title: Complex demodulation of cardiorespiratory dynamics preceding vasovagal syncope.
abstract: BACKGROUND
  The dynamic autonomic processes leading to vasovagal syncope are poorly understood.
  
  
  RESULTS
  We used complex demodulation to continuously assess changes in respiration, R-R interval, and arterial pressure (blood pressure) variability during 60 degree head-up tilt in 25 healthy subjects with tilt-induced vasovagal syncope and 25 age-matched nonsyncopal control subjects. Coherence and transfer function analyses were used to examine the relation between respiration and 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 mm Hg) necessitated termination of tilt. Approximately 90 seconds before syncope there was a sudden prolongation of R-R interval and increase in amplitude of high and low frequency R-R interval variability, indicating an abrupt enhancement of vagal tone. The increase in respiratory amplitude between 180 and 90 seconds before syncope was not accompanied by changes in R-R interval or R-R variability, suggesting a dissociation between respiration and the respiratory sinus arrhythmia. The coherence analysis showed fewer syncope subjects with coherence between respiratory and R-R interval variabilities and lower transfer magnitudes in syncope subjects compared with control subjects. Nonsyncopal subjects had no change in respiratory, R-R interval, or blood pressure dynamics during matched time periods before the time of syncope.
  
  
  CONCLUSIONS
  Vasovagal syncope is preceded by a period of hyperpnea and cardiorespiratory decoupling followed by an abrupt increase in cardiovagal tone. Respiratory pumping without inspiratory cardiac slowing may partially counteract preload reduction until sudden bradycardia precipitates syncope.
authors.affiliation: Hebrew Rehabilitation Center for Aged Research and Training Institute, Department of Medicine, Harvard Medical School, Boston, Mass 02131, USA. Lipsitz@mail.hrca.harvard.edu
authors.forename: L A
authors.initials: LA
authors.lastname: Lipsitz
authors.forename: J
authors.initials: J
authors.lastname: Hayano
authors.forename: S
authors.initials: S
authors.lastname: Sakata
authors.forename: A
authors.initials: A
authors.lastname: Okada
authors.forename: R J
authors.initials: RJ
authors.lastname: Morin
mesh_terms: D000328:Adult; D001146:Arrhythmia, Sinus / Q000503:physiopathology*; D001794:Blood Pressure / Q000502:physiology*; D003971:Diastole; D004562:Electrocardiography; D005080:Exercise Test; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011187:Posture; D012119:Respiration / Q000502:physiology*; D019462:Syncope, Vasovagal / Q000503:physiopathology*; D013599:Systole; D013997:Time Factors