Journal of Applied Physiology, Vol 60, Issue 5 1628-1637, Copyright © 1986 by American Physiological Society
Hemodynamics of miniature swine during +Gz stress with and without anti-G support
J. W. Burns, M. J. Parnell and R. R. Burton
Nine unanesthetized, chronically instrumented, female miniature swine (MS)
(avg wt, 39.7 kg) were exposed to head-to-tail inertial load (+Gz) levels
of +3, +5, and +7 Gz for 60 s, with and without anti-G-suit inflation.
Venous flow (VF) was measured by an electromagnetic flow sensor around the
inferior thoracic vena cava at the diaphragm. Central venous pressure
(CVP), abdominal venous pressure (AVP), eye-level blood pressure (ELBP),
and esophageal pressure (EP) were also measured before, during, and after
+Gz. There was a progressive significant decrease from control of both ELBP
(P less than 0.001) and VF (P less than 0.05) during the three +Gz
exposures, both with and without G-suit inflation. Without G-suit
inflation, most of the MS were unable to tolerate +5 and +7 Gz. Although VF
was significantly (P less than 0.02) improved by G-suit inflation during
+Gz there was no significant difference in VF between the three +Gz levels,
with or without G-suit inflation. The MS does a spontaneous straining
maneuver (cyclic Valsalva) during +Gz with G-suit support. Using EP as a
trigger, the data were grouped as strain or no strain (relaxation). A
continuous AVP-to-CVP gradient existed during G-suit inflation, which
increased dramatically during no strain with increasing +Gz, and was
associated with an increase in VF. Thus, the majority of VF occurred during
relaxation between strains, even though relaxation time was shortened as
+Gz increased. Although ELBP is obviously dependent on cardiac output and
venous return, the progressive reduction in ELBP with increased +Gz loads
was not significantly related to changes in VF at the diaphragm which was
maintained, although at a reduced rate, by the AVP-to-CVP gradient during
G-suit inflation.
