• Acid-base status can be assessed using arterial or venous pH level, BE (base excess) and PaCO2/ PvCO2 (conversion from VBG to ABG values covered in Blood Gases section), and serum HCO3.
Acidemia: defined as low blood pH due to either abnormally high PaCO2/ PvCO2 (respiratory acidosis), or abnormally low serum HCO3 (metabolic acidosis), or both.
Alkalemia: defined as high blood pH due to either abnormally low PaCO2/ PvCO2 (respiratory alkalosis), or abnormally high serum HCO3 (metabolic alkalosis), or both.
• Acid-base regulation involves maintaining the balance of HCO3 ions and CO2 gas molecules in the blood by the renal and respiratory systems, respectively (illustrated below).
• To workup acidemia, the first step is to determine whether it’s caused by metabolic acidosis (HCO3 < 22), respiratory acidosis (PaCO2 > 45), or both.
• In metabolic acidosis (low HCO3), there should be a compensatory mechanism in which CO2 is actively removed from the blood via hyperventilation, resulting in lower than normal PaCO2-- this drop in PaCO2 should be proportional to the fall in HCO3 and the ∆pCO2/ ∆HCO3 ratio should be close to 1.3 (if respiratory compensation is appropriate). Lower or higher than 1.3 might signify concurrent respiratory acidosis or alkalosis, respectively.
• In respiratory acidosis (high PaCO2), there should be a compensatory mechanism in which HCO3 is actively reabsorbed by the kidney, resulting in higher than normal HCO3-- this rise in HCO3 should be proportional to the rise in pCO2 and the ∆HCO3/ ∆pCO2 ratio should be close to 0.1 (if acute respiratory acidosis) or 0.4 (if chronic respiratory acidosis). Lower or higher than 0.1 (or 0.4) might signify concurrent metabolic acidosis or alkalosis, respectively.
• To workup alkalemia, the first step is to determine whether it’s caused by metabolic alkalosis (HCO3 > 28), respiratory alkalosis (PaCO2 < 35), or both.
• In metabolic alkalosis (high HCO3), there should be a compensatory mechanism in which CO2 is actively retained in the blood via hypoventilation, resulting in higher than normal PaCO2-- this rise in PaCO2 should be proportional to the rise in HCO3 and the ∆pCO2/ ∆HCO3 ratio should be close to 0.7 (if respiratory compensation is appropriate). Lower or higher than 0.7 might signify concurrent respiratory alkalosis or acidosis, respectively.
• In respiratory alkalosis (low PaCO2), there should be a compensatory mechanism in which HCO3 is actively excreted by the kidney, resulting in lower than normal HCO3-- this fall in HCO3 should be proportional to the fall in pCO2 and the ∆HCO3/ ∆pCO2 ratio should be close to 0.2 (if acute respiratory acidosis) or 0.5 (if chronic respiratory acidosis). Lower or higher than 0.2 (or 0.5) might signify concurrent metabolic alkalosis or acidosis, respectively.
