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Values of stroke volume calculated from five different formulas utilising parameters derived from the aortic-pressure pulse waveform were compared with stroke-volume values calculated from the thoracic electrical-impedance waveform, which was recorded simultaneously with the aortic-pressure waveform. A total of 195 beats from three closed-chest anaesthetised dogs were compared. During the recording of the impedance and blood-pressure waveforms, the cardiac output was measured by dye and saline dilution. The mean of stroke-volume values from the impedance method was 39% greater than the mean ‘stroke volume’ calculated by dividing the dye cardiac-output value by the mean heartrate measured during the recording of the dye-dilution curve. The correlation between impedance and dye cardiac-output values was 0·879. The correlation of dye ‘stroke volume’ with values calculated by all four formulas was about 0·89. Individual stroke volumes could be markedly affected by variations in the beat-by-beat values of the total systemic resistance.

Three approximate methods for the computation of cardiac output from dye-dilution curves have been analysed using the classical Stewart-Hamilton technique as a reference in a total of 89 curves obtained from 11 dogs. The gamma-function method has a tendency to overestimate on the average (+7·1%±1·6%), with an 82% probability of falling within ±20% of the Stewart-Hamilton value. The forward-triangle and compartmental methods are approximately equivalent, the first giving a small overestimation of (0·3%±1·2%), and the second a small underestimation of (−3%±1·2%); both methods have a probability of better than 92% of being within ±20% of the Stewart-Hamilton value. The analysis of human radiocardiograms by the compartmental method was encouraging. A number N of equivalent compartments greater than 2·2 for either the right-or left-heart peaks was associated with an abnormal radiocardiogram. In eight normal subjects, N yielded an average of 1·73 for the right heart and 1·69 for the left heart. The compartmental analysis of radiocardiograms, together with a study of other parameters of the curves, would seem to provide a useful assessment of the cardio-vascular function.