Measurement of gastrointestinal intramucosal pH is a poor guide to tolerable levels of anemia during isovolemic hemodilution in a canine model of coronary stenosis
M. Layland, M. G. Mythen, K. Y. Wang, P. Albaladejo and B. J. Leone
Department of Anesthesiology, Duke University Medical Center, Durham, NC, USA.
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 +/- 18 g/L to 82 +/- 14 g/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.
