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1 Physiology Research Branch,
We
measured hemodynamic responses during 4 days of head-down tilt (HDT)
and during graded lower body negative pressure (LBNP) in invasively
instrumented rhesus monkeys to test the hypotheses that exposure to
simulated microgravity increases cardiac compliance and that decreased
stroke volume, cardiac output, and orthostatic tolerance are associated
with reduced left ventricular peak
dP/dt. Six monkeys underwent two 4-day
(96 h) experimental conditions separated by 9 days of ambulatory
activities in a crossover counterbalance design:
1) continuous exposure to 10°
HDT and 2) ~12-14 h per day
of 80° head-up tilt and 10-12 h supine (control condition). Each animal underwent measurements of central venous pressure (CVP),
left ventricular and aortic pressures, stroke volume, esophageal pressure (EsP), plasma volume,
1- and
1-adrenergic responsiveness, and tolerance to LBNP. HDT induced a hypovolemic and hypoadrenergic state with reduced LBNP tolerance compared with the control condition. Decreased LBNP tolerance with HDT was associated with reduced stroke
volume, cardiac output, and peak
dP/dt. Compared with the control
condition, a 34% reduction in CVP (P = 0.010) and no change in left ventricular end-diastolic area during
HDT was associated with increased ventricular compliance
(P = 0.0053). Increased cardiac
compliance could not be explained by reduced intrathoracic pressure
since EsP was unaltered by HDT. Our data provide the first direct
evidence that increased cardiac compliance was associated with headward
fluid shifts similar to those induced by exposure to spaceflight and
that reduced orthostatic tolerance was associated with lower cardiac
contractility.
head-down tilt; central venous pressure; adrenergic function; blood pressure
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