EFFECTS OF LEFT ATRIAL DILATATION ON THE ENDOCARDIAL ATRIAL DEFIBRILLATION THRESHOLD - A STUDY IN AN OVINE MODEL OF PACING-INDUCED DILATED CARDIOMYOPATHY

Left atrial (LA) dilatation is a common finding in patients with chron ic atrial fibrillation (AF). Progressive dilatation may alter the atri al defibrillation threshold (ADFT). In our study, epicardial electrode s were implanted on the LA free wall and right ventricular apex of eig ht adult sheep. Large surface area, coiled endocardial electrodes were positioned in the coronary sinus and right atrium (RA). LA dilatation was induced by rapid ventricular pacing (190 beats/min) for 6 weeks a nd echocardiographically assessed weekly along with the ADFT (under pr opofol anesthesia). LA effective refractory period (ERP) was measured every 2-3 days using a standard extra stimulus technique and 400 ms dr ive. The AF cycle length (AFCL) was assessed from LA electrograms. Dur ing the 6 weeks of pacing the mean LA area increased from 6.1 +/- 1.5 to 22.3 +/- 2.4 cm(2). There were no significant changes in the mean A DFT (122 +/- 25 V), circuit impedance (46 +/- 5 Omega), or LA AFCL (13 6 +/- 23 ms). There was a significant increase in the mean LA ERP (206 +/- 20 ms at day 0, and 120 +/- 13 ms at day 42 of pacing). In this s tudy, using chronically implanted defibrillation leads, the minimal en ergy requirements for successful AF were not significantly altered by ongoing left atrial dilatation. This finding is a further endorsement of the efficiency of the coronary sinus/RA shock vector. Furthermore, the apparent stability of the AF present may be a further indication o f a link between the type of AF and the ADFT.