Multiresolution wavelet analysis of time-dependent physiological responses in syncopal youths

Abstract

Our prior studies indicated that postural fainting relates to thoracic hypovolemia. A supranormal increase in initial vascular resistance was sustained by increased peripheral resistance until late during head-up tilt (HUT), whereas splanchnic resistance, cardiac output, and blood pressure (BP) decreased throughout HUT. Our aim in the present study was to investigate the alterations of baroreflex activity that occur in synchrony with the beat-to-beat time-dependent changes in heart rate (HR), BP, and total peripheral resistance (TPR). We proposed that changes of low-frequency Mayer waves reflect sympathetic baroreflex. We used DWT multiresolution analyses to measure their time dependence. We studied 22 patients, 13 to 21 yr old, 14 who fainted within 10 min of upright tilt (fainters) and 8 healthy control subjects. Multiresolution analysis was obtained of continuous BP, HR, and respirations as a function of time during 70°upright tilt at different scales corresponding to frequency bands. Wavelet power was concentrated in scales corresponding to 0.125 and 0.25 Hz. A major difference from control subjects was observed in fainters at the 0.125 Hz AP scale, which progressively decreased from early HUT. The alpha index at 0.125 Hz was increased in fainters. RR interval 0.25 Hz power decreased in fainters and controls but was markedly increased in fainters with syncope and thereafter corresponding to increased vagal tone compared with control subjects at those times only. The data imply a rapid reduction in time-dependent sympathetic baroreflex activity in fainters but not control subjects during HUT. baroreflex; syncope; variance SYNCOPE (FAINTING) IS DEFINED by a sudden transient loss of consciousness and postural tone due to cerebral hypoperfusion (8). Simple postural faint is identified with upright vasovagal syncope in which vasodilation-induced hypotension and bradycardia are relieved by recumbency (18). Upright orthostatic stress testing and specifically upright tilt table testing have been used to provoke postural fainting episodes. We recently reported on the time course of physiological changes during upright tilt using a system of fiducial points. Fiducial points allowed for the comparison of relevant physiological events across patients with simple postural fainting. The fiducial points were baseline, 1-min after head-up tilt (HUT), the onset of gradual blood pressure (BP) decline, mid decline, late decline just prior to the actual faint, and the faint itself, which was the final fiducial point comprising the rapid fall in BP and heart rate (HR) (Fig. 1). These events were consistently present in all young syncopal patients.During early tilt we demonstrated a time-dependent increase in HR coinciding with an initially stable arterial pressure (AP) and followed by a slow decrease in AP. The slow decline corresponded to a decrease in cardiac output (CO) and in central hypovolemia caused by failure of splanchnic vasoconstriction and enhanced splanchnic blood pooling (34, 36). However, total peripheral resistan...

Respiration drives phase synchronization between blood pressure and RR interval following loss of cardiovagal baroreflex during vasovagal syncope

“…The intervals selected corresponded on the average to the intervals measured in fainting subjects. Previously, we showed that this time period was sufficient for comparison with fainters and does not invoke a false-positive fainting response (24,25,41). Additionally, most of our fainters faint during a 10-min tilt, and the duration of the time intervals is similar between groups (see RESULTS).…”

“…Both are driven and entrained by the same strong oscillator. We have previously shown that high frequency (HF) HR variability power, characteristic of respiratory arrhythmia (variations in HR that occurs during breathing cycles), is increased for both AP and RR interval at the time of faint (24). Additionally, we (24) and Bernardi's group (31) have previously demonstrated that respiratory power increases at the time of faint.…”

“…Previously, using multiresolution wavelet analysis during fainting, we showed a decrease in ␣-index of heart rate (HR) variability, suggesting impairment of cardiovagal baroreflex function (24). Initially during orthostatic stress, the cardiovagal baroreflex appears intact since changes in AP elicit appropriate changes in RR interval (RR).…”

“…As faint approaches, total peripheral resistance (TPR) decreases coincident with a gradual decrease in mean arterial blood pressure (MAP) in some young vasovagal fainters ( Fig. 1) (24,41). This leads inexorably to a stage of overt hypotension, bradycardia, and circulatory collapse, coinciding with increased respiratory tidal volume, hyperpnea, and hypocapnia (31,41).…”

“…The reasons for this are unknown but may relate to the loss of the baroreflex. We have shown in fainters that there is excessive gravitational pooling of blood within the dependent vasculature with resultant thoracic hypovolemia and impaired compensatory splanchnic vasoconstriction and venoconstriction (24,39,41). Splanchnic pooling does not necessarily provoke fainting and can occur in forms of chronic orthostatic intolerance, in which fainting typically does not occur (40).…”