A shorter R-R interval causes a larger endocardial R wave amplitude in cases with atrial fibrillation

Endocardial electrograms were recorded with a Medtronic programmer model (9742A) in 15 patients with implanted pacemakers required for bradycardia and atrial fibrillation. Telemetered unipolar ventricular endocardial electrograms were recorded for 2 minutes, and the R wave amplitudes of the shortest R-R intervals were compared to that of the longest R-R intervals. Eleven of the 15 cases showed R wave amplitudes of the shortest R-R intervals an average of 2.1 mV larger than the R wave amplitudes of the longest R-R intervals. Three cases showed no difference, and one the opposite condition. In all four, the average recorded R-R interval exceeded 670 ms. The shortest R-R interval among the 11 cases with a larger R wave amplitude was 545 +/- 100 ms. The endocardial R wave amplitude of the shortest R-R intervals were larger than the R wave amplitudes of the longest R-R intervals. Shorter intervals were followed by larger endocardial R wave amplitudes (P < 0.005 with the Wilcoxon's rank sum test).