the hyperventilatory response is fading, hence the higher than calculated PCO
2
and the low PO
2
. Coma, if untreated, would add a second reason of acidosis (respiratory) and would prove lethal (the patient was treated with insulin and survived).
7) Primary hyperaldosteronism. Woman, 50 years old, complains weakness and recent onset hypertension, with headaches.
Laboratory findings:
sodium = 150 mEq/L
potassium = 2.2 mEq/L
chloride = 103 mEq/L
bicarbonate = 32 mMoles/L
arterial blood pH = 7.50
PCO
2
= 43 mmHg
Comment: hypersodiemia and hypokalemia are characteristic of hyperaldosteronism - prescribe measurement of aldosterone, cortisol and ACTH. The patient also presents metabolic (hypercapnic) alkalosis, a finding that is common in hyperaldosteronism because of excess urinary excretion of hydrogen ions.
Application of
Winter's formula for metabolic alkalosis
tells us that the expected PCO
2
under these conditions is: 0.7x32+20 = 42.4 mmHg, essentially identical to the measured value; thus optimal pulmonary compensation is present, and there is non indication of lung disfunction.
8) Infectious gastroenteritis. 22 year old male presenting with fever, abdominal pain, severe vomiting and diarrhhoea.
Laboratory findings:
sodium = 140 mEq/L
potassium = 3 mEq/L
chloride = 86 mEq/L
bicarbonate = 38 mMoles/L
arterial blood pH = 7.60
PCO
2
= 40 mmHg
Comment: the patient has metabolic (hypercapnic) alkalosis, most probably due to vomiting. Electrolyte inbalances are most probably due to excess water loss. Application of
Winter's formula for metabolic alkalosis
tells us that the expected PCO
2
is 0.7x38+20 = 46.6 mmHg, significantly higher than the actual measured value. This suggests that the patient has a superimposed respiratory alkalosis (eliminates more CO
2
than expected). Moderate respiratory alkalosis is not uncommon in fever, due to hyperventilation. Further laboratory tests: look for pathogens in the faeces and in the blood; measure glycemia, lactate and osmolarity of blood; if necessary rehydrate the patient orally or parenterally.
COMMON CAUSES OF ACIDOSES AND ALKALOSES
ACIDOSES AND ALKALOSES OF THE ADULT
Acidoses and alkaloses appearing around or after puberty are usually acquired (see below for the congenital acidoses of the newborn and infant), and may be due to several possible causes. Some common examples are listed in the Table below:
pH disturbance
possible diseases
acute respiratory acidosis
interstitial pneumonia
lung intoxication by fumes or other substances
depression of respiratory centers (e.g. eroine abuse, brainstem infarction, coma)
chronic respiratory acidosis
chronic obstructive pulmonary disease (in heavy smokers!)
lung emphisema
impaired ventilation
lung fibrosis and penumoconioses
metabolic acidosis
diabetic ketoacidosis
lactic acidosis (sepsis!)
kidney failure and several kidney diseases (acquired Fanconi's syndrome)
respiratory alkalosis
hyperventilation due to neurological damage
mountain sickness
metabolic alkalosis
prolonged vomiting
diuretic therapy, some defects of tubular function
In the pH disturbances of the adult and the elderly it is important to pay attention to the possible coexistence of multiple pathological conditions: e.g. diabetic ketoacidosis is of the metabolic type, but may cause coma, which in turn depresses the respiratory centers and cause respiratory acidosis: thus two causes of acidosis are present at the same time, one metabolic and one respiratory. In an elderly patient suffering of BPCO and chronic respiratory acidosis, an episode of acute vomiting may superimpose a metabolic alkalosis; thus two opposite alterations of the blood pH may be present at the same time (and the pH may appear almost normal, but with severely altered bicarbonate and total CO
2
).
