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I believe the article by Pianosi (February 1997)1 concerning the measurement of cardiac output using impedance cardiogra¬ phy (ICG) illustrates a potentially serious defect in the evaluation of measurement techniques, related to results calculated using a formula with parameters not directly determined by the pro¬ posed measurement. Specifically, the formula for calculation of cardiac output, Q (stroke volume X heart rate) contains variables for the resistivity of blood, r; the distance between electrodes, L (which is an indirect measurement of height); ventricular ejection time, VET; heart rate; and the measured parameter [dZ/dt]/ZoThe authors compare Q determined from this formula to Q determined from C02 rebreathing and from oxygen consump¬ tion. I suspect that in this experiment, VET and heart rate contribute much more significantly to the described correlation between differently determined Qs than does [dZ/dt]/Zo, the presumed basis for the measurement. Without presentation of the behavior of the impedance data related to the alternatively measured cardiac output, it is not possible to establish the contribution of the impedance measurement itself to the final result, and thus it is not possible to conclude that the ICG is any better for measuring changes in Q than, for example, a value of Q calculated on the basis of VET and heart rate alone.