Exercise or atropine always increase the rate to normal. Exercise or atropine often fail to increase the rate to normal. P upright in L2 and negative in AVR with identical or slightly different size and contour from beat to beat. Tachycardia usually appears and subsides gradually. Vagal stimulation produces gradual slowing which reverses when pressure is released.
Exercise further increases the rate.
The further from the SA node the ectopic focus is, the more abnormal will be P' configuration. PR interval normal or slightly prolonged. The pause between APB and the next sinus beat is usually noncompensatory, i.e., the RR interval between two QRS enclosing APB is less than twice the normal RR interval. Atrial activity may be represented by upright P in L2 or by P superimposed on T or ST segment of the preceding beat. Onset and termination of AT is often abrupt.