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 til t-back were investigated in six healthy subjects (aged 30-58 years) an d sis patients with orthostatic hypotension due to pure autonomic fail ure (aged 33-65 years), 2. Continuous responses of finger arterial pre ssure and heart rate were recorded by Finapres, A pulse contour algori thm applied to the arterial pressure waveform was used to compute stro ke 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 pronounce d and rapid circulatory responses upon tilt-back. 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 mmH g 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 dec reased 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 compensat ory mechanisms are very effective in maintaining arterial pressure at heart level, The gradual circulatory adjustments to head-up tilt in he althy subjects contrast with the pronounced and abrupt circulatory cha nges on tilt-back. In patients with a lack of neural circulatory refle x adjustments, gradual blood pressure decreases to head-up tilt and gr adual increases to tilt-back are observed.