INHERITED METABOLIC ACIDOSES OF THE NEWBORN
Several inherited metabolic disorders may cause metabolic acidosis because of the accumulation of metabolytes whose elimination is impaired. These must be diagnosed early because appropriate treatment or diet may prevent brain damage and progression of the disease. Acidosis may not be the most prominent symptom, and may actually be mild (in many cases neurological abnormalities may be prominent); however it provides an important diagnostic clue. Acidosis is of the metabolic type, with increased anion gap. Aciduria is usually present (the urine is acidic). Each of these defects is uncommon, but they are many and thus their cumulative incidence is significant, possibly as high as 1 every 100 births.
The nutrients of animals are compounds containing reduced carbon (sugars, aminoacids, fatty acids). Carbon is oxidized using air O
2
, and since it is a non-metal its oxides are acidic (CO
2
, carboxylic acids). Blockade of metabolism because of the inherited deficiency of an enzyme may cause carboxylic acid intermediates to accumulate in the blood and in the urine. Each specific defect is discussed in the appropriate lecture or chapter, but it is useful to collect at least the most common defects in a cumulative Table.
disease
metabolyte and pathway
orotic aciduria
orotic acid (pyrimidine biosynthesis)
methylmalonic aciduria
methylmalonic acid (catabolism of propionyl-CoA)
maple sirup urine disease
branched chain α-ketoacids (catabolism of branched chain aminoacids)
phenylketonuria
phenylpiruvic acid (catabolism of Phe and Tyr)
alkaptonuria
homogentisic acid (catabolism of Phe and Tyr)
isovaleric aciduria
isovaleric acid (catabolism of Leu)
argininosuccinic aciduria
argininosuccinic acid (urea cycle)
mevalonic aciduria
mevalonic acid (biosynthesis of cholesterol)
lactic aciduria
lactic acid (several causes, genetic or acquired)
Fanconi's syndrome
defect of tubular reabsorption of bicabonate and other solutes (genetic or acquired)
Further readings
Pasipoularides A.
Historical Perspective: Harvey's epoch-making discovery of the Circulation, its historical antecedents, and some initial consequences on medical practice
; Journal of Applied Physiology
Paul K. Hamilton, Neal A. Morgan, Grainne M. Connolly and Alexander P. Maxwell:
Understanding Acid-Base Disorders
.
Alan W. Grogono
Acid-base tutorial
Ole Siggaard-Andersen maintains a
very informative website
on the respiratory and pH properties of blood.
HK Walker, WD Hall, and JW Hurst,
Clinical Methods
Butterworths, Boston, MA, USA.
JH Galla
Metabolic alkalosis
JASN
JW Severinghaus
The invention and development of the blood gas analysis apparatus
Anesthesiology 2002; 97: 253-56.
Krbec et al.
Non-carbonic buffer power of whole blood is increased in experimental metabolic acidosis: An in-vitro study
Front Physiol. 2022; 13: 1009378.
CS Breathnach
The development of blood gas analysis
.
Jubran A
Pulse oximetry
Crit Care. 1999; 3: R11–R17.
Arbus GS
An in vivo acid-base nomogram for clinical use.
Can. Med. Assoc. J. 1973; 109: 291.
A collection of clinical cases can be found at this website:
https://geekymedics.com/abg-examples-and-case-studies/
Questions and exercises:
1) The normal parameters of arterial O
2
are:
PO
2
> 110 mmHg; O
2
content 20 mL/dL; O
2
saturation > 95%
PO
2
> 90 mmHg; O
2
content 15 mL/dL; O
2
saturation > 95%
PO
2
> 90 mmHg; O
2
content 20 mL/dL; O
2
saturation > 95%
2) Normal values for serum CO
2
and bicarbonate are:
PCO
2
=40 mmHg (corresponding to 1,3 mM); bicarbonate=26 mM
PCO
2
=26 mmHg (corresponding to 0,8 mM); bicarbonate=26 mM
PCO
2
=40 mmHg (corresponding to 1,3 mM); bicarbonate=40 mM
3) The characteristic features of acute respiratory acidosis are
markedly increased PCO
2
, markedly decreased pH, normal or moderately increased bicarbonate
markedly increased PCO
2
, moderately decreased pH, markedly increased bicarbonate
reduced PCO
2
, markedly decreased pH, reduced bicarbonate
4) The anion gap is increased in
acute respiratory acidosis
diabetic ketoacidosis
adaptation to high altitude.
