The effect of a progressive decrease in the circulating blood volume of the dog on the transthoracic impedance.

The correlations between the haemodynamic and transthoracic electrial impedance changes resulting from a progressive reduction in the circulating blood volume were studied in four intact mongrel dogs artificially ventilated with a mixture of halothane in nitrous oxide-oxygen. The cardiac output of the dogs was measured by both the electrical impedance and the fibre optic dye dilution techniques. It was found that significant correlations existed between the blood lloss and the arterial blood pressure, the maximum first derivative of the transthoracic impedance, the Heather Index, the transthoracic impedance, the maximum rate of change of aortic pressure and the cardiac stroke work. There was also a good correlation between the dye and impedance cardiac output values, the impedance value always being higher than the corresponding dye value. The correlation between the Heather Index and the PEP/LVET ratio and 1/PEP2 varied markedly from dog to dog.