Factors influencing the cardiovascular response to +G(z): Implications on the design of life support systems for acceleration protection

Abstract

Tactical aircraft pilots can be exposed to high levels of headward acceleration. Resultant gravitoinertial forces (+Gz) impair vision and, if prolonged, may cause loss of consciousness (G-LOC). Current G-LOC countermeasures include a +Gz actuated regulator (G-valve) pressurizing a lower body garment (G-suit). Protection may also be augmented with positive pressure breathing during +Gz (PBG). Before an optimal strategy can be proposed, a comprehensive investigation is required into how the various countermeasures and their timing affect the cardiovascular response and alter +Gz tolerance.

Six subjects were exposed to acceleration profiles ranging from +2.5 to +5.0 Gz in a human centrifuge. Subjects wore one of three lower body garments pressurized according to one of three different schedules. Pressurization was coincident with +Gz or was delayed in 1.5 second intervals (maximum lag of 4.5 seconds). When PBG was used, pressure was applied according to one of two schedules. Tolerance to +Gz, based on changes in beat-by-beat heart level systolic blood pressure (SBP) indicated no significant improvements from pressure schedules for the G-suits or PBG. Significant increases in SBP occurred after a 3 to 4 second delay, and were delayed further with any lag in G-suit pressurization. The largest sustained improvements in SBP were recorded with the G-suit providing the greatest coverage to the lower body.

An inferential analysis procedure estimated whole body blood flow (Q _est) and resistive (R_est) changes. Flow dropped simultaneously with G-suit inflation, but increased above baseline values after 2 to 3 seconds, primarily from an increased heart rate. Resistance rose rapidly with lower body pressurization, but immediately fell to values approaching pre-acceleratory levels. The data suggested that lower body pressurization resulted in an increased afterload concomitant with the forced emptying of the underlying venous vasculature. Heterometric autoregulation and a Bainbridge reflex compensated for the increased afterload and augmented venous return, respectively. R_est decreased as blood settled into venous vessels not compressed by the lower body garment and Q_est remained elevated from a “rightward shift” in vascular functioning. With the countermeasures G-LOC employed, the blood pressure control system seems to function more as a poor regulator than a servomechanistic system.