From the Hebrew Rehabilitation Center for Aged Research and Training
Institute (L.A.L., R.J.M.), the Beth Israel/Deaconess Medical Center
Department of Medicine, Harvard Medical School, Boston, Mass (L.A.L.); and
Nagoya City University Medical School, Nagoya, Japan (J.H., S.S., A.O.).
Correspondence to Lewis A. Lipsitz, MD, Hebrew Rehabilitation Center for Aged, 1200 Centre St, Boston, MA 02131. E-mail Lipsitz{at}mail.hrca.harvard.edu
Background—The dynamic autonomic
processes leading to vasovagal syncope are poorly understood.
Methods and 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.
© 1998 American Heart Association, Inc.
Clinical Investigation and Reports
Complex Demodulation of Cardiorespiratory Dynamics Preceding Vasovagal Syncope
Key Words: respiration • heart rate • Fourier analysis
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