Dynamics of circulatory adjustments to head-up tilt and tilt-back in healthy and sympathetically denervated subjects.

1. The initial circulatory adjustments induced by head-up tilt and tilt-back were investigated in six healthy subject (aged 30-58 years) and six patients with orthostatic hypotension due to pure autonomic failure (aged 33-65 years). 2. Continuous responses of finger arterial pressure and heart rate were recorded by Finapres. A pulse contour algorithm applied to the arterial pressure waveform was used to compute stroke volume responses. 3. In the healthy subjects, head-up tilt induced gradual circulatory adjustments. After 1 min upright stroke volume and cardiac output had decreased by 39 +/- 9% and 26 +/- 10% respectively. Little change in mean blood pressure at heart level (+1 +/- 7 mmHg) indicated that systemic vascular resistance had increased by 39 +/- 24%. The gradual responses to head-up tilt contrasted with the pronounced and rapid circulatory responses upon tiltback. After 2-3 s a rapid increase in stroke volume (from 62 +/- 8% to 106 +/- 10%) and cardiac output (from 81 +/- 11% to 118 +/- 20%) was observed with an overshoot of mean arterial pressure above supine control values of 16 +/- 3 mmHg at 7 s. In the patients a progressive fall in blood pressure on head-up tilt was observed. After 1 min upright mean blood pressure had decreased by 59 +/- 8 mmHg. No change in systemic vascular resistance and a larger decrease in stroke volume (60 +/- 7%) and cardiac output (53 +/- 8%) were found. On tilt-back a gradual recovery of blood pressure was observed. 4. In healthy humans upon head-up tilt neural compensatory mechanisms are very effective in maintaining arterial pressure at heart level. The gradual circulatory adjustments to head-up tilt in healthy subjects contrast with the pronounced and abrupt circulatory changes on tilt-back. In patients with a lack of neural circulatory reflex adjustments, gradual blood pressure decreases to head-up tilt and gradual increases to tilt-back are observed.