Abstract
BackgroundThe purpose of the present study was to compare sinusoidal versus constant lower body negative pressure (LBNP) with reference to very mild whole-body heating. Sinusoidal LBNP has a periodic load component (PLC) and a constant load component (CLC) of orthostatic stress, whereas constant LBNP has only a CLC. We tested two sinusoidal patterns (30-s and 180-s periods with 25 mmHg amplitude) of LBNP and a constant LBNP with −25 mmHg in 12 adult male subjects.ResultsAlthough the CLC of all three LBNP conditions were configured with −25 mmHg, the mean arterial pressure (MAP) results showed a significantly large decrease from baseline in the 30-s period condition (P <0.01). In contrast, the other cardiovascular indices (heart rate (HR), stroke volume (SV), cardiac output (CO), basal thoracic impedance (Z0), total peripheral resistance (TPR), the natural logarithmic of the HF component (lnHF), and LF/HF (ln(LF/HF))) of heart rate variability (HRV) showed relatively small variations from baseline in the 30-s period condition (P <0.01). The result of the gain and phase of transfer function at the sinusoidal period of LBNP showed that the very mild whole-body heating augmented the orthostatic responses.ConclusionThese results revealed that the effect of the CLC of LBNP on cardiovascular adjustability was attenuated by the addition of the PLC to LBNP. Based on the results of suppressed HRV response from baseline in the 30-s period condition, we suggest that the attenuation may be caused by the suppression of the vagal responsiveness to LBNP.
Highlights
The purpose of the present study was to compare sinusoidal versus constant lower body negative pressure (LBNP) with reference to very mild whole-body heating
Wholebody heating significantly increased the baseline values of Tre (P
The major finding of this study was that the cardiovascular responses, except the mean arterial pressure (MAP), were attenuated in the sinusoidal (PLC plus constant load component (CLC)) LBNP compared to the constant (CLC only) LBNP, the average gauge pressure (CLC) during the LBNP period was set at −25 mmHg in all conditions
Summary
The purpose of the present study was to compare sinusoidal versus constant lower body negative pressure (LBNP) with reference to very mild whole-body heating. Lower body negative pressure (LBNP) is used as a perturbation to the cardiovascular system and has been applied to simulate the gravitational stress of orthostatic blood shift in humans [1,2]. A few previous studies used sinusoidal LBNP as the perturbation stimulus [12,13], only Levenhagen et al (1994) showed the result of an analysis of the actual gauge pressure of sinusoidal LBNP. They investigated the frequency characteristics of hemodynamic changes induced by sinusoidal LBNP in humans across a range of 10- to 250-s periods (that is, 0.1 to 0.004 Hz) [13]. They revealed that the cardiovascular adjustability to sinusoidal LBNP was maintained at a period slower than 50-s (that is, 0.02 Hz) oscillation
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