Estimation of left ventricular stroke volume by impedance cardiography: its relation to the aortic reservoir

Impedance cardiography is a non-invasive technique used to estimate left ventricular (LV) stroke volume (SV) using the change in thoracic impedance (ΔZ). It remains controversial, partly because impedance cardiographic parameters have not been successfully related to haemodynamic events. We hypothesized that the change in ΔZ may be proportional to the variation in thoracic (primarily aortic) blood volumes. Nine anaesthetized and ventilated dogs were divided into the following two groups: the 'aortic volume group' (n = 5), in which aortic and IVC (inferior vena caval) dimensions were measured ultrasonically; and the 'reservoir volume group', in which aortic and IVC reservoir volumes were calculated using the reservoir-wave model. Measurements were made in control conditions, in the presence of nitroprusside and methoxamine and after volume loading. In both the aortic volume group and the reservoir volume group, the maximal rate of increase in ΔZ [(dZ/dt)max] strongly correlated with the maximal rate of change in aortic/reservoir blood volume (R(2) = 0.85 and 0.95, respectively), which in turn was proportional to the LV SV. The LV and IVC contributions to ΔZ were small in control conditions (∼5 and 1%, respectively), but the LV contribution increased slightly (to 7%) with administration of methoxamine and after volume loading (to 10%). It is concluded that the change in thoracic impedance (ΔZ) during the cardiac cycle is proportional to the change in aortic reservoir (i.e. Windkessel) volume, which provides a mechanistic explanation for previously demonstrated good correlations with standard measures of cardiac output.