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Thank you Professor (lecture on bilirubin and jaundice).
The fourth recorded part, the one on hyper and hypoglycemias is not working.
Bellelli: I checked and in my computer it seems to work. Can you better specify
the problem you observe?
This Presentation (electrolytes and blood pH) feels longer than previous lectures
Bellelli: it is indeed. Some subjects require more information than others. I was
thinking of splitting it in two nest year.
Bellelli in response to a question raised by email: when we compare the blood pH
with the standard pH we do not mean to compare the "normal" blood pH (7.4)
with the standard pH. Rather we compare the actual blood pH of the patient, with
the pH of the same blood sample equilibrated under standard conditions.
Thus, if we say that standard pH is lower than pH we mean that equilibriation with
40 mmHg CO2 has caused absorption of CO2 and has lowered the pH with respect
to its value before equilibration.
(Lipoproteins) Is the production of leptin an indirect cause of type 2 diabetes since
it works as a stimulus to have more adipose tissue that produces hormones?
Bellelli: in a sense yes, sustained increase of leptin causes the hypothalamus to adapt
and to stop responding. Obesity ensues and this in turn may cause an increase in the
production of resistin and other insulin-suppressing protein hormones produced by the
adipose tissue. However, this is quite an indirect link, and most probably other factors
contribute as well.
(Urea cycle) what is the meaning of "dissimilatory pathway"?
Bellelli: a dissimilatory pathway is a catabolic pathway whose function is not to produce
energy, but to produce some terminal metabolyte that must be excreted. Dissimilatory
pathways are necessary for those metabolytes that cannot be excreted as such by the
kidney or the liver because they are toxic or poorly soluble. Examples of metabolytes
that require transformation before being eliminated are heme-bilirubin, ammonia,
sulfur and nitrogen oxides, etc.
Talking about IDDM linked neuropathy can be the C peptide absence considered a cause of it??
Bellelli: The C peptide released during the maturation of insulin, besides being an indicator
of the severity of diabetes, plays some incompletely understood physiological roles. For
example it has been hypothesized that it may play a role in the reparation of the
atherosclerotic damage of the small arteries. Thus said, I am not aware that it plays a direct
role in preventing diabetic polyneuropathy. Diabetic neuropathy has at least two causes: the
microvascular damage of the arteries of the nerve (the vasa nervorum), and a direct
effect of hyperglycemia and decreased and irregular insulin supply on the nerve metabolism.
Diabetic neuropathy is observed in both IDDM and NIDDM, and requires several years to
develop. Since the levels of the C peptide differ in IDDM and NIDDM, this would suggest
that the role of the C peptide in diabetic neuropathy is not a major one. If you do have
better information please share it on this site!
In acute intermitted porphyria and congenital erythropoietic porphyria why do the end product
of the affected enzymes accumulate instead of their substrate??
Bellelli: First of all, congratulations! This is an excellent question.
Remember that a condition is which the heme is not produced is lethal in the foetus; thus
the affected enzyme(s) must maintain some functionality for the patient
to be born and to come to medical attention. All known genetic defects of heme
biosynthesis derange but do not block this metabolic pathway.
Congenital Erythropoietc Porphyria (CEP) is a genetic defect of uroporphyrinogen
III cosynthase. This protein associates to uroporphyrinogen synthase (which is present
and functional in CEP) and guarantees that the appropriate uroporphyrinogen isomer is produced
(i.e. uroporphyrinogen III). In the absence of a functional uroporphyrinogen III
cosynthase other possible isomers of uroporphyrinogen are produced together with
uroporpyrinogen III, mostly uroporphyrinogen I. The isomers of uroporphyrinogen
that are produced differ because of the positions of propionate and acetate side chains,
and this in turn is due to the pseudo symmetric structure of porphobilinogen. Only
isomer III can be further used to produce protoporphyrin IX. Thus in the
case of CEP we observe accumulation of abnormal uroporphyrinogen derivatives, which, as
you correctly observed are the products of the enzymatic synthesis operated by
uroporphyrinogen synthase.
