Background: Cardiac function curves are widely accepted to apply to humans but are not established for the entire range of filling of the heart that can be elicited during head-up (HUT) and head-down tilt (HDT), taken to represent minimal and maximal physiological filling of the heart, respectively. With the supine resting position as a reference, we assessed stroke volume (SV), cardiac output (CO) and filling of the heart during graded tilt to evaluate whether SV and CO are maintained during an assumed maximal physiological filling of the heart elicited by 90° HDT in healthy resting humans.

Methods: In 26 subjects, central blood volume was manipulated with graded tilt from 60° HUT to 90° HDT. We measured SV, CO (Finometer®) and cardiac filling by echocardiography of the left ventricular end-diastolic volume (LVEDV; n=12).

Results: From supine rest to 60° HUT, SV and CO decreased 23 ml [confidence intervals (CI): 16–30; P<0.001; 23%] and 0.9 l/min (0.4–1.4; P<0.0001; 14%), respectively, but neither SV nor CO changed during HDT up to 70°. However, during 90° HDT, SV decreased 12 ml (CI: 6–19; P<0.0001; 12%), with an increase of 21 ml (9–33; P=0.002; 16%) in LVEDV because HR increased 3 bpm and CO decreased 0.5 l/min (ns).

Conclusion: This study confirmed that SV and CO are maximal in resting, supine, healthy humans and decrease during HUT. However, 90° HDT was associated with increased LVEDV and induced a reduction in SV.