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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Spontaneous beat-by-beat fluctuations of total peripheral and cerebrovascular resistance in response to tilt

¹Department of Kinesiology, University of Waterloo, Waterloo, Ontario N2L 3G1; ²Department of Community Health Sciences, Brock University, St. Catharines, Ontario L2S 3A1; and ³School of Kinesiology, University of Western Ontario, London, Ontario, Canada N6A 3K7

Submitted 21 July 2003 ; accepted in final form 27 April 2004

Beat-by-beat estimates of total peripheral resistance (TPR) can be obtained from continuous measurements of cardiac output by using Doppler ultrasound and noninvasive mean arterial blood pressure (MAP). We employed transfer function analysis to study the heart rate (HR) and vascular response to spontaneous changes in blood pressure from the relationships of systolic blood pressure (SBP) to HR (SBPHR), MAP to total peripheral resistance (TPR) and cerebrovascular resistance index (CVRi) (MAPTPR and MAPCVRi), as well as stroke volume (SV) to TPR in nine 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- (0.03–0.15 Hz) and high-frequency (0.15–0.35 Hz) regions. In contrast, MAPTPR transfer function gain was not affected by head-up tilt, but it did increase from low- to high-frequency 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 a 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.

baroreflex; power spectral analysis; Doppler ultrasound; cardiac output; myogenic; autoregulation

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