Paced rhythm with retrograde P waves

Paced rhythm with retrograde P waves

Paced rhythm with retrograde P waves: ECG shows a wide QRS rhythm with LBBB pattern in I, aVL and V1. Axis is leftward and V6 shows negative QRS complexes, indicating pacing from the right ventricular apex. Pacemaker artefacts or spikes (S) can be seen on close scrutiny in V4-V6, just before the onset of the wide QRS complex. Anterograde P waves (P) are dissociated from the QRS complexes. Hence the basic rhythm was complete heart block which needed a pacemaker. Retrograde P waves (P’) are seen after the paced QRS complex in the lead II rhythm strip, with a reasonable retrograde conduction time. This is an interesting situation in that anterograde complete heart block co-exists in the presence of fairly intact retrograde ventriculo-atrial conduction.

Here the retrograde P waves are negative in inferior leads, suggesting retrograde activation of the atrium. Negative P waves in V6 due to retrograde conduction has been reported by Venkataraman K [1], though usually negative P waves in V6 signifies activation starting from the left atrium (left atrial rhythm).

In single chamber pacemakers, retrograde P waves can cause reverse atrial kick as the atrial contraction occurs during ventricular systole leading to regular cannon waves. But in dual chamber pacemaker, retrograde conduction can lead on to pacemaker mediated tachycardia, which is prevented by programming the post ventricular atrial refractory period (PVARP) beyond the time of occurrence of the retrograde P wave.

Pacemaker mediated tachycardia or endless loop tachycardia occurs because the retrograde P wave is detected by the atrial circuit of the pacemaker and triggers a ventricular pacing after the programmed AV delay. This ventricular paced beat can again be conducted back into the atrium, forming a circus phenomenon. Programming PVARP makes the atrial circuit refractory when the retrograde P wave arrives and prevents triggering of ventricular pacing and hence, the endless loop tachycardia.

Reference

1. Venkataraman K. Negative P waves in lead V6 during right ventricular pacing. Angiology. 1979 Oct;30(10):716-20.