Autonomic and electrocardiographic changes in cardioinhibitory syncope.

AIMS: Cardioinhibitory syncope (CS) is a neurally mediated response causing bradycardia or asystole. This study reports on changes in blood pressure, heart rate variability (HRV), and ECG patterns before and after syncope with asystole. METHODS AND
RESULTS: Thirty-five patients with CS and a matched control group were submitted to 60 degrees head-up tilt for 20 min with the addition of nitroglycerin. Syncope developed after a tilt-duration of 1.082 (range 50-1.734 s). Asystole lasted for 21.3 s (range 3.4-80.2 s) and was preceded by sinus rhythm in 21, junctional rhythm in 10, and atrioventricular block in four. Asystole was followed by sinus rhythm in four, junctional rhythm in 24, atrioventricular block in four, and atrial fibrillation in three. The two groups did not differ with respect to supine heart rate, HRV or blood pressure. Prior to syncope, patients showed significant increases in total and low-frequency HRV with reductions in high-frequency HRV and a progressive shortening of the PR-interval.
CONCLUSION: Syncope was preceded by marked accentuation of sympathetic tone with a sudden shift in heart rate control to vagal dominance. Asystole was accompanied by vagally induced, benign arrhythmia in the majority of the patients.