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 degrees 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.

Figures

Fig. 1.

Arterial pressure (AP; top), RR interval (RR; middle), and total peripheral resistance (TPR; bottom) of a representative fainting patient during head-up tilt (HUT) to 70°. The fiducial points “baseline,” “1 min” after HUT, “early” onset of gradual BP decline, “mid” decline, “late” decline just prior to the actual faint, and “faint” are indicated. A time-dependent increase in heart rate (decrease in RR) is coupled with an initial increase in TPR, allowing for an initially stable AP. These events are followed by a decline in TPR and AP until faint.

Fig. 2.

Representative multiresolution analysis of systolic blood pressure (BP; SBP) of a healthy control subject during HUT is shown. Left panels from top down: original mean arterial pressure (MAP) signal, a “smooth” version of this signal representing larger scales corresponding to frequencies below 0.03125 Hz, and signal details derived from wavelet scale coefficients corresponding to 0.03125, 0.0625, 0.125, and 0.25 Hz as a function of time. Right panels from top down: original signal, total power summed over all scales corresponding to frequencies of 0.03125 Hz or higher, and individual power (wavelet variance) as a function of time at scales corresponding to 0.03125, 0.0625, 0.125, and 0.25 Hz. Low frequency (LF; 0.125 Hz) and high frequency (HF; 0.0625 Hz) contribute the large majority of total wavelet power. Tilt-up and Tilt-down are demarcated by solid lines. LF power (0.125 Hz) and HF power (0.25 Hz) are shown within a box.

Fig. 3.

Representative multiresolution analysis of systolic AP of a fainter during HUT is shown. Left panels from top down: the original MAP signal, a smooth version of this signal representing larger scales corresponding to frequencies below 0.03125 Hz, and signal details derived from wavelet scale coefficients corresponding to 0.03125, 0.0625, 0.125, and 0.25 Hz as a function of time. Right panels from top down: the original signal, total power summed over all scales corresponding to frequencies of 0.03125 Hz or higher, and individual power (wavelet variance) as a function of time at scales corresponding to 0.03125, 0.0625, 0.125, and 0.25 Hz. LF (0.125 Hz) and HF (0.25 Hz) contribute the large majority of total wavelet power. Tilt-up and Tilt-down are demarcated by solid lines. LF power (0.125 Hz) and HF power (0.25 Hz) are shown within a box.

Fig. 4.

Top: systolic and diastolic blood pressure during HUT in fainters and controls. Bottom from top down: systolic, mean, and diastolic pressure low-frequency (low freq.) power. ▪, Fainters; □, healthy controls. Systolic pressure initially increases above control and then gradually decreases until syncope when it abruptly falls. Diastolic pressure changes are less marked. With the exception of 1 min posttilt, LF systolic, mean, and diastolic power are significantly lower in fainters than in controls (P < 0.05). The difference in power widens as HUT progresses. Bsl, baseline; ERec, early recovery; LRec, late recovery. Pgroup, probability of group differences; Ptime, probability of time differences.

Fig. 5.

RR interval (RRI) as a function of time is shown at top left, LF wavelet power as a function of time at top right, HF wavelet power (variance) as a function of time at bottom right, and LF-to-HF ratio at bottom left. ▪, Fainters; □, healthy controls. RR interval was smaller (heart rate was higher) during tilt in fainters until fainting supervened, after which RR interval was larger in fainters than in control subjects. LF power changed similarly with time for fainters and controls alike. HF power decreased significantly and similarly with time in fainters and controls until fainting supervened, after which HF power was larger in fainters than in control subjects. LF-to-HF ratio tended to be increased in early HUT in fainters and was significantly decreased at the time of faint and during early recovery. Ptime*group, probability of group and time differences.

Fig. 6.

Changes in respiratory power (variance) during HUT in fainters (closed boxes) and control subjects (open boxes). Power started from a similar baseline and increased with HUT for fainters and control subjects. Power was greater in fainters throughout HUT (P < 0.01).

Fig. 7.

Alpha index computed as the square root of the ratio between LF RR interval power and LF systolic AP. Alpha index was used as an index of cardiovagal baroreflex sensitivity. ▪, Fainters; □, healthy controls. The index decreased significantly with HUT for fainters and control subjects. The index was larger in fainters compared with control subjects during HUT, at the faint itself and during recovery.