Key result
Head-up tilt reduced the gain of the systolic blood pressure to heart rate transfer function, but did not affect the mean arterial pressure to total peripheral resistance transfer function gain.
Why the study?
Does a 45-degree head-up tilt alter the transfer function relationships between blood pressure, heart rate, and vascular resistance in healthy subjects?
Observational (n=9)
Does a 45-degree head-up tilt alter the transfer function relationships between blood pressure, heart rate, and vascular resistance in healthy subjects?
Head-up tilt reduces the gain of the systolic blood pressure to heart rate transfer function but does not affect the mean arterial pressure to total peripheral resistance transfer function, reflecting complex baroreflex and myogenic interactions.
Supports selective baroreflex modulation during orthostasis; leaves open clinical relevance and mechanisms in humans.
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 (SBP-->HR), MAP to total peripheral resistance (TPR) and cerebrovascular resistance index (CVRi) (MAP-->TPR and MAP-->CVRi), as well as stroke volume (SV) to TPR in nine healthy subjects in supine and 45 degrees head-up tilt positions. The gain of the SBP-->HR 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, MAP-->TPR transfer function gain was not affected by head-up tilt, but it did increase from low- to high-frequency regions. The phase relationships between MAP-->TPR 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 MAP-->CVRi. The SV-->TPR had high coherence with a constant phase of 150-160 degrees. 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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O’Leary et al. (2004) conducted an observational in Healthy (n=9). 45 degrees head-up tilt vs. Supine position was evaluated on Heart rate and vascular response to spontaneous changes in blood pressure. Head-up tilt reduced the gain of the systolic blood pressure to heart rate transfer function, but did not affect the mean arterial pressure to total peripheral resistance transfer function gain.
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