Clinical Approach to Acid-Base Balance

Abstract

The assessment of acid-base balance is one of the most-fundamental aspects of the management of critically ill patients. Traditional teaching has been based on the Henderson-Hasselbalch equation, which is the mass action equation written in the form of the equilibrium relationship of H+ to CO₂ and HCO₃. The equation is

$$ pH = 6.1 + log([HC{O_3} - ]/[0.0301 \times PC{O_2}]) $$

where 0.031 is the constant to account for the solubility of CO₂ in water, and 6.1 the negative log of the pKa of the dissociation of carbonic acid. Using this approach, one first determines if there is an acid-base disturbance by observing whether there is a deviation in the pH from the physiological neutral value of 7.4. Increases or decreases in the pCO₂ are used to determine if there is a respiratory component, and variations in HCO₃ are used to determine if there is a metabolic component. A complex set of equations can then be used to determine if the changes are acute or chronic and compensated or uncompensated. This approach has by and large served clinicians adequately. However, it provides no insight into physiological mechanisms and potentially produces some very faulty reasoning.