The case of Acute Intermittent Porphyria (AIP) is similar, although there may be variants
of this disease. What happens is that either the affected enzyme is a variant that does not
properly associate with uroporphyrinogen III cosynthase or presents active site mutations
that impair the proper alignement of the phoprphobilinogen substrates. In either case
abnormal isomers of uroporphyrinogen are produced, as in CEP.
Also remark that in both AIP and CEP we observe accumulation of the porphobilinogen
precursor: this is because the overall efficiency of the biosynthesis of uroporphyrinogens is
reduced. Thus: (i) less uroporphyrinogen is produced, and (ii) only a fraction of the
uroporphyrinogen that is produced is the correct isomer (uroporphyrinogen III).
is it possible to take gulonolactone oxidase to synthesize vitamin C
instead of vitamin C supplement?
Bellelli: no, this approach does not work. The main reason is that
the biosynthesis of vitamin C, as almost all other metabolic processes, occurs intracellularly.
If you administer the enzyme it will at most reach the extracellular fluid but will not be
transported inside the cells to any significant extent. Besides, there are other problems
in this type of therapy (e.g. the enzyme if administered orally, may be degraded by digestive
proteases; if administered parenterally, may cause the immune system to react against a
non-self protein). In theory one could think of a genetic modification of the inactive human
gene of gulonolactone oxidase, but the risk and cost of this intervention would not be
justified. In addition to these considerations, except for cases of shipwreckage or
other catastrophes, a proper diet or administration of tablets of vitamin C is effective,
risk-free and unexpensive, thus no alternative therapy is reasonable. However, I express my
congratulations for your search on the biosynthesis pathway of ascorbic acid.
Resorption and not reabsorption would lead to hypercalcemia ie bone matrix being broken down.
Bellelli: I am not sure to interpret your question correctly. Resorption indicates destruction of the bone matrix and release of calcium and
phosphate in the blood, thus it causes an increase of calcemia. Reabsorption usually means active transport of calcium from the renal tubuli to the blood, thus
it prevents calcium loss. It prevents hypocalcemia, and thus complement bone resorption. To avoid confusion it is better use the terms "bone resorption" and "
renal reabsorption of calcium". If you have a defect in renal reabsorption, parthyroid hormone will be released to maintain a normal calcium level by means of
bone resorption; the drawback is osteoporosis.
In Reed and Frost model: I haven't understood what is the relationship
between K and R reproductive index. Thank you Professor!
Bellelli: in the Reed and Frost model K is the theoretical upper limit of
R
0
. R the reproductive index is the ratio (new cases)/(old cases) measured after
one serial generation time. R
0
is the value of R one measures at the beginning
of the epidemics, when in principle all the population is susceptible.
What is the link between nucleotide metabolism and immunodeficiencies and mental retardation?
Bellelli: the links may be quite complex, but the principal ones are as follows:
1) the immune response requires a replication burst of granulocytes and lymphocytes, which in turn requires
a sudden increase of nucleotide production, necessary for DNA replication. Defects of nucleotide metabolism
impair this phase of the immune defense. Notice that the mechanism is similar to the one responsible of
anemia which requires a sustained biosynthesis of nucleotides at a constant rate, rather than in a burst.
2) Mental retardation is mainly due to the accumulation of nulceotide precursors in the brain of the
newborn, due to the incompletely competent blood-brain barrier.
How can ornithine transaminase defects cause hyperammonemia? Is it due to the accumulation
of ornithine that blocks the urea cycle or for other reasons?
