Atrial Pacemaker: Complete Failure to Sense

Report:

Sinus/atrial pacemaker rhythm 58-72/min 5

Atrial pacemaker: failure to sense 3

Borderline low voltage in frontal leads 1

Nonspecific ST/T changes, V2-5, consistent with ischæmia 1

Comment:

The atrial pacing rate is much slower than the sinus rhythm but there are more paced than sinus beats in the trace. Having lost its ability to sense atrial events, viz. sinus P waves, the pacemaker hogs the rhythm with parasystolic constancy. The native P waves, on the other hand, keep getting reset by the early atrial captures, responding with the post-ectopic sinoatrial depression; by the time they recover – with their faster intrinsic rate – they are reset again by another early pacemaker capture. A repeat trace is shown below (Fig 3a). Note the (normally) long AV interval for atrial pacemaker captures: the AV node is caught by surprise.

The obvious thing to do is to switch the (AAI) pacemaker off, but this is often beyond the courage of the protocol-driven junior staff, especially out of hours (this trace was recorded at 22.45 hrs). Although the overall rate is over 70/min, due to the early pacemaker captures, it is more desirable to have natural P waves drive the atrial systole rather than their electronically induced counterparts. More importantly, any early beats may be unhealthy in those at high risk of atrial arrhythmias.

Most importantly, these recorded irregularities are often first discovered by the cardiac surgeons on their rounds and, perhaps rightly, held against the ICU consultant in charge. Cardiac surgeons can be surprisingly cluey in electrocardiographic matters affecting their operative outcomes and should never be provoked beyond what’s required to keep them in their place!

The ischæmic-looking T wave changes in this setting are almost invariably pre-operative and reflect the myocardial damage already sustained. They should still be reported on their “face value”, because that is what complete and accurate reporting is about; evaluation of their significance should be reserved for the comment.

Fig 3a.

Fig 4. 73 year old lady in CCU for unstable angina. She had a permanent pacemaker implanted 3 years previously for intermittent atrial fibrillation and syncope. Angiography showed normal left ventricular function with small antero-apical hypokinesia and a 95% LAD lesion suitable for angioplasty.

Fig 4. 84 year old lady in CCU with paroxysmal tachycardia and angina.