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 (S m O 2 ) was examined during (pre)syncope induced by head‐up tilt ( HUT ). Skin blood flow (Sk BF ) and oxygenation (S skin O 2 ) were determined because evaluation of S m O 2 may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (S c O 2 ) and middle cerebral artery mean blood flow velocity ( MCA v mean ). 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. S c O 2 and S m O 2 (near‐infrared spectroscopy), MCA v mean (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, S c O 2 decreased [73% (71–76; mean and 95% CI ) to 68% (65–71), P < 0.0001] along with MCA v mean [40 (37–43) to 32 (29–35) cm s −1 , P < 0.0001]. In contrast, S m O 2 increased [63 (56–69)% to 71% (65–78), P < 0.0001], while S skin O 2 [64% (58–69) to 53% (47–58), P < 0.0001] and Sk BF [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 S m O 2 despite reduced S skin O 2 and Sk BF , supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT .