ROLE OF AUTONOMIC REFLEXES IN SYNCOPE ASSOCIATED WITH PAROXYSMAL ATRIAL-FIBRILLATION

Objectives. The purpose of this study was to evaluate the role of auto nomic reflexes in the genesis of syncope associated with the onset of paroxysmal atrial fibrillation. Background. Syncope associated with pa roxysmal atrial fibrillation has been interpreted as an ominous findin g predictive of rapid ventricular rates. However, various mechanisms m ay be involved when heart rate is not particularly high.Methods. Forty patients (age 60 +/- 14 years, 20 men, 20 women) with syncope and atr ial fibrillation were compared with 16 control subjects (age 66 +/- 13 years, 8 men, 8 women) with atrial fibrillation without syncope. Caro tid sinus massage and head-up tilt testing (at 60-degrees for 60 min a t baseline and during isoproterenol infusion) were performed during si nus rhythm. A positive response was defined as the induction of syncop e. Atrial fibrillation was also induced on a tilt table at 60-degrees by means of short bursts of atrial pacing.Results. Results of carotid sinus massage were positive in 15 (37%) of 40 patients but in no contr ol subjects (p = 0.002). Head-up tilt test findings were positive in 2 5 (66%) of 38 patients and in 2 (12%) of 16 control subjects (p = 0.00 04). The induction of atrial fibrillation in the upright position elic ited syncope in 16 (42%) of 38 patients but in none of 16 control subj ects (p = 0.001). At the beginning of atrial fibrillation, systolic bl ood pressure was lower in patients than in control subjects (88 +/- 32 vs. 127 +/- 32 mm Hg), whereas mean heart rate was similar (142 +/- 3 5 vs. 134 +/- 25 beats/min). The correlation between heart rate and sy stolic blood pressure was weak (r = 0.35), and in five patients syncop e occurred at a heart rate less-than-or-equal-to 130 beats/min. At the time of syncope, heart rate decreased (-12 +/- 21 beats/min) in patie nts with induced syncope, whereas it remained unchanged in patients wi thout induced syncope (+1 +/- 17 beats/min, p = 0.04) or slightly incr eased in control subjects (+9 +/- 21 beats/min, p = 0.009). Conclusion s. Patients with syncope associated with paroxysmal atrial fibrillatio n are predisposed to an abnormal neural response during both sinus rhy thm and arrhythmia. In some patients the onset of atrial fibrillation triggers vasovagal syncope.