Temporary pacing ECG

Temporary pacing ECG

Temporary pacing ECG
Temporary pacing ECG

What are the findings in this ECG and possible explanations?

ECG shows a paced rhythm at around 60 per minute, with pacing spikes preceding each QRS complex. At first look it may seem that there is P wave before each QRS with a reasonable PR interval suggesting atrial sensed ventricular pacing (A sense – V pace sequence or tracking). But close scrutiny shows that PR interval is gradually varying. The atrial rate is also close to 60/minute, making false appearance of an AV synchronous pacing in some beats.

In analog ECGs the pacing spikes in temporary pacing are usually small as the pacing is bipolar with both electrodes within the ventricle. In digital ECGs such small spikes are usually ‘wiped’ out by the filter settings and the ECG appears like a left bundle branch block (LBBB) pattern. But switching on the pacing detection algorithm in digital electrocardiographs enhances the pacing spike detection and sometimes the spikes are prominent even with temporary bipolar pacing as in this case. Otherwise a large spike indicates unipolar pacing (between pacemaker can and lead tip electrode) which can occur only with a permanent pacemaker. The temporary pacemaker cover is non conducting and is situated outside the body without electrical contact.

Temporary pacemaker and lead schematic diagram
Temporary pacemaker and lead schematic diagram

The QRS pattern mimics LBBB pattern with left axis deviation as is usual with right ventricular apical pacing. If the lead tip slips and paces the right ventricular outflow region, it will give an LBBB pattern with inferior axis. Right bundle branch block (RBBB) pattern, if it occurs in the setting of temporary pacing may indicate lead tip perforating either the interventricular septum and pacing the left ventricle endocardially or the right ventricular free wall and pacing the left ventricular epicardium through the pericardial cavity.

Mechanisms of RBBB pattern in pacing
Mechanisms of RBBB pattern in pacing

Here the interval between the pacing spike and the QRS complex is very short so that there is good conduction at the lead – endocardium interface. If that interval is prolonged, it would indicate a first degree exit block at the lead – endocardium interface (pacemaker ventricular block [1]). If the block is severe it can lead to intermittent pacing (second degree pacemaker ventricular block) or loss of capture (exit block).

References

  1. Kistler PM, Mond HG, Vohra JK. Pacemaker ventricular block. Pacing Clin Electrophysiol. 2003 Oct;26(10):1997-9.