Bellelli: ornithine transaminase is required for the reversible interconversion of ornithine
and proline, and thus participates to both the biosynthesis and degradation of ornithine. The enzyme is
synthesized in the cytoplasm and imported in the mitochondrion. Depending on the metabolic conditions
the deficiency of this enzyme may cause both excess (when degradation would be necessary) or defect
(when biosynthesis would be necessary) of ornithine; in the latter case, the urea cycle slows down. Thus
there is the paradoxical condition in which alternation may occur between episodes of hyperammonemia
and of hyperornithinemia.
When we use the Berthelot's reaction to measure BUN do we also have to
measure the concentration of free ammonia before adding urease?
Bellelli: yes, in principle you should. Berthelot's reaction detects ammonia,
thus one should take two identical volumes of serum, use one to measure free ammonia,
the other to add urease and measure free ammonia plus ammonia released by urea. BUN is
obtained by difference. However, free ammonia in our blood is so much lower than urea that
you may omit the first sample, if you only want to measure BUN.
Why do we have abnormal electrolytes in hematological neoplasia e.g.
leukemia?
Bellelli: I do not have a good explanation for this effect, which may have
multiple causes. However, you should consider two factors: (i) acute leukemias cause a massive
proliferation of leukocytes (or lymphocytes depending on the cell type affected) with a very
shortened lifetime; thus you observe an excess death rate of the neoplastic cells. The dying
cells release in the bloodstream their content, which has an electrolyte composition different
from that of plasma: the cell cytoplasm is rich in K and poor in Na, thus causing hyperkalemia.
(ii) the kidney may be affected by the accumulation of neoplastic white cells or their lytic products.
Gaussian curve: If it is bimodal is it more likely to be a "certain diagnosis" than if it is
unimodal or does it only show the distinguishment from health?
Bellelli an obviously bimodal Gaussian curve indicates that the disease is clearly
separated from health: usually it is a matter of how precise and clear-cut is the definition of the disease.
For example tuberculosis is the disease caused by M. tuberculosis, thus if the culture of the sputum is
positive for this bacterium you have a "certain" diagnosis (caution: the patient may suffer of two diseases,
e.g. tuberculosis and COPD diagnosis of the first does not exclude the second). However, in order to have
a "certain" diagnosis it is not enough that distribution of the parameter is bimodal, it is also required that the
patient's parameter is out of the range of the healthy condition: this is because a distribution can be
bimodal even though it is composed by two Gaussians that present a large overlap, and the patient's
parameter may fall in the overlapping region. Thus, in order to obtain a "certain" diagnosis you need to
consider not only the distribution of the parameter(s) but also the patient's values and the extent of the
overlapping region.
Prof can you please elaborate a bit more on the interhuman variability and its difference
with the interpopulation variability please?
Bellelli: every individual is a unique combination of different alleles of the same genes;
this is the source of interindividual variability. Every population is a group of individuals who intermarry and
share the same gene pool (better: allele pool). Every allele in a population has its own frequency. Two
population may differ because of the diffferent frequencies of the same alleles; in some cases one
population may completely lack some alleles. The number and frequencies of alleles of each gene
determine the variance. If you take two populations and calculate the cumulative interindividual variance
of the population the number you obtain is the sum of two contributions: the interindividual variance within each population, plus the interpopulation variance
between the means of the allele frequencies. For example, there are human population in which the frequency of blood group B is close to 0% and other populati
ons in which it is 30% or more.
Prof can you please explain again the graph you have showed us in class about thromboplastin?
(Y axis=abs X axis= time)
Bellelli: the graph that I crudely sketched in class represented the signal
of the instrument (an absorbance spectrophotometer) used to record the turbidity of the
sample (turbidimetry). The plasma is more or less transparent, before coagulation starts.
When calcium and the tissue factor (or collagen) are added. thrombin is activated and begins
digesting fibrinogen to fibrin; then fibrin aggregates. The macroscopic fibrin aggregates cause
the sample to become turbid, which means it scatters the incident light. The instrument reads
this as a decrease of transmitted light (i.re an increase of the apparent absorbance) and the
time profile of the signal presents an initial lag phase, which is called the protrombin or
thromboplastin time depending on the component which was added to start coagulation
(tissue factor or collagen).
