COMPLEX DEMODULATION OF CARDIORESPIRATORY DYNAMICS PRECEDING VASOVAGAL SYNCOPE

Background-The dynamic autonomic processes leading to vasovagal syncop e are poorly understood. Methods and Results-We used complex demodulat ion to continuously assess changes in respiration, R-R interval, and a rterial pressure (blood pressure) variability during 60 degree head-up tilt in 25 healthy subjects with tilt-induced vasovagal syncope and 2 5 age-matched nonsyncopal control subjects. Coherence and transfer fun ction analyses were used to examine the relation between respiration a nd R-R interval variability before syncope, Baseline blood pressure, R -R, and ventilation were similar between syncope subjects and control subjects. Syncope subjects experienced an increase in tidal volume and decrease in BP beginning 3 minutes before impending syncope (systolic blood pressure <80 mmHg) necessitated termination of tilt. Approximat ely 90 seconds before syncope there was a sudden prolongation of R-R i nterval and increase in amplitude of high and low frequency R-R interv al variability, indicating an abrupt enhancement of vagal tone. The in crease in respiratory amplitude between 180 and 90 seconds before sync ope was not accompanied by changes in R-R interval or R-R variability, suggesting a dissociation between respiration and the respiratory sin us arrhythmia. The coherence analysis showed fewer syncope subjects wi th coherence between respiratory and R-R interval variabilities and lo wer transfer magnitudes in syncope subjects compared with control subj ects. Nonsyncopal subjects had no change in respiratory, R-R interval, or blood pressure dynamics during matched time periods before the tim e of syncope. Conclusions-Vasovagal syncope is preceded by a period of hyperpnea and cardiorespiratory decoupling followed by an abrupt incr ease in cardiovagal tone. Respiratory pumping without inspiratory card iac slowing may partially counteract preload reduction until sudden br adycardia precipitates syncope.