Atrial refractoriness and spontaneous or induced atrial fibrillation.

Atrial refractoriness and vulnerability were studied in 10 patients with paroxysmal atrial fibrillation (PAF) and in 12 age-matched normal subjects (N). Effective and functional refractory periods were measured at three sites of the right atrium: high, middle and low in the lateral wall, both in sinus rhythm and during atrial pacing (120/min). Twice threshold stimuli were applied. Dispersion of refractoriness (D) was measured as the longest minus the shortest refractory period. Atrial fibrillation (AF) was induced in 5 of the PAF; in each of these 5 only one atrial site proved vulnerable (middle in one case, low in the other 4). In every case the shortest refractory period was located at the vulnerable atrial site. In vulnerable patients coupling intervals only slightly different from those which induced AF determined an abrupt change in the atrial electrogram recorded at the vulnerable site, suggesting a modified and in some way abnormal behaviour of the atrial activation wave. At the same time the interval between the beginning of the electrogram at the vulnerable site and of that obtained by the electrode positioned near the A-V node lengthened, suggesting a lower conduction velocity of the atrial activation wave. PAF evidenced significantly higher refractoriness and D than did N during sinus rhythm. Atrial pacing significantly reduced refractoriness but not D, which remained significantly higher than that of N at the same driven frequency. In conclusion, lower cycle length (paced rhythm), a short refractory period and the possibility of delivering extrastimulus at shorter coupling intervals seem conditions favourable to the induction of irregular activation of atrial myocardium. The increased D might be connected to the particular pathophysiological condition of our patients.