Prof can you please explain the concept you have described in class about
the simultaneous hypercoagulation and hemorrhagic syndrome? How can this occur?
Bellelli: The condition you describe is observed only in the Disseminated
Intravascular Coagulation syndrome. Suppose that the patient experiences an episode of
acute pancreatitis: tripsin and chymotripsin are reabsorbed in the blood and proteolytically
activate coagulation causing an extensive consumption of fibrinogen and other coagulation
factors. Tripsin and chymotripsin also damage the vessel walls and may cause internal
hemorrages, but at that point the consumption of fibrinogen may have been so massive that
not enough is left to form the clot where the vessel has been damaged, causing an internal
hemorrage. Pancreatitis is a very severe, potentially lethal condition, and DIC is only one of
the reasons of its severity.
You said that certain drugs (ethanol, cocaine, cannabis, opiates...) cause a
necessity of higher and higher dosage, for two reasons: the enzyme in the liver is inducible and
the receptors in the brain are expressed less and less. So, first, I am not sure I got it right, and
second I did not understand how expressing less receptors leads to a necessity of higher
dosage.
Bellelli: You got it correctly, but the detailed mechanism of resistance may
vary among different substances, and not all drugs cause adaptation.
The reason why reducing the number of receptors may require an increased dosage of the drug
is as follows: suppose that a certain cell has 10,000 receptors for a drug. When bound to its
agonist/effector, each receptor produces an intracellular second messenger. Suppose that in
order for the cell to respond 1,000 receptors must be activated. The concentration of the
effector required is thus the concentration that produces 10% saturation. You can easily
calculate that this concentration is approximately 1/10 of the equilibrium dissociation constant
of the receptor-effector complex (its Kd), the law being
Fraction bound = [X] / ([X]+Kd)
where [X] is the concentration of the free drug.
After repeated administration, the subject becomes adapted to the drug, and his/her cells
express less receptors, say 5,000. The cell response will in any case require that 1,000
receptors are bound to the effector and activated, but this now represents 20% of the total
receptors, instead of 10%. The drug concentration required is now 1/4 of the Kd.
Continuing administration of the drug further reduces the cell receptors, but the absolute
number of activated receptors required to start the response is constant; thus the fewer
receptors on the cell membrane, the higher the fraction of activated receptors required.
Why does hyperosmolarity happen in type 2 diabetes and not in type 1?
Bellelli: Hyperosmolarity can occur also in type 1 diabetes, albeit
infrequently. The approximate formula for plasma osmolarity is reported in the lecture on
electrolytes:
osmolarity = 2 x (Na
+
+ K
+
) + BUN/2.8 + glucose/18
this is expressed in the usual clinical laboratory units (mEq/L for electrolytes, g/dL for non-
electrolytes). The normal values are:
osmolarity = 2 x (135 + 5) + 15/2.8 + 100/18 = 280 + 5.4 + 5.6 = 291 mOsmol/L
Let's imagine a diabetic patient having normal values for electrolytes and BUN, and glycemia=400 mg/dL:
osmolarity = 280 + 5.4 + 22.4 = 307.8 mOsmol/L
The hyperosmolarity in diabetes is mainly due to hyperglycemia, even though other factors
may contribute (e.g. diabetic nefropathy); however the contribution of glucose to osmolarity is
relatively small. As a consequence in order to observe hyperosmolarity the hyperglycemia
should be extremely high; this is more often observed in type 2 than in type 1 diabetes, for
several reasons, the most relevant of which is that in type 1 diabetes all cells are starved of
glucose, and the global reserve of glycogen in the body is impoverished: there is too much
glucose in the blood and too few everywhere else, thus reducing, but not abolishing, the risk of
extreme hyperglycemia. Usually in type 2 diabetes the glycogen reserve in the organism is not
impoverished, thus the risk of extreme hyperglycemia is higher.
Hemostasis and Thrombosis lecture: I don't understand why is sodium citrate
added to the serum solution to measure the prothrombin time.
