AJP - Heart and Circulatory Physiology, Vol 256, Issue 1 302-H310, Copyright © 1989 by American Physiological Society
Errors in microsphere determination of cardiac output: a computer simulation in fetal sheep
I. D. Hope, F. J. Huikeshoven, R. D. Gilbert, G. G. Power and L. D. Longo
Department of Physiology, School of Medicine, Loma Linda University, California 92350.
Through use of a compartmental model, we simulated the measurement of
cardiac output and distribution by means of radioactively labeled
microspheres in fetal lambs with weights between 0.5 and 3 kg. A systematic
error in measured cardiac output caused by artifactual changes in blood
volume caused by the injection and withdrawal of fluids during the
procedure was less than 5% for fetal weights greater than or equal to 1 kg
but increased for fetal weights less than 1 kg and when hypovolemia was
simulated at all fetal weights. Sensitivity analysis disclosed no
significant effect of changes in vascular resistance. We examined the
effects of recirculation of microspheres and found no significant increase
in error in the measured value of cardiac output due to 20% recirculation
of all spheres entering a single isolated organ system; however, errors
between 7 and 14% were observed with simulations of 20% recirculation in
more than one compartment simultaneously. Recirculation also introduced
significant errors in the measured distribution of cardiac output in
certain cases. The effect on the measured cardiac output of a temporary
change in the true cardiac output was dampened by the artifactual blood
volume changes mentioned above and the fact that the measurement is a
time-weighted average. We also evaluated four different experimental
designs. We conclude that the microsphere technique provides a remarkably
reliable means of quantifying cardiac output and individual organ flow in
the fetus. The nonrandom errors inherent in the procedure examined in this
study are of the order of 10%, which is likely to be less than the
moment-to-moment variation in the true cardiac output.
