July 2018

Pre-syncope in a dependent pacemaker patient

Sam Griffiths – Senior Cardiac Physiologist


A 10 year old boy with a CRT-pacemaker implanted for congenital complete heart block attended pacing clinic after an episode of pre-syncope. The pacemaker mode was DDD with a lower rate limit of 80bpm. On interrogation the presenting rhythm was atrial sensing with biventricular pacing (AS/BiVP). The underlying rhythm was complete heart block with no ventricular escape at 30 bpm.  All lead measurements were within normal and acceptable ranges and overall there appeared to be normal pacemaker function.  There was one EGM captured of a ventricular high rate episode displayed below which happen to correlate to the timing of symptoms.

Figure 1
Stored EGM strip of a ventricular high rate episode. Top to bottom: Atrial EGM (5 mm/mV), right ventricular EGM (1 mm/mV), marker channel.


Which of the following best explains the episode of pre-syncope?

  • Electromagnetic interference (EMI)

  • Lead fracture

  • Ventricular tachycardia

  • Myopotentials

  • Minute ventilation sensor

Minute ventilation sensor

The EGM displayed in figure 1 is an example of pacing inhibition due to oversensing of the minute ventilation sensor signal.  The EGM starts with two tracked sinus p waves at a rate of approximately 100 bpm followed by biventricular pacing (AS/BiVP).  There is then a sudden onset of high frequency signals on the ventricular EGM that are sensed by the device at 150, 200 and 300ms intervals. Ventricular oversensing of the high frequency signals results in ventricular pacing inhibition which essentially causes a 4.5 second episode of ventricular standstill resulting in a pre-syncopal episode.

On closer inspection, the signals on the ventricular channel occur in 50ms intervals.  They are clearly too fast to be physiological ruling out ventricular tachycardia.  The signals are only present on the RV electrogram which would tend to rule out an external source of EMI which is more typically seen on both the atrial and ventricular channels.  The signals are not consistent with those seen with a lead fracture which tend to be more erratic and of higher amplitude than the signals in this episode. They also do not look typical of myopotential signals which would be expected to be displayed with high frequency but with a ‘crescendo-decrescendo’ appearance.

In cases where there is a lead integrity issue that creates high impedance states, minute ventilation signals can be sensed by some pacemakers. A recent field safety notice and medical device alert explaining this malfunction and advice on action to take is available here.