AJP - Heart and Circulatory Physiology, Vol 256, Issue 2 527-H532, Copyright © 1989 by American Physiological Society
Effect of hemoglobin concentration on critical cardiac output and oxygen transport
F. Heusser, J. T. Fahey and G. Lister
Department of Pediatrics, Yale University School of Medicine, New Haven, Connecticut 06510.
We previously found limited tolerance to acute reduction in cardiac output
in lambs at the nadir of their physiological anemia [Am. J. Physiol. 253
(Heart Cir. Physiol. 12): H100-H106, 1987]. To determine the effect of
hemoglobin concentration [Hb] on critical cardiac output, critical systemic
O2 transport, and peripheral O2 extraction, we performed 31 experiments in
12 one-mo-old lambs at four [Hb] (means +/- SD in g/dl): 7.4 +/- 0.6, 10.5
+/- 0.5, 14.5 +/- 0.5, and 16.5 +/- 0.6. Desired [Hb] was obtained by
exchange transfusion with packed red cells or plasma. Cardiac output was
reduced by inflation of a balloon-tipped catheter in the right atrium, and
critical levels were defined at the point where O2 consumption decreased
and/or arterial blood lactate concentration increased in response. With
lower [Hb], cardiac output was unchanged, systemic O2 transport was
reduced, and fractional O2 extraction was increased, keeping O2 consumption
constant at base line. As [Hb] was reduced, critical cardiac output was
significantly higher, whereas critical systemic O2 transport was
independent of [Hb], as were fractional O2 extraction and mixed venous PO2
at the critical point. Thus peripheral O2 extraction was not affected by
changes in [Hb] during progressive decreases in cardiac output. We conclude
that 4-wk-old lambs have decreased tolerance to reductions in cardiac
output and systemic O2 transport because their relative anemia provides
them with a base-line cardiac output and systemic O2 transport close to the
critical level.
