Cardiac output by impedance cardiography: two alternative methodologies compared with thermodilution.

One critical component of the Kubicek (8) stroke volume equation, used to measure cardiac output by impedance cardiography, is dZ/dt(max). The present study, conducted on six anesthetized dogs, compared impedance cardiac output derived using two alternative methods of quantifying dZ/dt(max), with simultaneous thermodilution measures. Values for dZ/dt(max) quantified relative to dZ/dt = 0 baseline and relative to the dZ/dt B-point were entered into the Kubicek equation to generate cardiac output measures designated as impedance-A and impedance-B, respectively. Emphasis was placed upon evaluating impedance cardiac output within its accepted limits as a relative change measure. Systematic alterations in cardiac output were produced by intravenously administered isoproterenol, phenylephrine and nitroprusside. Drug-induced changes in cardiac output measured by the impedance-A method were statistically identical to thermodilution. For the impedance-B method, similarity to thermodilution was limited to two of the three drug conditions. Correlations of impedance cardiac output with thermodilution tended to be higher for the impedance-A method. Together with theoretical and practical considerations, these results support quantification of dZ/dt(max) relative to dZ/dt = 0 when used in the Kubicek stroke volume equation.