65 ATRIAL SIZE AFFECTS ABILITY TO REVERT ATRIAL FIBRILLATION VIA ATRIAL CHILLING

Previous work has shown the ability of iced saline to easily revert acetylcholine-induced atrial fibrillation in the 25-30 kg pig (Sus domesticus). The present study investigated whether larger-sized subjects would show a similar response. Two subjects of 50-55 kg underwent an identical protocol to that used previously in the smaller sized animals of the same species. The left atria were exposed by left thoracotomy and atrial fibrillation induced with local application of 2-3 drops of 1:100 acetylcholine solution and gentle stroking of the tissue. The time durations of the episodes were measured, and iced saline compresses applied in an attempt to shorten the arrhythmia duration. In one case, synchronized cardioversion was used to revert an atrial episode which had lasted over 30 minutes, and in the other, a defibrillation shock was used to rescue from an untoward episode of ventricular fibrillation which had occurred. In both animals, we were initially able to get control episodes lasting 7.5 minutes. Repeat inductions lasted longer, and none of the applications of iced saline were effective in reverting the atrial arrhythmias. In one case it lasted over 30 minutes, and in the other case unexpected ventricular fibrillation was produced. Countershocks were given to both animals, but the hemodynamic status quickly deteriorated in both cases. We observed that the hearts of these subjects were larger than those in the preceding study. Since in these large animals, we were still cooling approximately the same area of atrium (estimated at 10-15 cm2) as in the smaller ones, it is likely that we were not able to reduce the fibrillating area to below the critical mass (i.e. that bulk of myocardium required for fibrillatory wavefronts to continue propagating). We conclude that larger areas of atrium will probably need to be chilled in bigger subjects in order to revert atrial fibrillation by this technique.