TISSUE-ARTERIAL PCO(2) DIFFERENCE IS A BETTER MARKER OF ISCHEMIA THANINTRAMURAL PH (PHI) OR ARTERIAL PH-PHI DIFFERENCE

Gastric intramucosal pH (pHi) is often calculated by the Hwenderson-Ha sselbalch equation, using arterial plasma [HCO3-]a(p) and PCO2 measure d in saline obtained from a silastic balloon tonometer after equilibra tion 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 alt ernative measure is tissue PCO2 (PtCO2) - PaCO2 difference [P(t - a)CO 2]. The idea is that PtCO2 will increase slightly relative to PaCO2 as O-2 Supply decreases, and then increase strikingly when flow decrease s to a critical value, because of liberation of CO2 from tissue HCO3- by anaerobically generated strong acid. A third method is arterial pla sma 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 infor mation as does P(t - a) CO2. Using the Van Slyke version of the arteri al whole blood [standard base excess] ([SBE]a(WB)) equation, it was fo und that a change in [SBE]a(WB) at constant PaCO2 and constant PtCO2 p roduces a change in calculated pHi (P = 0), such that the relation bet ween changing [SBE]a(WB) and changing pHi is predictable by a single p olyomial 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)CO2, depending on th e magnitude of pH(a(p) - i), and on the PaCO2 at which P(t - a)CO2 is measured, We conclude that p(t - a)CO2 is a more reliable index of gas tric oxygenation than is pHi alone or pH(a(p) - i). Copyright (C) 1996 by W.B. Saunders Company