CHANGES IN CARDIAC REPOLARIZATION FOLLOWING SHORT PERIODS OF VENTRICULAR PACING

Cardiac Memory. Introduction: ''Cardiac memory'' (primary T wave chang e) is thought to occur after 15 minutes to several hours of right vent ricular (RV) pacing, The two components of the temporal change in repo larization are memory and accumulation, The purpose of this study was to examine quantitatively the effect of short periods of ventricular p acing on the human cardiac action potential, using monophasic action p otential (MAP) recordings. Methods and Results: Thirty-one patients (a ges 43 +/- 14 years) with structurally normal hearts undergoing a clin ically indicated electrophysiologic procedure were enrolled, Catheters were placed in the right atrium (RA) and RV, and a MAP catheter was p ositioned at the RV septum, APD(90) was calculated from digitized MAP recordings, MAP morphology comparisons were performed using the root m ean square (RMS) of the difference between complexes, All pacing was a t 500-msec cycle length, There were four pacing protocols: (1) RA paci ng was performed for approximately 15 minutes to evaluate temporal sta bility of the MAP recordings (5 pts); (2) to evaluate the memory pheno menon, four successive 1-minute episodes of RV pacing were intersperse d with 2 minutes of RA pacing (5 pts); (3) the accumulation phenomenon was evaluated by assessing the effects of 1, 5, 10, and 15 minutes of RV pacing on the MAP during RA pacing (16 pts); and (4) 20 minutes of RV pacing was followed by 10 minutes of RA pacing to correlate visual ly apparent T wave changes with changes in MAP recordings (5 pts), In the control patients, no changes in APD(90) or RMS analysis were noted during 14.9 +/- 1.4 minutes of RA pacing, In the second protocol, RMS of the difference between the baseline MAP complexes and the signal a verage of the first 50 beats following each of four 1-minute RV pacing trains demonstrated progressively greater differences in morphology a fter successive episodes of RV pacing, In protocol 3, RMS analysis ide ntified a progressively greater difference between the baseline MAP re cording and the average of the first 50 beats after 1, 5, 10, and 15 m inutes of RV pacing, In protocol 4, visually apparent changes in T wav es occurred in parallel with the RMS of the difference between the bas eline MAP recordings and the average of the first 50 beats after 20 mi nutes of RV pacing, Similar changes also were demonstrated by APD(90) analysis. Conclusion: This study is the first to demonstrate that epis odes of abnormal ventricular activation as short as 1 minute in durati on may exert lingering effects on the repolarization process once norm al ventricular activation resumes.