Natalija Vuković1 , Milan Elenkov2
1Clinic for Anesthesiology and Intensive Care, Clinical Center Niš,
2Department for Anesthesiology and Intensive Care, General hospital Pirot
Blood gas analysis can be driven from capillary, peripheral arterial, peripheral and central venous blood sampling and from pulmonary artery.  It provides the assessment of acid-
In this review we will revise three basic steps in the interpretation of the acid-
What is the primary abnormality, metabolic or respiratory?
The primary metabolic disorder occurs in these two cases :
a) If the pH and PaCO2 change in the same direction
b) If the pH changes, while the PaCO2 stays normal
In this way, the patient will have metabolic acidosis if the pH and PaCO2 fall, or the pH falls and the PaCO2 is normal (Figure 1).
If the pH and PaCO2 rise, or the pH rises and the PaCO2 is normal, the patient will have metabolic alkalosis (Figure 2).
The primary respiratory disorder is present if the pH and PaC
O2 change in the opposite direction. An increase in PaCO2 is respiratory acidosis, while a decrease in PaCO2 is respiratory alkalosis.
In the situation where the patient has metabolic acidosis the anion gap should be calculated.
The anion gap is the assumption that the negatively charged anions and positively charged cations in the serum equal electrical neutrality. [6.7] It represents the difference between unmeasured anions and cations.  It is calculated on the basis of electrolyte measurements as follows in the equation1.1:
1.1. AG = (Na+ + K+) – (Cl-
The normal value of AG is 12±3mmol/L.  Nowadays, with more accurately measured electrolytes, the normal range of AG has decreased up to 7 ± 4mmol/L. 
Metabolic acidosis can be divided into those having an elevated AG and those with a normal AG (10). The high AG acidosis occurs after the addition of acids or decreased excretion of endogenous acids. In those situations hydrogen ions combine with bicarbonate, decrease it and produce an increase in the AG (Figure 3).
The normal AG acidosis is characterized by the loss of bicarbonate (diarrhea etc.) or addition of chloride (rapid administration of isotonic saline etc.), (Figure 4).
Is there a compensation or superimposed secondary abnormality?
This question should be answered for all the primary disorders. So, if the patient has metabolic alkalosis we should follow the explanation for compensation in metabolic alkalosis, for the patient with respiratory acidosis, we follow the interpretation for respiratory acidosis etc.
Respiratory compensation for metabolic acidosis is hyperventilation with reduction of PaCO2. . The expected PaCO2 for each patient is calculated from equation 1.2.  In this equation, the concentration of bicarbonate that is used is from the patient’s acid-
1.2. Expected PaCO2= (1.5 x HCO3-
If the PaCO2 from the analysis is the same as the expected one, the patient has compensated metabolic acidosis. If the measured PaCO2 exceeds the expected one, then the patient has metabolic acidosis as the first derangement and the respiratory acidosis as the second one. If the measured PaCO2 is lower than the expected one, then the patient has primary metabolic acidosis with secondary respiratory alkalosis.
The expected PaCO2 in patients that have metabolic alkalosis as primary disorder is calculated from the equation 1.2.  Again, like in the previous case, the concentration of bicarbonate is the one we have from the patient’s blood gas analysis.
1.3. Expected PaCO2= (0.7 x HCO3-
In the situation where the PaCO2 from the analysis is the same as the expected one, the patient has compensated metabolic alkalosis. If the measured PaCO2 exceeds the expected one, there is no compensation and metabolic alkalosis occurs as the first disorder and the respiratory acidosis as the second one. If the measured PaCO2 is lower than the expected one, then the patient has primary metabolic alkalosis with secondary respiratory alkalosis.
Compensation for raised PaCO2 in respiratory acidosis is the increased reabsorption of HCO3-
● If the result is > 0.008 then there is associated metabolic acidosis
● If the result is between 0.003-
● If the result is < 0.003 there is associated metabolic alkalosis
If the result of ∆pH/∆PaCO2 is 0.003-
Compensation in respiratory alkalosis is decreased reabsorption of HCO3-
● If the result is > 0.008 then there is associated metabolic alkalosis
● If the result is between 0.002-
● If the result is < 0.002 there is associated metabolic acidosis
If the ∆pH/∆PaCO2 result is between 0.002-
This was just one way of interpreting results of the acid-
analysis like Lactate, Alb and electrolytes are additional parameters which can be analyzed. All of the previous enhance diagnostic process and better treatment plan of diseases.
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Natalija VUKOVIĆ, MD, PhD
Clinical Centre Niš
Clinic for Anesthesiology and Intensive Care
Bulevar Zorana Djindjića 48, Niš
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