Factors influencing the cardiovascular response to +G¦z, implications on the design of life support systems for acceleration protection

grantor: University of TorontoTactical 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 (Rest) 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. Rest decreased as blood settled into venous vessels not compressed by the lower body garment and Qest 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.Ph.D