These ECGs are from a 78 year old female with left ventricular hypertrophy, congestive heart failure and ventricular tachycardia. Intravenous procainamide was administered to control the arrhythmia. The tracing on the left was recorded before the administration of the drug. The tracing on the right was recorded after its administration. The R-R interval is the same in both (0.82 seconds). Note the PR interval, QRS duration and QT interval are longer after the drug. The prolongation of the PR interval and QRS duration reflect the effect of procainamide on the sodium channel while the prolongation of the QT interval reflects the drug's effect on the potassium channels.