Physiological and fluid-dynamic investigations of the transthoracic impedance plethysmography method for measuring cardiac output: Part I—A fluid-dynamic approach to the theory using an expansible tube model

The method of Nyboer and Kubicek for measuring stroke volume (s.v.) was used with an expansible tube model to analyse the relationship between the impedance variation and the actual flow. A preliminary correlation study between the method and electromagnetic flowmetry was conducted on 28 dogs: The correlation coefficient and the percentage of decentralisation within a ±25% error for the individual dogs were 0·79 and 53·5%, while those for a dog whose s.v. was controlled by drugs were 0·88 and 82·4%. The s.v. value and volume flow rate of a sinusoidal flow, as well as of a pulsatile flow, in the tube with an occluded outflow can be measured exactly by Nyboer's equation. In the tube with an opened outflow the s.v. value of the pulsatile flow can be measured only approximately by Kubicek's equation. In both tubes, the hydraulic frequency of the flow and the tube length are the most important factors governing the working frequency range of the method. Based on the results, we have discussed how to utilise the theory with a living subject.