Effects of gravitational acceleration on cardiovascular autonomic control in resting humans.

PURPOSE: Previous studies of cardiovascular responses in hypergravity suggest increased sympathetic regulation. The analysis of spontaneous heart rate variability (HRV) parameters and spontaneous baroreflex sensitivity (BRS) informs on the reciprocal balance of parasympathetic and sympathetic regulations at rest. This paper was aimed at determining the effects of gravitational acceleration (a g) on HRV and BRS. METHODS: Eleven healthy subjects (age 26.6 ± 6.1) were studied in a human centrifuge at four a g levels (1, 1.5, 2 and 2.5 g) during 5-min sessions at rest. We evaluated spontaneous variability of R-R interval (RR), and of systolic and diastolic blood pressure (SAP and DAP, respectively), by power spectral analysis, and BRS by the sequence method, using the BRSanalysis(®) software. RESULTS: At 2.5 g, compared to 1 g, (1) the total power (P TOT) and the powers of LF and HF components of HRV were lower, while the LF/HF ratio was higher; (2) normalized units for LF and HF did not changed significantly; (3) the P TOT, LF and HF powers of SAP were higher; (4) the P TOT and LF power of DAP were higher; and (5) BRS was decreased. CONCLUSIONS: These results do not agree with the notion of sympathetic up-regulation supported by the increase in HR and DAP (tonic indices), and of SAP and DAP LF powers (oscillatory indices). The P TOT reduction leads to speculate that only the sympathetic branch of the ANS might have been active during elevated a g exposure. The vascular response occurred in a condition of massive baroreceptive unloading.

Effects of acceleration in the Gz axis on human cardiopulmonary responses to exercise.

The aim of this paper was to develop a model from experimental data allowing a prediction of the cardiopulmonary responses to steady-state submaximal exercise in varying gravitational environments, with acceleration in the G(z) axis (a (g)) ranging from 0 to 3 g. To this aim, we combined data from three different experiments, carried out at Buffalo, at Stockholm and inside the Mir Station. Oxygen consumption, as expected, increased linearly with a (g). In contrast, heart rate increased non-linearly with a (g), whereas stroke volume decreased non-linearly: both were described by quadratic functions. Thus, the relationship between cardiac output and a (g) was described by a fourth power regression equation. Mean arterial pressure increased with a (g) non linearly, a relation that we interpolated again with a quadratic function. Thus, total peripheral resistance varied linearly with a (g). These data led to predict that maximal oxygen consumption would decrease drastically as a (g) is increased. Maximal oxygen consumption would become equal to resting oxygen consumption when a (g) is around 4.5 g, thus indicating the practical impossibility for humans to stay and work on the biggest Planets of the Solar System.

Determinants of oxygen consumption during exercise on cycle ergometer: the effects of gravity acceleration.

The hypothesis that changes in gravity acceleration (a(g)) affect the linear relationships between oxygen consumption VO2 and mechanical power (w ) so that at any w, VO2 increases linearly with a(g) was tested under conditions where the weight of constant-mass legs was let to vary by inducing changes in a(g) in a human centrifuge. The effects of a(g) on the VO2/w relationship were studied on 14 subjects at two pedalling frequencies (f(p), 1.0 and 1.5 Hz), during four work loads on a cycle ergometer (25, 50, 75 and 100 W) and at four a(g) levels (1.0, 1.5, 2.0 and 2.5 times normal gravity). VO2 increased linearly with w. The slope did not differ significantly at various a(g) and f(p), suggesting invariant mechanical efficiency during cycling, independent of f(p) and a(g). Conversely, the y-intercept of the VO2/w relationship, defined as constant b, increased linearly with a(g). Constant b is the sum of resting VO2 plus internal metabolic power (E (i)). Since the former was the same at all investigated a(g), the increase in constant b was entirely due to an increase in E (i). Since the VO2 versus w lines had similar slopes, the changes in E (i) entirely explained the higher VO2 at each w, as a(g) was increased. In conclusion, the effects of a(g) on VO2 are mediated through changes in E (i), and not in w or in resting VO2.