DDD pacing with psuedo fusion

ECG taken in the midst of a bradycardia alarm.

INTRODUCTION:

I was asked to review a 12 lead ECG of a patient post abdominal surgery with a Dual Chamber Permanent Pacemaker and no myocardial ischaemic symptoms.

The indication for the 12 lead was to investigate a monitor alarm for bradycardia to 40bpm, discussion around a PPM check was beginning as medical and nursing staff were concerned the PPM was dysfunctional.

This ECG was taken in the midst of one of these alarms. The patient was asymptomatic, indeed the patient was wondering what all the fuss was about.


A quick look through the patients history/documentation showed no real details of the settings of the pacemaker.

A review of the ECG shows: 

• the rhythm which I would inelegantly call sinus rhythm with premature atrial ectopics, atrial and atrio-ventricular sequential pacing (a bit of a mouth full).
• pacemaker rate 75, but overall heart rate around 80.
• Intrinsic PR is around 230ms Measured in V4 V5
• Pacemaker AV interval is meassured at 240ms
• Incomplete RBBB pattern (? something for a future post) V1, early R wave progression, normal axis, normal width QRS complexes.
• Atrial capture of pacing is seen evidenced by p wave artifact after each atrial pacing spike, implied atrial capture is seen when atrial pacing occurs and a QRS complex follows at same interval as the intrinsic PR intervalon the 9th and the 13th.
• Ventricular capture cannot be verified, as despite ventricular pacing spikes occuring they fall on intrinsically conducted QRS complexes. Despite ventricular pacing the QRS is not deformed at all. This is called psuedofusion (Normal in ventricular pacing).
• Possibly half a mm of ST elevation in aVF and lead II but wander in the baseline could easily be the cause of this.
• T waves look unremarkable.

Judging by the ECG no evidence of a bradycardia or a dysfunctional PPM was found.

DISCUSSION:

Fusion and Psuedofusion

This is a strip showing various types of ventricular activation. The first beat is the patients own intrinsic conduction. The red arrows indicate where ventricular pacing spikes occur. The second, third, forth and fifth complexes show varying degrees of fusion. Note that the second complex is minimally distorted by activation by pacing. The third, forth and fifth complexes show increasing distortion of the QRS complex, this distortion is secondary to greater amounts of activation of the ventricular tissue via the pacemaker rather than the intrinsic conduction. The final three complexes see the ventricles completely activated by the pacemaker.

Normal  conduction in the ventricles produces a narrow QRS complex, isoelectric ST segment and normal polarity T wave. 
When the ventricles are depolarised from a stimulus from a pacemaker a wide QRS complex, discordant ST segments and T waves.
Fusion beats are a common occurance in pacing, the delivery of a pacing stimulus occuring at the same time as that of intrinsic conduction has the ventricles partly depolarised by the pacing stimulus and partly by intrinsic conduction.

Some examples of psuedofusion: Intrinsic QRS complexes not distorted by pacing.  Little to no  activation of the ventricle by the pacing stimulus. Therefore psuedofusion.

Psuedofusion is seen when a pacing stimulus is sent at the same time that intrinsic conduction occurs yet no deformation of the QRS complex occurs. This implies that little to no ventricular tissue was activated by the pacing stimulus.


Monitors and Pacing:

Monitors often have difficulties picking up changes in the ECG produced by pacing. Whilst viewing the monitor during an alarm it was noted that any time a psuedofusion complex occurred the monitor failed to register a QRS complex. The image above demonstrates that three complexes in a 6 second strip that the monitor would have missed in it's rate count. 

LEARNING POINTS:

• Pacemakers evoke fear in healthcare providers and a low threashold for PPM check exists.
• Fusion beats are common in pacing and this is an example of pseudofusion.
• Monitors are prone to being confused by pacing artifact or indeed changes in QRS morphology in response to pacing.
• Atrial capture is often difficult to appreciate on ECG although this shows big p waves and capture is implied as the 9^th and the 13^th cardiac cycle has a QRS following atrial pacing.

QUESTIONS THAT REMAIN TO ANSWER:

• Is this incomplete RBBB? No S V5, V6, or I. I need to define this a little better.