Tissue-arterial PC0 2 difference is a better marker of ischemia than intramural pH (pHi) or arterial pH-pHi difference

Abstract

Gastric intramucosal pH (pHi) is often calculated by the Henderson-Hasselbalch equation, using arterial plasma [HCO 3 −]a p and PCO 2 measured in saline obtained from a silastic balloon tonometer after equilibration in the lumen of the stomach. A pHi value less than approximately 7.3 pH units is often taken as evidence of intestinal ischemia. An alternative measure is tissue Pco 2 (PtCO 2) - PaCO 2 difference [P(t − a)CO 2]. The idea is that Ptco 2 will increase slightly relative to PtCO 2 as O 2 supply decreases, and then increase strikingly when flow decreases to a critical value, because of liberation of CO 2 from tissue HCO 3 − by anaerobically generated strong acid. A third method is arterial plasma pH (pHa p) - pHi difference [pH(a p - i)]. We used mathematical simulations to test the hypotheses that calculated pHi is independent of arterial acid-base status; and pH(a p - i) provides the same information as does P(t - a) CO 2. Using the Van Slyke version of the arterial whole blood [standard base excess] ([SBE]a WB) equation, it was found that a change in [SBE]a WB at constant PaCO 2 and constant PtCO 2 produces a change in calculated pHi ( P = 0), such that the relation between changing [SBE]a WB and changing pHi is predictable by a single polyomial equation (R 2 = .999). pH(a p - i) avoids this confounding influence of [SBE]a WB. However, it was further shown that pH(a p - i) can be associated with a wide range of P(t - a)CO 2, depending on the magnitude of pH(a p - i), and on the PaCO 2 at which P(t - a)CO 2 is measured. We conclude that P(t - a)C0 2 is a more reliable index of gastric oxygenation than is pHi alone or pH(a p - i).