A COMPARISON OF THE ESTIMATION OF THE BASAL CARDIAC OUTPUT FROM A LINEAR FORMULA AND THE “CARDIAC INDEX” 1

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Abstract
The relationship between cardiac output and surface area was examined in a series of observations carried out with high-frequency ballistocardiograph, ethyl iodide method, acetylene method (drawn from the literature) and with the low-frequency, critically damped ballistocardiograph and acetylene method corrected to direct Fick levels (Laboratory of Physiol. Hygiene). The high-frequency ballistocardiograph and ethyl iodide procedures gave estimates of cardiac outputs not related to body size (surface area). The low-frequency, critically damped ballistocardiograph, the acetylene method, the direct Fick (catheter) procedure and the corrected acetylene all distinguished between large and small individuals in a satisfactory manner. The relative efficiencies of the cardiac index and the regression line as methods for prediction of the cardiac output from body size (surface area) were examined in data obtained with 4 methods for the detn. of cardiac output which are in current use. When the cardiac index was used as a standard for data obtained with the low-frequency, critically damped ballistocardiograph, the corrected acetylene and the direct Fick (catheter) procedure, the variance of the standard increased by only 3.6% or less (less than 2% in terms of the root mean square) over that found when the regression line was used. The use of the cardiac index did not impose an unacceptable penalty for prediction of individual values. Important errors could arise if large groups of different mean surface areas were to be compared by the use of the cardiac index.
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