Electrical impedance tomography (EIT) is a non-invasive imaging technique for detecting blood volume changes that can visualize pulmonary perfusion. The two studies reported here tested the hypothesis that the size of the pulmonary microvascular bed, rather than stroke volume (SV), determines the EIT signal. In the first study, the impedance changes relating to the maximal pulmonary pulsatile blood volume during systole (ΔZsys) were measured in ten healthy subjects, ten patients diagnosed with chronic obstructive pulmonary disease, who were considered to have a reduced pulmonary vascular bed, and ten heart failure patients with an assumed low cardiac output but with a normal lung parenchyma. Mean ΔZsys (SD) in these groups was 261 (34)×10-5, 196 (39)×10-5 (P<0.001) and 233 (61)×10-5 arbitrary units (AU) (P=NS), respectively. In the second study, including seven healthy volunteers, ΔZsys, was measured at rest and during exercise on a recumbent bicycle while SV was measured by means of magnetic resonance imaging. The ΔZsys at rest was 352 (53)×10-5 and 345 (112)×10-5 AU during exercise (P=NS), whereas SV increased from 83 (21) to 105 (34) ml (P< 0.05). The EIT signal likely reflects the size of the pulmonary microvascular bed, since neither a low cardiac output nor a change in SV of the heart appear to influence EIT.