Diurnal Variation in the Mechanical and Neural Components of the Baroreflex

Abstract

Abstract-Diminished baroreflex sensitivity in the morning negatively influences morning coronary blood flow and blood pressure control in hypertensive patients. Our aim was to determine the contribution of the mechanical and neural components of the cardiac baroreflex to diurnal variation in blood pressure control. In 12 healthy participants, we used the modified Oxford method to quantify baroreflex sensitivity for rising (G up ) and falling (G down ) pressures in the morning (7:00 AM) and afternoon (4:00 PM). Beat-to-beat blood pressure, R-R intervals, and carotid artery diameter measurements were recorded. Integrated sensitivity was determined by plotting R-R intervals against systolic blood pressure. The mechanical component was carotid artery diameter plotted against systolic blood pressure, and the neural component was R-R intervals plotted against carotid artery diameter. Linear mixed models were used to compare the integrated, mechanical, and neural sensitivities between morning and afternoon. We found significant diurnal variation in integrated sensitivity, with an attenuated response in the morning (G up ϭ13.0Ϯ0.6; G down ϭ6.3Ϯ0.4 ms/mm Hg) when compared with the afternoon (G up ϭ15.1Ϯ0.6; G down ϭ12.6Ϯ0.4 ms/mm Hg). For rising pressures, the diminished integrated sensitivity in the morning was caused by a reduction in mechanical sensitivity, whereas for falling pressures it was caused by a reduction in neural sensitivity. Our findings explicate the mechanisms underlying diurnal variation in baroreflex function. Pharmacological and lifestyle interventions targeted specifically at the diminished component of the cardiac baroreflex in the morning may lead to better management of hypertension. (Hypertension. 2011;58:51-56.)Key Words: circadian rhythms Ⅲ baroreflex Ⅲ blood pressure Ⅲ heart rate Ⅲ carotid artery B lood pressure (BP) exhibits a circadian rhythm, with the lowest pressures occurring during nocturnal sleep and a rapid "morning surge" after waking. 1 BP is most reactive to a given level of physical activity in the morning, 2 and risk of orthostatic intolerance is also greater at this time compared with the afternoon. 3 In a number of prospective studies (reviewed by Kario 4 ), the morning surge in BP has been found to be an independent risk factor for cardiovascular and cerebrovascular events, which also peak between 6:00 AM and 12:00 PM. 5,6 There is a wealth of evidence that BP control varies significantly over a 24-hour period, with studies in the frequency domain indicating the presence of a circadian rhythm in sympathovagal balance. 7,8 However, investigations directed at unraveling the autonomic mechanisms governing these time-of-day differences are lacking.The arterial baroreflex control of vagal neural outflow is quantified by the responses in R-R interval to changes in systolic BP. 9 Previous studies have identified diurnal variation in baroreflex sensitivity in both normotensive and hypertensive people, with reduced sensitivities after waking compared with during sleep. 10 -13 Moreover,...

Morning blood pressure surge is associated with arterial stiffness and sympathetic baroreflex sensitivity in hypertensive seniors

“…All of them demonstrated that a reduction of cardiovagal BRS occurred concomitant with BP increase in the morning, and it was suggested that cardiovagal BRS may be one of the determining factors for morning surge (4,33,36). Given the fact that a reduction of cardiovagal BRS and its correlation with morning BP was also observed in healthy normotensive subjects (4,33) and that this relationship disappeared in hypertensive seniors in the current study, cardiovagal BRS may not be a predominant factor for morning BP increase in elderly hypertensive subjects. Indeed, multiple regression analysis showed that the lower cardiovagal BRS tended to be the factor for the higher morning surge in normotensive subjects, but not in elderly hypertensive subjects whose morning surge was significantly impacted by sympathetic BRS.…”

“…Cardiovagal baroreflex sensitivity (BRS), which regulates BP at the heart through the autonomic nervous system, was negatively correlated with morning SBP (4). Moreover, it was found that a decreased cardiovagal BRS was caused by a reduction in distensibility of the carotid artery, especially during the BP rise in the morning (33). Therefore, it seems that hypertensive patients with greater arterial stiffness have a lesser ability to buffer against BP increases in the morning.…”

Neurocardiological differences between musicians and control subjects

“…Measurement sessions were done in the morning [23] in a quiet room in the Leiden University Medical Center. Room temperature was kept at 23°C.…”

The Role of Ultrasonography in the Assessment of Arterial Baroreflex Function

“…We have shown that 1) the morning rise in blood pressure is related to overall reductions in integrated baroreflex gain, 2) that for falling pressures the lower integrated gain in the morning was caused by reduced neural gain compared with the afternoon, and 3) for rising pressures the lower integrated gain in the morning was caused by reduced mechanical gain compared with the afternoon 25 . These unique findings hold the prospect of guiding the development of future treatment strategies aimed at lowering cardio-and cerebrovascular events that occur more frequently in the morning.…”

“…However, mathematical modeling procedures can be applied for more objective analysis. For example, a piecewise linear regression can be applied to the raw data points to statistically identify breakpoints that occur at the upper and lower ends of the data set ( Figure 5) 24,25 . Other approaches for the objective identification of cardiac-vagal BRS have been reported in the literature including the use sigmoid 26 , logistic 27 and elliptical functions 28 .…”