Blood resistivity and its implications for the calculation of cardiac output by the thoracic electrical impedance technique.

A relationship has been established between the haematocrit and the resistivity of whole blood at 37 degrees C for neonates and adults not suffering from renal failure. Values of resistivity obtained in this way were substituted into Kubicek's equation for stroke volume by the electrical impedance technique, the signal pick-up electrodes being placed in standard positions. The calculated cardiac outputs were then compared with those obtained simultaneously from other techniques such as the dye dilution method for adults and the measurement of pulmonary effective capillary blood flow by rebreathing nitrous oxide in neonates. It was found that the impedance method overrestimates under these circumstances, and other workers have shown that this is likely to be due to a contribution from the right heart. A haemotocrit-dependent correction factor has been employed in the case of neonates to align the impedance and nitrous oxide results.