Errors in microsphere determination of cardiac output: a computer simulation in fetal sheep.

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 i...