1. The immediate heart rate and blood pressure changes upon standing have been well documented. It has been recognized, that blood pressure transients elicit baroreflex responses contributing to the complex mechanism of post standing heart rate fluctuations. On the other hand the influence of heart rate changes on orthostatic blood pressure control is not well understood. Therefore we have studied the blood pressure regulation of 28 pacemaker dependent subjects with fixed ventricular pacemaker rhythm during active orthostasis, and their responses were compared to that of 10 elderly (66 ± 11 year), and 12 young (35 ± 5 year) volunteers. 2. The young volunteers exhibited the characteristic biphasic heart rate response on standing, with a maximum acceleration of 29 ± 12 beats. The heart rate response of the elderly volunteers was very limited, and no response was seen among pacemaker subjects. A significantly greater (-37 ± 15 mmHg) systolic blood pressure drop was seen in the pacemaker group than in the group of young volunteers (-22 ± 13 mmHg). The systolic blood pressure overshoot of the young volunteers (36 ± 17 mmHg) was significantly greater than that of the pacemaker patients' (11 ± 22 mmHg). The blood pressure transients of the elderly volunteers and pacemaker subjects were very similar. Significant correlation was detected between the extent of maximum heart rate acceleration and the magnitude of the subsequent blood pressure overshoot (R = 0.68, p < 0.0005) among healthy volunteers. 3. Our results indicate that certain post standing blood pressure transients are heart rate dependent. The chronotrop incompetency of healthy elderly volunteers and pacemaker subjects result in similar alteration of the orthostatic blood pressure regulation, however this modified response does not interfere with a satisfactory hemodynamic stabilization.
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