The circadian rhythm of blood pressure is masked by postural changes

Abstract

It is well known that blood pressure (BP) varies throughout the day with lower levels observed during nocturnal sleep. This diurnal rhythm is partly explained by the BP-lowering effect of sleep and by an endogenous circadian rhythm of BP with a nocturnal through. As BP is acutely responsive to postural changes, the present study aimed at clarifying how posture affects the circadian variation of BP. Eleven healthy subjects (10 men, 1 woman; mean age ¡À SD: 22.6 ¡À 3.4 years old) were studied individually in time isolation for 6 consecutive days. After 2 baseline days on their regular sleep schedule, subjects underwent a 72-h ultradian sleep–wake cycle (USW) procedure, which consists of 60-min wake periods in dim light ( < 10 lux) alternating with 60-min naps in darkness ( < 0.3 lux). Subjects remained in a semi- recumbent posture throughout the first 48 h of the USW procedure. During the last 24 h of the USW procedure, they slept in a supine position during naps and changed into a sitting and standing position at 5 and 10 min after lights on, respectively. They returned to a sitting then supine position at 22 and 56 min after light on, respectively. BP was measured 1, 6, 11, 16 and 21 min after lights on. Circadian phase was assessed from core body temperature (CBT) (4x/min) which minimum was determined by a dual-harmonic regression and assigned a circadian phase of 0 degree. BP measures were averaged per subject by 60-degree bins and analyzed using non-linear mixed models. As expected, postural changes affected BP levels. No BP change was observed throughout wake periods when subjects remained in a semi-recumbent posture ( P ¡Ý0.38). In comparison, during the last 24 h of the USW procedure, diastolic (DBP) and mean arterial BP (MAP) increased by 11.97 ¡À 2.11 and 10.11 ¡À 2.34 mmHg, respectively, when participants changed from the supine to the standing position (measured 1 and 21 min after lights on, respectively; P < 0.001). Moreover, we observed a significant circadian rhythm of systolic BP (SBP), DBP, and MAP ( P ¡Ü0.001) during wake periods without postural change, with an acrophase occurring 7.4 ¡À 0.2 h, 8.6 ¡À 0.3 h and 7.0 ¡À 0.3 h after subjects¡ ¯ regular wake time, respectively. In comparison, during the postural change segment of the USW procedure, none of these rhythms were of sufficient amplitude to be significant, even though average values were comparable. Our observations indicate that postural changes significantly mask the endogenous circadian rhythm of BP. These results have practical implications for the measurement and interpretation of BP data collected in field conditions. Even when sleep is controlled for, the determination of circadian phase and amplitude of BP in ambulatory conditions is of limited value due to the confounding effects of changes in activity levels and posture. This research was supported by an operating grant from the Canadian Institutes of Health Research (CIHR). P. Boudreau was supported by a fellowship from the Institut de recherche Robert-Sauv ¨ ∣ en sant ¨ ∣ et en s ¨ ∣ curit ¨ ∣ du travail (IRSST).