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 SmO2may 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°HUTfor 1 h or until (pre)syncope. ScO2and 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 inMAP,SV,COandTPR, whileHRremained elevated. During (pre)syncope, ScO2decreased [73% (71–76; mean and 95%CI) to 68% (65–71),P< 0.0001] along withMCAvmean[40 (37–43) to 32 (29–35) cm s−1,P< 0.0001]. In contrast, SmO2increased [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 inMAPduringHUTis associated with a reduction in indices of cerebral perfusion. (Pre)syncope was associated with an increase in SmO2despite reduced SskinO2and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension duringHUT.