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1 Department of Clinical Physiology, Kuopio University Hospital and University of Kuopio, Kuopio, Finland
2 Department of Medicine, Kuopio University Hospital and University of Kuopio, Kuopio, Finland
3 Laboratoire de Physiologie de l'Environnement, Universite Claude Bernard and Hospices Civils de Lyon, Hopital E. Herriot, Lyon, France
* To whom correspondence should be addressed. E-mail: Tomi.Laitinen{at}kuh.fi.
In elderly subjects, heart rate responses to postural change are attenuated, while their vascular responses are augmented. Altered strategy in maintaining blood pressure homeostasis during upright position may result from various cardiovascular changes including age-related cardiovascular autonomic dysfunction. This exploratory study was conducted to evaluate impact of age on cardiovascular autonomic responses to head-up tilt (HUT) in healthy subjects covering a wide age range. The study population consisted of 63 healthy, normal-weighted, non-smoking subjects aged 23-77 years. Five minutes electrocardiogram and finger blood pressure recordings were performed in the supine position and in the upright position 5 minutes after 70 °HUT. Stroke volume was assessed from non-invasive blood pressure signals by the arterial pulse contour method. Heart rate variability (HRV) and systolic blood pressure variability (SBPV) were analyzed using spectral analysis and baroreflex sensitivity (BRS) was assessed using sequence and cross spectral methods. Cardiovascular autonomic activation during HUT consisted of decreases in HRV and BRS and an increase in SBPV. These changes became attenuated with aging. Age correlated significantly with amplitude of HUT-stimulated response of high frequency component (r= -0.59, P<0.001) and low frequency to high frequency ratio of HRV (r=-0.30, P<0.05) and indices of BRS (Local BRS: r=-0.60, P<0.001, 
LF: r=-0.28, P<0.05). Blood pressure in the upright position was maintained well irrespective of age. However, the HUT-induced increase in heart rate was more pronounced in the younger subjects, whereas the increase in peripheral resistance was predominantly observed in the older subjects. Thus it is likely that while the dynamic capacity of cardiac autonomic regulation decreases, vascular responses related to vasoactive mechanisms and vascular sympathetic regulation become augmented with increasing age.
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