Bellelli: in order to measure PT or PTT you want to be able to start the
coagulation process at an arbitrary time zero, and measure the increase in turbidity of the
serum sample. To do so you need (i) to prevent spontaneous coagulation with an anticoagulant;
and (ii) to be able to overcome the anticoagulant at your will. Citrate (or oxaloacetate; or EDTA)
has the required characteristics: it chelates calcium, and in this way it prevents coagulation;
but you can revert its effect at your will by adding CaCl
2
in excess to the amount
of citrate. You cannot obtain the same effect with other anticoagulants (e.g. heparin) whose
action cannot be easily overcome.
Dear professor I cannot do the self evaluation test because it says the the
time has expired It is not possible because I havent even started them
Bellelli: this is due to the fact that the program registers your name and
matricola number from previous attempts. I shall fix this bug. Meanwhile try to use a fake
matricola number.
How is nephrotic syndrome associated hypoalbuminemia as you described
in methods of analysis of protein because seems counterintuitive
Bellelli: nephrotic syndrome is an autoimmune disease in which the
glomerulus is damaged and the filtration barrier is disrupted; diuresis is normal but there is
loss of proteins (mostly albumin) in the urine.
I m sorry i confused polyurea with hypoalbuminemia but my question still
stands during glomerulonephritis you mentioned something of polyurea as compensation
i could not follow how this compensation mechanism works and collapse after some time in
glomerulonephritis
Bellelli: the condition you describe is NOT characteristic of acute
glomerulonephritis. In glomerulonephritis there is damage of the glomerulus and severely
impaired GFR. Thus the diuresis is severely reduced, and due to impaired filtration proteins
appear in the urine.
The condition you describe corresponds to the initial stage of chronic kidney failure,
usually due to atherosclerosis, diabetes, hypertension or other type of damage of the kidney
tissue. In this case GFR is impaired, albeit to a lesser extent than in glomerulonephritis, and the
excretion of urea is reduced. This leads to increased BUN. However the increased concentration
of urea reduces the ability of the tubuli to reabsorb water, because of osmotic reasons, yielding
compensatory polyuria. The patient has reduced GFR but normal or increased diuresis (urine
volume in 24 hours). To some extent this effect is beneficial, as it favors the elimination of
urea; however it cannot completely solve the problem and in any case the progression of the
disease leads to kidney insufficiency. In its essence the point is that a moderately reduced GFR
can be partially compensated by reduced tubular reabsorption; a severely reduced GFR cannot.
Lecture on Hemogas analysis interpretation of complex cases standard pH
Why if PCO2 is less than 40 mmHg it is absorbed during equilibration? Thank you in advance
Bellelli: if PCO2 of the patient's blood sample is less than 40 mmHg, when
the machine equilibrates with 40 mmHg CO2 the gas is absorbed: i.e. the new PCO2 becomes
40 mmHg and the total CO2 of the sample increases; as CO2 is the acid of the buffer, the
standard pH (in this case) decreases, whereas standard bicarbonate will slightly increase.
Professor I don't understand how we arrive to this formula: Accuracy =
sensitivity x prevalence specificity x (1-prevalence)
Bellelli: ok, this relationship is poorly explained in your text, I shall improve its explanation.
We use the following definitions:
prevalence = sick individuals / total population;
accuracy = (true+ + true-) / total population;
sensitivity = true+ / sick individuals = true+ / total population x prevalence;
specificity = true- / healthy individuals = true- / total population x (1-prevalence);
thus we can rewrite:
sensitivity x prevalence = true+ / total population;
specificity x (1-prevalence) = true- / total population;
accuracy = sensitivity x prevalence + specificity x (1-prevalence)
Why do we use RNA primer in PCR and not DNA primers? I thought the
beginning of the sequence of the gene segment that is going to be formed is made of DNA
Bellelli: DNA polimerases require the RNA primers that are synthesized by
the enzyme primase. DNA primers do not exist in vivo and would not be recognized by DNA
polimerases.
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