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. Vagal stimulation may have no effect or slow ventricular rate by increasing AV block or terminate flutter.
Exercise increases HR by decreasing the block.
Only some f waves pass through the AV junction and activate ventricles in a completely irregular fashion. Vagal stimulation may have no effect or may slow the ventricular rate by increasing the AV block. The SA pacemaker is suppressed or its impulses are blocked and the heart is controlled by impulses originating at the AV junction. P waves are negative L2 and positive AVR and occur regularly at a rate 40-60. Depending upon whether retrograde conduction to the atria is faster, slower or similar to antegrade conduction to the ventricles, P waves either precede QRS complexes, the PR interval is less than 0.12 sec or follow QRS complexes by no more than 0.20 sec or are buried in the QRS complexes.