Basic principles of electrohaemodynamics

Electrohaemodynamics (EHD) is a method of measuring cardiac output continuously, also cardiac index, stroke volume, blood flow through limbs, vascular resistance, useful work output from the heart, elastic properties of arterial walls, and contractile and rate of energy generation properties of heart muscle. The method is based on simultaneous measurements of the time variations of arterial blood pressure and electrical conductance in a portion of human body. The measurements, in the form of analogue signals, are processed by means of a microprocessor or a mini-computer. The processing is based on equations which combine haemodynamics with measurements of electrical conductance and arterial pressure variation. Direct comparison of simultaneous cardiac output measurements using EHD and thermal dilution, as well as indirect comparisons with the Fick and dye dilution methods, show that EHD is at least as accurate as these methods. The reproducibility of EHD is within one percent when the subjects are in the steady state, which appears to be better than any other known technique.