No Pacing and 2:1 Failure to Sense

Report:

Sinus rhythm 55/min 1

Demand pacemaker: failure to pace 2

2:1 failure to sense 5

LVH voltage (SV2 25mm) 0.5

Left atrial abnormality (LAA) 0.5

Old inferior infarct 0.5

Possible old anterior infarct (loss of R height from V2 to V3) 0.5

Comment:

The sinus P waves march, normally conducted and regular, across the page – but alternate P waves are preceded by a pacemaker spike, raising the possibility that they are in fact paced by an atrial electrode. That would imply an escape-capture situation where escape and capture cycles have equal lengths; further, what can be seen of the slightly distorted P waves looks exactly like P waves unassociated with pacemaker spikes. Atrial pacing is extremely unlikely.

A second pacemaker spike falls between the P and T waves, equally without any observable effect on the native rhythm. The pacemaker then remains silent until well after the next QRS complex, when it fires just at the onset of sinus P wave. The pacemaker spikes form a regular, but ghostly, bigeminy.

Now, it can be seen that the first spike escapes from the preceding QRS complex by the same distance as the second spike escapes from the first spike. The unit is set to pace at 70/min; its first spike senses the preceding QRS but the second does not – it only knows its electronic predecessor. Hence the fixed relationship between the sinus and the pacemaker rhythms – hence the bigeminy.

The trace was observed by the ward staff and the pacemaker was turned off. Sinus rhythm continued as before (Fig 108a).