Publication Abstracts

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

Self DA, White CD, Shaftstall RM, Mtinangi BL, Croft JS, Hainsworth R.
Aviat space environ Med 1996; 67;547-54.

Abstract

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 dependant 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 test 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 -40mm Hg. In the tilt/LBNP group, there was a net increase of 1022 +- 269.8ml 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 anti-G garment, even when it remains uninflated.

Information on subscribing, and on obtaining copies of an article or of an entire issue.

Table of Contents for Volume 67, Number 6 of the ASEM journal.