Muscle oxygen saturation increases during head‐up tilt‐induced (pre)syncope

Aim To evaluate whether muscle vasodilatation plays a role for hypotension developed during central hypovolaemia, muscle oxygenation (SmO2) was examined during (pre)syncope induced by head‐up tilt (HUT). Skin blood flow (SkBF) and oxygenation (SskinO2) were determined because evaluation of SmO2 may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (ScO2) and middle cerebral artery mean blood flow velocity (MCAvmean). Methods Twenty healthy male volunteers (median age 24 years; range 19–38) were subjected to passive 50° HUT for 1 h or until (pre)syncope. ScO2 and SmO2 (near‐infrared spectroscopy), MCAvmean (transcranial Doppler) along with mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR) (Modelflow®) were determined. Results (Pre)syncopal symptoms appeared in 17 subjects after 11 min (median; range 2–34) accompanied by a decrease in MAP, SV, CO and TPR, while HR remained elevated. During (pre)syncope, ScO2 decreased [73% (71–76; mean and 95% CI) to 68% (65–71), P < 0.0001] along with MCAvmean [40 (37–43) to 32 (29–35) cm s−1, P < 0.0001]. In contrast, SmO2 increased [63 (56–69)% to 71% (65–78), P < 0.0001], while SskinO2 [64% (58–69) to 53% (47–58), P < 0.0001] and SkBF [71 (44–98) compared to a baseline of 99 (72–125) units, P = 0.020] were reduced. Conclusion We confirm that the decrease in MAP during HUT is associated with a reduction in indices of cerebral perfusion. (Pre)syncope was associated with an increase in SmO2 despite reduced SskinO2 and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT.