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1 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada; Department of Community Health Sciences, Brock University, St. Catharines, Ontario, Canada
2 Neurovascular Research Laboratory, University of Western Ontario, School of Kinesiology, London, Ontario, Canada
3 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: hughson{at}uwaterloo.ca.
Beat-by-beat estimates of total peripheral resistance (TPR) can be obtained from continuous measurements of cardiac output (Q) using Doppler ultrasound and noninvasive mean arterial blood pressure (MAP). We employed transfer function analysis to study the heart rate and vascular response to spontaneous changes in blood pressure from the relationships of systolic blood pressure to heart rate (SBP
HR), MAP to total peripheral resistance and cerebrovascular resistance index (MAPTPR and MAPCVRi)as well as stroke volume (SV) to TPR in 9 healthy subjects in supine and 45° head-up tilt positions. The gain of the SBPHR transfer function was reduced with tilt in both the low (LF: 0.03-0.15 Hz) and high (HF: 0.15-0.35 Hz) frequency regions. In contrast, MAPTPR transfer function gain was not affected by head-up tilt but it did increase from LF to HF regions. The phase relationships between MAPTPR were unaffected by head-up tilt, but consistent with an autoregulatory system changes in MAP were followed by directionally similar changes in TPR, just as observed for the MAPCVRi. The SVTPR had high coherence with constant phase of 150-160°. Together, these data that showed changes in MAP preceded changes in TPR as well as a possible link between SV and TPR are consistent with complex interactions between the vascular component of the arterial and cardiopulmonary baroreflexes and intrinsic properties such as the myogenic response of the resistance arteries.
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