Thoracic bioimpedance

Impedance changes originate in the thoracic aorta when blood is ejected from the left ventricle. This effect is used to determine stroke volume from formulae utilising the electrical field size of the thorax, baseline thoracic impedance and fluctuation related to systole, and ventricular ejection time. A correction factor for sex, height and weight is also introduced. The technique simply utilises four pairs of electrodes placed in proscribed positions on the neck and thorax; these are connected to a dedicated monitor which measures thoracic impedance to a low amplitude, high (70kHz) frequency 2.5mA current applied across the electrodes.

Advantages

Quick, safe, totally non-invasive, reasonably accurate in normal, spontaneously breathing subjects.

Disadvantages

Discrepancies in critically ill patients (especially those with arrhythmias, tachycardias, intrathoracic fluid shifts, anatomical deformities, aortic regurgitation), metal within the thorax, inability to verify signal.

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