Relationship between the apparent dissociation constant of blood carbonic acid and severity of illness.

The Henderson-Hasselbalch equation is commonly used to calculate plasma bicarbonate and CO2 content (tCO2) from blood gas measurements and an assumed constant value of the apparent dissociation constant of blood carbonic acid (pK'). Several studies have reported pK' to be variable in critically ill patients. We prospectively compared the pK' of patients in an intensive care unit to their severity of illness. Blood specimens were analyzed for pH, Pco2, and tCO2, and the results were used to calculate pK'. The tCO2 was also calculated from this equation by means of the measured pH and Pco2 and from an assumed constant pK'. Severity of illness was evaluated with the acute physiology score and the Therapeutic Intervention Scoring System. A total of 2004 specimens were analyzed; they had a mean pK' of 6.126. A strong correlation was shown between calculated and measured tCO2; however, there was essentially no correlation between disease severity and pK'. We conclude that bicarbonate and tCO2 can be accurately calculated in critically ill patients.