Pacing-induced atrial electrical remodeling (AER) is characterized by shortening of atrial effective refractory period (A-ERP) and its altered rate adaptation. In paroxysmal atrial fibrillation (AF), periods of AF occur with interveneing normal sinus rhythm (NSR) when atria recover from the preceding AER. Previous episodes of AF may precondition the atrial myocardium and cause different time course of AER in subsequent episodes of AF. But the influence of the preceding AER on the subsequent AER has not been described.

Four mongrel dogs were anesthetized with enflurane. After thoracotomy, silicon band with 3 pairs of electrodes was sutured to the lateral wall of the left atrium. Atrial pacing was performed after 2 wks of recovery and autonomic blockade. Pacing protocol consisted of rapid atrial pacing (RAP) at 500 bpm (for 60 min) and recovery in NSR (for 60 min) which was repeated three times. A-ERP was measured every 10 min. The same pacing protocol was repeated after pretreatment with verapamil (0.1 mg/kg/hr).

1) With 60 min of RAP, A-ERP decreased significantly (126±6 ms vs. 105±7 ms, p<0.005). 2) After cessation of pacing, A-ERP returned to 98% of baseline value in 15 minutes. Recovery from AER occurred faster than AER (78 vs 21 ms/h). 3) After pretreatment with verapamil, RAP decreased A-ERP from 127±5 ms to 116±5 ms. AER, the reduction in A-ERP, was significantly attenuated by pretreatment with verapamil (deltaERp=17±7 vs. 9±0.2 %, p<0.05). 4) When RAPs were repeated, AER showed a tendency of acceleration, but it was not statistically significant (deltaERp=22 ms, 24 ms, 28 ms at the end of 60 min pacing for the 1st, 2nd, 3rd pacing).

RAP induced AER in conscious dog atria and it was reduced by pretreatment with calcium channel blocking agent, verapamil. Upon repeated atrial stimulations, AER did not accelerate or decelerate when the atria recovered from the preceding AER.