ZONES OF ATRIAL VULNERABILITY - RELATIONSHIPS TO BASIC CYCLE LENGTH

Background Bradycardia is commonly found in individuals at risk for pa roxysmai atrial fibrillation (AF). However, a clear relationship betwe en lengthening of basic cyclic length (BCL) and AF has nor been demons trated. Methods and Results In 20 open-chest dogs, atrial refractorine ss, AF vulnerability, and atrial activation limes (ACTs) were determin ed in sinus rhythm and at BCLs of 400, 300, and 200 ms, and the findin gs at the same coupling intervals and stimulus strengths were compared . As BCL increased, AFV zone lengthened, and its outer limit occurred later in diastole. The outer limit of the AF vulnerability zone for a BCL was its relative refractory period; the inner limit, however, was not its effective refractory period. A border zone, defined by the inn er limit of the AF vulnerability zone and the effective refractory per iod for a BCL, decreased as BCL lengthened, offsetting the increase in the AF vulnerability zone. The border zone was characterized by parad oxical stimulus current strength propagation relations and features su ggesting supernormal conduction. ACT also increased with BCL lengtheni ng. When AF induced by rapid atrial burst pacing was contrasted with A F induced by an extrastimulus, it tended to have a more disorganized p attern and lasted longer. Conclusions Lengthening of BCL increases the AF vulnerability zone, extending its outer limit later in diastole an d comprising an increasing component of the total duration of the rela tive refractory period. Very short BCLs create conditions that also fa vor AF vulnerability.