MEASUREMENT OF GASTROINTESTINAL INTRAMUCOSAL PH IS A POOR GUIDE TO TOLERABLE LEVELS OF ANEMIA DURING ISOVOLEMIC HEMODILUTION IN A CANINE MODEL OF CORONARY STENOSIS

Objective: To determine the relationship between gastrointestinal intr amucosal pH and myocardial oxygenation during isovolemic hemodilution in dogs with critical coronary artery stenoses. Design: Prospective se quential evaluation of ileal intramucosal pH and regional myocardial f unction of a critically perfused area of myocardial tissue in a canine model of normovolemic hemodilution. Setting: A research laboratory. S ubjects: Fifteen dogs. Interventions: A micrometer snare was placed ar ound a main coronary artery (8 left anterior descending artery, 7 righ t coronary artery). Three pairs of sonomicrometer crystals were placed in the heart to measure regional myocardial contraction. A gastrointe stinal tonometer was placed in the ileum and used to measure luminal P CO2. This PCO2 value was used to calculate the ileal intramucosal pH. The animals underwent normovolemic hemodilution until myocardial ische mia occurred in the region supplied by the snared vessel. Measurements were continued for a further 40 minutes. Measurements and Main Result s: Following instrumentation, stabilization, and critical constriction of a coronary vessel, percentage changes in systolic shortening of my ocardial tissue in the region of critical perfusion were determined ev ery 20 minutes. Ileal intramucosal pH and commonly measured cardiovasc ular variables were determined at the same time points. Myocardial isc hemia occurred after 80 minutes of hemodilution, when the mean (+/-SD) hemoglobin concentration had fallen from a baseline level of 123+/-18 gn to 82+/-14 g/L (P<.01). From the start of hemodilution to 40 minut es after myocardial ischemia occurred, there were no significant chang es in heart rate, cardiac output, oxygen consumption, arterial acid-ba se balance, or arterial PCO2. Oxygen delivery decreased by approximate ly 45% (5.99+/-1.66 to 3.41+/-0.90 mL/kg per minute; P<.01) but there were no changes in ileal intramucosal pH (7.31+/-0.08 to 7.30+/-0.08; P=.90). Conclusions: Myocardium compromised by coronary stenosis is mo re sensitive to normovolemic hemodilution-induced ischemia than the no rmally perfused gut mucosa. This limits the potential utilization of t he measurement of gastrointestinal intramucosal pH as a guide to toler able levels of anemia in critically ill patients.