May 1997

Measurement of Gastrointestinal Intramucosal pH Is a Poor Guide to Tolerable Levels of Anemia During Isovolemic Hemodilution in a Canine Model of Coronary Stenosis

Author Affiliations

From the Department of Anesthesiology, Duke University Medical Center, Durham, NC.

Arch Surg. 1997;132(5):547-552. doi:10.1001/archsurg.1997.01430290093019

Objective:  To determine the relationship between gastrointestinal intramucosal pH and myocardial oxygenation during isovolemic hemodilution in dogs with critical coronary artery stenoses.

Design:  Prospective sequential evaluation of ileal intramucosal pH and regional myocardial function of a critically perfused area of myocardial tissue in a canine model of normovolemic hemodilution.

Setting:  A research laboratory.

Subjects:  Fifteen dogs.

Interventions:  A micrometer snare was placed around a main coronary artery (8 left anterior descending artery, 7 right coronary artery). Three pairs of sonomicrometer crystals were placed in the heart to measure regional myocardial contraction. A gastrointestinal tonometer was placed in the ileum and used to measure luminal Pco2. This Pco2 value was used to calculate the ileal intramucosal pH. The animals underwent normovolemic hemodilution until myocardial ischemia occurred in the region supplied by the snared vessel. Measurements were continued for a further 40 minutes.

Measurements and Main Results:  Following instrumentation, stabilization, and critical constriction of a coronary vessel, percentage changes in systolic shortening of myocardial tissue in the region of critical perfusion were determined every 20 minutes. Ileal intramucosal pH and commonly measured cardiovascular variables were determined at the same time points. Myocardial ischemia occurred after 80 minutes of hemodilution, when the mean (±SD) hemoglobin concentration had fallen from a baseline level of 123±18g/L to 82±14g/L (P<.01). From the start of hemodilution to 40 minutes after myocardial ischemia occurred, there were no significant changes in heart rate, cardiac output, oxygen consumption, arterial acid-base balance, or arterial Pco2. Oxygen delivery decreased by approximately 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 more sensitive to normovolemic hemodilution—induced ischemia than the normally perfused gut mucosa. This limits the potential utilization of the measurement of gastrointestinal intramucosal pH as a guide to tolerable levels of anemia in critically ill patients.Arch Surg. 1997;132:547-552