Differences between syncope resulting from rapid onset acceleration and orthostatic stress.

BACKGROUND AND HYPOTHESIS

Orthostatically-induced syncope is accompanied by venous pooling and vasodilatation. Loss of consciousness during head-to-foot acceleration (G-LOC) in aviators may be caused by a different mechanism, as venous pooling should be prevented through the use of an anti-G suit. This research was conducted to test the hypothesis that in individuals wearing a well-fitted anti-G garment, no important changes occur in the volume of dependent regions during loss of consciousness resulting from rapid onset acceleration stress. Further, this work compares venous pooling patterns in G-LOC subjects to patterns seen during syncope in volunteers and patients subjected to orthostatic stress. We conducted the tilt/LBNP tests to establish what level of venous pooling was required to induce syncope in the absence of a hydrostatic component (other than 1 G) and to confirm that our equipment was sensitive enough to detect volume changes large enough to cause syncope.

METHODS

Shifts in blood volume to the calf, thigh and abdominal segments were compared in subjects with G-LOC to those in subjects taken to presyncope with orthostatic stress created by upright tilt and lower body negative pressure (LBNP). Centrifuge subjects were exposed to a 15 s rapid onset (6 G.s-1) + 5 Gz exposure on the centrifuge while remaining relaxed and wearing a well-fitting anti-G suit, but with the anti-G suit pressure inactivated.

RESULTS

Blood volume decreased an average of 14.9 +/- 22.1 ml in the calf segment; increased an average of 64.1 +/- 7.9 ml in the thigh segment, and decreased an average of 80.1 +/- 29.7 ml in the abdominal segment. The mean net change in volume of the three combined regions was not significantly different from zero. Presyncope was induced in subjects by a progressive exposure to upright tilt, and then addition of LBNP at -20 mm Hg and -40 mm Hg. In the tilt/LBNP group, there was a net increase of 1022 +/- 269.8 ml for the combined segments. Changes in all three segments were significantly different than the mean segmental volume changes seen in centrifuge subjects at G-LOC endpoints. Significant changes from baseline mean arterial pressure, but not heart rate were also seen within, but not between the 2 groups, with mean eye level blood pressures (ELBP) falling an average of 45.6 +/- 7.7 mm Hg in the tilt/LBNP group at syncope and 105.1 +/- 15.5 mm Hg in the centrifuge subjects at G-LOC.

CONCLUSIONS

These differences suggest that G-LOC may be due entirely to hydrostatic effects, with venous pooling being prevented by the wearing of an ant-G garment, even when it remains uninflated.