The Journal of General Physiology, Vol 94, 567-590, Copyright © 1989 by The Rockefeller University Press
Exchange of oxygen across the epicardial surface distorts estimates of myocardial oxygen consumption
DS Loiselle
Department of Physiology, School of Medicine, University of Auckland, New Zealand.
The rate of oxygen consumption of isolated, Langendorff-circulated,
saline-perfused hearts of guinea pigs, rats, and rabbits was measured using
the classical Fick Principle method. The heart was suspended in a glass
chamber the oxygen partial pressure, PO2, of which could be varied. The
measured rate of oxygen consumption was found to vary inversely with the
ambient (heart chamber) PO2. This result prevailed whether the chamber was
filled with air, saline, or oil, and whether the pericardium was present or
the heart was wrapped in Saran. The effect varied inversely with heart size
both within and across species. It is concluded that the epicardial surface
is permeable to oxygen which will diffuse either into or out of the heart
as the PO2 gradient dictates. In either case the classically measured rate
of oxygen consumption will be in error. The error can be large in studies
of cardiac basal metabolism. A simple model is developed to describe the
observed rate of oxygen consumption as classically measured. The measured
rate is partitioned into two components: the true rate of oxygen
consumption of the heart, and the rate of loss of oxygen by diffusive
exchange across the epicardial surface. The latter component is
proportional to the gradient of oxygen partial pressure from myocardium to
environment and to the diffusive oxygen conductance of myocardial tissue.
Application of the model allows the true rate of oxygen consumption of the
heart to be recovered from measured values which may be considerably in
error.
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