Evidence for increased cardiac compliance during exposure to simulated microgravity

Abstract

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/d t. 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, α1- and β1-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/d t. 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.

Long-term dry immersion: review and prospects

“…Surprisingly, cardiac output did not decrease. Several hypotheses have been suggested, which include thoracic (Foldager et al 1996) and cardiac (Koenig et al 1998) biomechanical changes that are specific to real conditions of weightlessness. During weightlessness, the chest relaxes and thus the volume of the closed chest cavity increases.…”

Effectiveness of intermittent -Gx gravitation in preventing deconditioning due to simulated microgravity

“…Our previous work (52,55) showed the nature and time course of depression in contractility of papillary muscle of rats subjected to 4 and 13 wk of tail suspension. Koenig et al (25) demonstrated with invasively instrumented rhesus monkeys that, after 4 days of exposure to head-down tilt, the reduced orthostatic tolerance during graded lower body negative pressure was associated with a lowered Values are means Ϯ SE; n, number of animals. CSA, cross-sectional area of papillary muscle.…”

Operational point of neural cardiovascular regulation in humans up to 6 months in space

“…The absence of gravitational stimuli during spaceflight induces a number of adaptive changes within the cardiovascular system that might affect crew health and safety, especially upon return to Earth (2,36). Most imperative, cardiovascular modifications occurring in microgravity consist of altered blood volume distribution (26,48), impaired myocardial properties (13,24,32), and/or end-organ (i.e., vascular) remodeling (11,39,49). In addition, the baroreflex in space is chronically unchallenged due to removal of intravascular hydrostatic pressure gradients.…”