Background: The push-pull effect has been defined previously as decreased Gz tolerance caused by previous baseline zero or -Gz exposure. Earlier work
indicates that the delay in BP (BP) recovery during + Gz is a function of
time at -Gz, and is due to the lengthened time-course of sympathetically me
diated peripheral vasoconstriction. Hypothesis: The purpose of this study w
as to retrospectively determine whether heart rate (HR) varies with BP as d
uration at preceding -Gz increased. Methods: Continuous ECG R-R interval da
ta from 15 s of +2.25Gz after preceding 2, 5, 10, or 15 s at -2Gz obtained
from previous experiments were analyzed and compared with the previously re
ported BP data. Repeated measures ANOVA and regression analyses were used t
o compare +2.25Gz HR responses after the four -Gz conditions and one contro
l +2.25Gz condition. Results: An initial rapid rise in HR was observed for
all conditions with a consistent steady-state plateau achieved after the fi
rst 7 s of +2.25Gz. However, there were significant differences in mean HR
attained during the +2.25Gz plateau for preceding 15 s -2.0 Gz vs. the cont
rol,, 5, and 10s -Gz conditions (109 +/- 1.1 vs. 102 +/- 1.8, 100 +/- 2.0,
97 +/- 1.1 and 101 +/- 1.1, bpm, respectively; p < 0.05). Conclusions: HR,
unlike BP, increases briskly across all preceding -Gz time conditions, adap
ting within the initial baroreflex-compensatory time frame typically expect
ed for +Gz exposures. These results suggest there may be a threshold effect
for HR response. Consequently, vasoconstrictor response is a critical adap
tive mechanism during + Gz when preceded by long (>10 s) -Gz exposures.