1. The purpose of this investigation was to determine non-invasively the changes in autonomic cardiovascular control observed in normal subjects submitted to acute cardiopulmonary blood volume expansion by 100° head-down tilt. The effect of head-down tilt on finger blood pressure and heart rate fluctuations was studied by means of power spectral analysis in 12 healthy men.

2. Amplitude spectra of heart rate and blood pressure rhythmicity were estimated at the low-frequency (60–140 mHz, 10-s rhythm) and high-frequency (area under the curve at mean respiration rate ± 50 mHz) component. Transfer gain and phase were calculated between systolic blood pressure and heart rate. Forearm vascular resistance was estimated to validate the head-down procedure.

3. Forearm vascular resistance decreased significantly from 19.82 (16.34–26.46) mmHg ml−1 min 100 ml to 18.05 (13.69–22.88) mmHg ml−1 min 100 ml (P < 0.01) during head-down tilt (values are medians and 25 and 75 percentiles). The overall variability (total area under the curve of the spectrum from 20 to 500 mHz) of blood pressure and heart rate time series was consistently reduced with head-down tilt.

4. The spectral pattern of systolic blood pressure showed a diminution of the absolute and relative low-frequency component during head-down tilt: absolute log-transformed values, 2.86 (2.80–2.94) mmHg/Hz1/2 versus 2.77 (2.72–2.82) mmHg/Hz1/2 (P < 0.05); relative values, 35% (32–37%) versus 32% (29–32%) (P < 0.05). In heart rate spectra only the absolute low-frequency component decreased. There was no change in the high-frequency component in all time series or in the transfer gain and phase during head-down tilt.

5. It is concluded that head-down tilt is a simple manoeuvre to diminish the 10-s rhythm in systolic blood pressure, which may reflect the reduced sympathetic vasomotor control after cardiopulmonary baroreceptor loading.

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