Impedance cardiography (ICG) is a noninvasive technique used to estimate left ventricular stroke volume (SV_LV) using thoracic impedance (ΔZ). It remains controversial, partly because ICG parameters have not been successfully related to hemodynamic events. We hypothesized that the change in ΔZ may be proportional to the variation in thoracic blood volumes, primarily aortic blood volume.

Nine anaesthetized dogs were divided into two groups: the “Aortic Volume Group” (n=5), where aortic and IVC (inferior vena cava) dimensions were measured ultrasonically, and the “Reservoir Volume Group”, where aortic and IVC reservoir volumes were calculated using the reservoir-wave approach. Measurements were made under control conditions, with nitroprusside, with methoxamine (Mtx), and after volume loading.

In both groups, the maximum rate of increase in ΔZ, (dZ/dt)_max, strongly correlated with the maximum rate of change in aortic/reservoir blood volume (r²= 0.85 and 0.95, respectively), which in turn were proportional to SV_LV. As shown in the figure, the aortic reservoir volume (V_{Ao reservoir}) explains SV_LV in that measured aortic flow (Q_Ao) equals the sum of dV_{Ao reservoir}/dt and calculated Q_out. The LV and IVC contributions to ΔZ were small under control conditions (∼5 and 1%, respectively), but the LV contribution increased slightly with administration of Mtx and after volume loading (to 7 and 10%, respectively).

We conclude that the change in thoracic impedance (ΔZ) during the cardiac cycle is proportional to the change in aortic reservoir volume, which mechanistically explains why ICG and standard measures of cardiac output have sometimes been shown to correlate well.