AUTONOMIC MODULATION OF THE ATRIAL CYCLE LENGTH BY THE HEAD-UP TILT TEST - NONINVASIVE EVALUATION IN PATIENTS WITH CHRONIC ATRIAL-FIBRILLATION

Objective-To determine the effects of upright posture compared with su pine position on the dominant atrial cycle length (DACL) in patients w ith chronic atrial fibrillation. Design-The power/frequency spectrum o f QRST suppressed lead V1 ECG was studied in 14 patients in the supine position and during the head up tilt table test. The DACL changes wer e compared with changes in heart rate and blood pressure,Results-Compa red with the supine position, the upright position reduced the DACL fr om 160 to 150 ms (p, < 0.01). The DACL was increased after returning t o the supine position from the upright position, from 147 to 154 ms (p < 0.01). Heart rate increased from 91 beats/min in the supine positio n to 106 in the upright position (p < 0.01). There was a decrease in h eart rate from 109 beats/min in the upright position to 93 after retur ning to the supine position (p < 0.01), No significant changes were se en in systolic or diastolic blood pressure. There were indications of an inverse relation between DACL and heart rate when comparing the sup ine position before and after tilt with the upright position (p < 0.00 1). Conclusions-The sympathetic stimulation and vagal withdrawal induc ed by rising to upright body position are associated with a decrease i n DACL during chronic atrial fibrillation. Thus a reflex increase in s ympathetic discharge after induction of atrial fibrillation could favo ur the persistence of the arrhythmia.