The Use of Noninvasive Bioelectric Impedance to Determine Cardiac Output: Factors Affecting Its Accuracy

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Abstract
Cardiac output was measured simultaneously by bioelectric impedance and thermodilution techniques in 11 anesthetized male hounds. Impedance was calculated using the Kubicek equation from the following three dZ/dt signals: (1) the peak inspiratory dZ/dt signal on the calibration baseline; (2) the baseline dZ/dt signal (the stable second or third signal postinspiration on the calibration baseline), and (3) the end-expiratory dZ/dt signal. Each calculated cardiac output was compared to the simultaneously measured thermodilution cardiac output. Impedance cardiac outputs from the inspiratory and expiratory waveforms were significantly different (p < 0.05) than the simultaneous thermodilution cardiac outputs (mean ± SD in ml/min/kg was 339 ± 76, 169 ± 55 and 219 ± 52, respectively). There was no significant difference (p > 0.05) between thermodilution cardiac output and impedance baseline signal cardiac output. Mean values were 219 ± 52 and 231 ± 52 ml/min/kg, respectively. In each of these cases, impedance cardiac outputs were calculated using the actual value for blood resistivity (rho) as determined from the hematocrit. However, we did compare impedance values derived by using the baseline dZ/dt signal, and an assumed normal rho =150 ohm-cm, to thermodilution values. No significant differences between the means were found, with the mean impedance cardiac output equal to 247 ± 53 ml/min/kg. Within any one hound actual rho values can deviate from normal and can adversely affect the accuracy of cardiac output measurements. These results indicate that baseline dZ/dt signals combined with the absolute value of rho accurately predict cardiac output when compared to thermodilution determinations.

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