Noninvasive cardiac output monitoring based on thoracic impedance

The maintenance of optimal cardiac output is mandatory to reduce the incidence of postoperative complications and length of hospital stay. Despite the advantages of cardiac output monitoring, most of the modalities to measure cardiac output had been invasive thereby preventing their liberal use. However, nowadays, cardiac output can be measured noninvasively using a thoracic bioimpedance technique. Assuming that the human thorax is a cylinder consisting of resistor and capacitor, the periodic change (in accordance with a cardiac cycle) in the thoracic impedance generated by low-amplitude alternating current can be measured. The maximal rate of change in the magnitude of thoracic impedance is correlated with stroke volume by which heart rate multiplied is cardiac output. However, the accuracy of cardiac output measured by the magnitude of thoracic impedance is significantly influenced by several factors, such as electrode positioning, electrocautery, or static fluids in the thorax. In contrast, the phase angle of thoracic impedance is not affected by those factors and the maximal rate of its change is also correlated with stroke volume. Therefore, the phase angle of thoracic impedance is more useful for the noninvasive cardiac output monitoring than the magnitude of thoracic impedance.