Cardiac memory in canine atrium - Identification and implications

Background-Memory is a diverse biological phenomenon whose importance in th e ventricle has been demonstrated. We hypothesized its occurrence in the at rium, contributing to the modulation of cardiac rhythm. Methods and Results-We analyzed P and Ta waves in conscious chronically ins trumented dogs with complete heart block. Animals were atrioventricularly s equentially paced at 5% greater than the sinus rate from the lateral right atrium (RA) during control, followed by 2 periods of 1-hour test pacing at 50% greater than the sinus rate, or by equivalent test pacing from the left atrial appendage (LAA) at 54b or 50% greater than the sinus rate. Recovery RA pacing periods of 20- and 30-minute duration, respectively, succeeded e ach test pacing period. RA test pacing at either rate did not affect the va riables measured, but changing the pacing site from RA to LAA altered the P and Ta waves. Displacement of the spatial atrial gradient vector occurred during recovery from LAA pacing, was more marked at rapid pacing rates, and manifested accumulation and resolution consistent with cardiac memory. Con currently, the right effective refractory period decreased. Conclusions-Memory is demonstrable in canine atrium, showing rapid onset, a ccumulation during successive pacing periods, and resolution on cessation o f pacing. Given its association with a reduced effective refractory period, it may contribute to the substrate for atrial arrhythmias.