Double Coupling

Report:

Sinus rhythm 100/min 1

VEBs 1

Ventricular demand pacemaker 3

Apparent intermittent failure to sense 4

Right bundle branch block 1

Comment:

The odd thing about the pacemaker’s lack of sensing is that all the paced beats occur after – and seem coupled to – VEBs, the latter being coupled to sinus beats. This is not necessarily a sign of pacemaker failure: “a normally functioning pacing system at times fails to detect atrial or ventricular extrasystoles47.” This is due to inadequate signal at the site of the endocardial electrode: it may be too small, or at right angle to the electrode, or rising (as VEBs often do, slowly through the myocardium) at too low slew rate. The slew rate is voltage change over time (dV/dT) and both temporary and permanent, unipolar or bipolar electrodes ignore low-rate events in order to distinguish the QRS complex from the T wave.

Below (Fig 86a) is the result of adjusting the pacemaker’s sensitivity: in the lower strip, the VEBs are now “recognised” and the pacemaker beats now “keep a respectful distance”48.