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Transmission

The tranmission shows a RED lead alert referring to the low sensing values on the RV lead. While a single value has limited meaning, the observation can be confirmed when looking at the ventricular sensing histograms. These bar graphs often follow a Gaussian distribution meaning that a small part of the R-waves (left most blue bar) are to be expected to have an amplitude lower than the sensitivity (red line and grey bar). Also when looking at the Detection trend curve, we can see that sensing has been diminishing since the generator change. Finally, the “Ventricular run” episode (which is probably just an acceleration of conducted atrial fibrillation, and now a true ventricular arrhythmia) shows two undersensed ventricular events. These small-amplitude R-waves are not sensed and therefore the pacemaker reacts with inappropriate ventricular pacing. The pacing occurs during the vulnerable phase of ventricular repolarisation, associated with a small but real risk of inducing ventricular arrhythmia.

Next steps

Ventricular undersensing is a true challenge for pacemakers and ICDs as it can only suspect the issue due to low-amplitude sensing, but never confirm the diagnosis. Only by observing longer strips of EGM can the diagnosis of undersensing be made. As undersensing is associated with inappropriate pacing and therefore induction of potentially lethal arrhythmia, it can never be accepted and monitored. Programming changes can be proposed such as lowering the ventricular sensitivity but it is associated with increased risk of oversensing of noise artifacts (with inappropriate inhibition) and also it is not certain whether it fixes the problem as we can not know the amplitude values of the undersensed events. As we see on the Sensing curve the progressive decline of the R-wave amplitude, the risk of ventricular undersensing may be fixed temporarily but may re-occur in the future. In this case we decided not to take the risk and we implanted a new ventricular lead.