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 O ) was examined during (pre)syncope induced by head-up tilt (HUT). Skin blood flow (SkBF) and oxygenation (S O ) were determined because evaluation of S O may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (S O ) and middle cerebral artery mean blood flow velocity (MCAv ).

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 O and S O (near-infrared spectroscopy), MCAv (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 O decreased [73% (71-76; mean and 95% CI) to 68% (65-71), P < 0.0001] along with MCAv [40 (37-43) to 32 (29-35) cm s , P < 0.0001]. In contrast, S O increased [63 (56-69)% to 71% (65-78), P < 0.0001], while S O [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 S O despite reduced S O and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT.