Accuracy of intramucosal pH calculated from arterial bicarbonate and the Henderson-Hasselbalch equation: Assessment using simulated ischemia

Objectives: To determine the accuracy of intramucosal pH (pHi) calculated u sing arterial bicarbonate instead of mucosal capillary bicarbonate in the H enderson-Hasselbalch equation. Design: Simulation of progressive ischemia in mucosal capillary blood, Setting: University research laboratory, Subjects: Normal human blood diluted with plasma, Interventions: Three venous blood specimens were heparinized and diluted to a mean hemoglobin concentration of 5.0 (+/-0.9) g/dL by addition of plasma (2:1, vol:vol), Mucosal capillary aerobic flow stagnation was simulated by multiple exposures of each cooled specimen to a gas mixture containing 90% nitrogen and 10% CO2, When PCO2 measured at 37 degrees C (98.6 degrees F) was approximately 120 torr (16 kPa), the assigned anaerobic threshold, subs equent anaerobic flow stagnation was simulated by mixing the hypercapnic sp ecimens in sealed syringes with five to six successive small aliquots (<100 mu L) of lactic acid (10 g/L), Measurements and Main Results: The relationship between PCO2 and pH in the specimens was compared with the relationship between the same PCO2 values a nd pHi calculated by substituting bicarbonate concentrations of 22 and 26 m mol/L in the Henderson-Hasselbalch equation. As PCO2 rose from 50 torr (8 k Pa), conventionally calculated pHi increasingly underestimated simulated mu cosal capillary ph, with bias >0.1 pH unit at the simulated anaerobic thres hold of 120 torr (16 kPa), As PCO2 rose further the values converged, becom ing equivalent at PCO2 similar to 150 torr (20 kPa), From PCO2 greater than or equal to 200 torr (26.7 kPa), conventional pHi progressively overestima ted simulated mucosal pH, The difference was >0.3 pH units at PCO2 = 250 to rr (33.3 kPa), Conclusions: In the mucosal PCO2 range usually encountered clinically, the arterial bicarbonate substitution causes underestimation of mucosal capilla ry pH, With moderate mucosal capillary lactic acidosis the error becomes sm all, and in severe regional ischemia there is significant overestimation of mucosal capillary pH.