Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC202
Diurnal variation in the circulatory changes associated with initial orthostatic hypotension during active standing: sleep vs. circadian influences
N. Lewis1,2, H. Jones1, P. Ainslie2, A. Thompson1, K. Marrin3
1. Research Institute for Sport & Exercise Sciences, Liverpool John Moores University, Liverpool, United Kingdom. 2. Department of Human Kinetics, University of British Columbia Okanagan, Kelowna, British Columbia, Canada. 3. Sport & Exercise Research Group, Edge Hill University, Liverpool, United Kingdom.
During everyday activities, the incidence of vasovagal-related syncope is substantially higher in the morning compared to other times of day. In 3-10% of the general population, standing is one such activity which has been identified as a trigger of syncope. In physiological terms, a critical time is during initial orthostatic hypotension (IOH), involving a transient fall in systolic and/or diastolic blood pressure of >40 and >20 mm Hg respectively, within the first 15 s of standing (Wieling et al., 2007). We have reported that there is a lack of diurnal variation in the physiological responses associated with this IOH (Lewis et al., 2011). However, subjects in this study slept normally before the morning trial but not before the afternoon trial. Therefore, the aim of this study is to examine the diurnal variation in the cardiorespiratory and cerebrovascular physiological responses during IOH using a protocol in which the amount of prior sleep is controlled. Nine healthy individuals, aged 27 ± 3 yrs, completed the protocol at 06:00 h and 16:00 h in a randomized order. At each time of day continuous beat-to-beat recordings of mean MCAv, MAP, SV, cardiac output (Q), heart rate (HR), and end-tidal carbon dioxide (PETCO2) were obtained during supine rest and active standing (3 min). Importantly, approximately 4 h of sleep preceded both the morning and afternoon test. Differences between times of day were analysed using paired t-tests and reported as mean±SE. All variables measured at baseline when resting supine did not differ between times of day (P≥0.10), except HR which was 4 ± 2 beats●min-1 higher in the afternoon (P=0.02). All individuals (n=8) experienced IOH during active standing at both times of day. The initial decline in MAP occurred within 9 ± 1 s at both times of day (P=1.0); however, the magnitude of decline in the afternoon (-47 ± 3 mm Hg) was greater than the morning (-34 ± 3 mm Hg; P=0.01). The magnitude of decline in mean MCAv was similar in the morning (-24 ± 8 cm●s) and afternoon (-27 ± 9 cm●s-1; P=0.51). No diurnal variation was evident in MAP measured 30 s after standing, although Q was 1.2 ± 0.4 L●min-1 greater in the afternoon at this time (P=0.03). These data indicate that the magnitude but not the time-course of the transient decline in MAP during IOH alters with time of day. Because we controlled the amount of sleep prior to each time of day, the greater decline in MAP during active standing in the afternoon may be under the control of endogenous circadian rhythms. Nevertheless, despite the greater hypotension in the afternoon, cerebral perfusion was well controlled at both times of day. The finding is suggestive of possible compensatory interactions between blood pressure and mechanisms of cerebral blood flow control.
Where applicable, experiments conform with Society ethical requirements