Diurnal variation in time to presyncope and associated circulatory changes during a controlled orthostatic challenge

Abstract

Epidemiological data indicate that the risk of neurally mediated syncope is substantially higher in the morning. Syncope is precipitated by cerebral hypoperfusion, yet no chronobiological experiment has been undertaken to examine whether the major circulatory factors, which influence perfusion, show diurnal variation during a controlled orthostatic challenge. Therefore, we examined the diurnal variation in orthostatic tolerance and circulatory function measured at baseline and at presyncope. In a repeated-measures experiment, conducted at 0600 and 1600, 17 normotensive volunteers, aged 26 Ϯ 4 yr (mean Ϯ SD), rested supine at baseline and then underwent a 60°head-up tilt with 5-min incremental stages of lower body negative pressure until standardized symptoms of presyncope were apparent. Pretest hydration status was similar at both times of day. Continuous beat-to-beat measurements of cerebral blood flow velocity, blood pressure, heart rate, stroke volume, cardiac output, and end-tidal PCO 2 were obtained. At baseline, mean cerebral blood flow velocity was 9 Ϯ 2 cm/s (15%) lower in the morning than the afternoon (P Ͻ 0.0001). The mean time to presyncope was shorter in the morning than in the afternoon (27.2 Ϯ 10.5 min vs. 33.1 Ϯ 7.9 min; 95% CI: 0.4 to 11.4 min, P ϭ 0.01). All measurements made at presyncope did not show diurnal variation (P Ͼ 0.05), but the changes over time (from baseline to presyncope time) in arterial blood pressure, estimated peripheral vascular resistance, and ␣-index baroreflex sensitivity were greater during the morning tests (P Ͻ 0.05). These data indicate that tolerance to an incremental orthostatic challenge is markedly reduced in the morning due to diurnal variations in the time-based decline in blood pressure and the initial cerebral blood flow velocity "reserve" rather than the circulatory status at eventual presyncope. Such information may be used to help identify individuals who are particularly prone to orthostatic intolerance in the morning. cerebrovascualr circulation; syncope; diurnal variation SYNCOPE REFERS TO A TRANSIENT loss of consciousness as a result of cerebral hypoperfusion with subsequent spontaneous recovery (41). The typical symptoms experienced just prior to syncope (presyncope) are wooziness, light-headedness, and visual dimming, and are fundamentally caused by insufficient oxygen supply to the brain which, if sustained, results in syncope. The prevalence of syncope in the general population is clinically significant, accounting for 3-5% of all emergency room visits (39). Neurally mediated (i.e., vasovagal) syncope is the most common form (20), and the incidence of syncope is nearly twice as common in people aged under 40 yr compared with individuals aged over 60 yr (35). Although syncope is also common in individuals with autonomic dysfunction, it can occur in healthy individuals after a period of bed rest (26) or after prolonged endurance exercise (27).Data from recent epidemiological studies have revealed a distinct increase in the frequency of neurally mediat...

Validity of auscultatory and Penaz blood pressure measurements during profound heat stress alone and with an orthostatic challenge

“…Criteria for test termination during LBNP or upright tilt often includes a consistent SBP of Ͻ70 -90 mmHg (6,8,18,20,31,35,36), as well as a 15-to 20-mmHg decrease in SBP (8,15). Therefore, the validity of both absolute BPs and the changes in BP are important to appropriately identify presyncope and thus test termination during LBNP or similar orthostatic testing.…”

“…For this test, LBNP began at 20 mmHg and was progressively reduced by 10 mmHg every 3 min until presyncope. Presyncope was defined by the following criteria: continued self-reporting by the subject of feeling faint or nauseous, continued SBP of Ͻ80 mmHg [measured by BP AUSC, given that this is the approach often used by others (6,8,20,31,35,36)], and/or relative bradycardia accompanied by narrowing of pulse pressure.…”

“…In conclusion, auscultatory SBP and Penaz SBP failed to track the decrease in intra-arterial SBP that occurred during the profound heat stress, while decreases in arterial BP during an orthostatic challenge are comparable between methodologies. lower body negative pressure; passive heat stress; arterial blood pressure VARIOUS METHODS ARE USED TO measure arterial blood pressure (BP) in the passively heat-stressed human (6,8,18,20,31,35,36). The most valid approach is an intra-arterial catheter connected to a calibrated transducer (BP INTRA ).…”

“…VARIOUS METHODS ARE USED TO measure arterial blood pressure (BP) in the passively heat-stressed human (6,8,18,20,31,35,36). The most valid approach is an intra-arterial catheter connected to a calibrated transducer (BP INTRA ).…”

The Role of Ultrasonography in the Assessment of Arterial Baroreflex Function

“…For example, given increases in blood pressure such as those that occur in the morning, may predispose to cerebral haemorrhage 47 , hypertensive patients may benefit most from clinical interventions that focus augmenting the mechanical component by enhancing vascular properties. Conversely, individuals suffering from orthostatic intolerance, for which the risk is also greatest in the morning 48 , may benefit from interventions aimed at enhancing the neural component 49 .…”