Constancy of blood carbonic acid pK' in patients during cardiopulmonary resuscitation.

Previous studies have suggested that the apparent dissociation constant of blood carbonic acid (pK') may actually vary in acutely ill patients. We prospectively compared the pK' of healthy control subjects to that of patients undergoing cardiopulmonary resuscitation (CPR). Arterial blood obtained from 20 patients undergoing CPR and from 30 healthy volunteers was analyzed for Na+, pH, PCO2, and total CO2 content (tCO2). pK' was calculated from this data, using the Henderson-Hasselbalch equation. Total CO2 was then calculated in the CPR patients, using this equation and the control pK'. Mean pK' was 6.109 +/- 0.004 (SEM) for the control group and 6.123 +/- 0.007 for the CPR group (p = NS). In the CPR group, calculated tCO2 was not significantly different from measured from tCO2, and the correlation between calculated and measured tCO2 was 0.99. In patients undergoing CPR, pK' does not differ significantly from normal, and tCO2 can be accurately estimated with the Henderson-Hasselbalch equation.