Stroke volume pL2Z2Zm

where L is the mean separation of the inner pair of electrodes.

The sensing electrodes are usually applied to either side of the neck root and to opposite sides of the body in a midcoronal plain at xiphisternum level. Two pairs of transmitting electrodes are placed 5 cm above the neck sensors and 5 cm below the thoracic sensors respectively. Accurate electrode placement is crucial. A clear ECG trace is required to allow gating of data acquisition; displacement of a single lead will falsely reduce cardiac output.

The original Kubicek equation consistently overestimated stroke volume. As the thorax behaves electrically more like a truncated cone than a cylinder, correction factors have been introduced, notably by Sramek and Bernstein. Computer analysis can also eliminate variations due to gross movement and respiration. Under optimal conditions, bio-impedance cardiac output correlates reasonably with other techniques. However, shivering, valvular regurgitation, pacemaker devices, atrial fibrillation, positive end-expiratory pressure, marked tachycardia, different brands of electrode and, possibly, poor left ventricular function all impede reliability. Cardiac output may be overestimated in low-flow states, and underestimated in hyperdynamic states and soon after cardiac surgery. The frequent presence of one or more of these confounding factors in the ICU patient has precluded its routine adoption.

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