T waves oversensing

Oversensing (lead failure, myopotential oversensing, T-wave oversensing)... 

Sensing in an ICD is quite dissimilar to that in a pacemaker. Why Recall that the R wave signals of a typical pacemaker system may fall between about 5-25 millivolt (mV). In contrast, an ICD needs to be able to detect VF wavelets that may be of very small amplitude, on the order of tenths of a millivolt. Simply setting a fixed sensitivity level at such a low level in an ICD without any other adjustments would likely cause T wave oversensing in any baseline rhythm. A ventricular refractory period would not solve this issue as the ICD needs to be alert during this time period for any tachycardia events. 

The marker channel shows frequent T wave oversensing on the VEGM. 

Management Solution A parameter spedhc to this manufacturer s device, the "decay delay" was altered so as to eliminate the native QRS double counting. Also, decreased ventricular sensitivity to eliminate T wave oversensing, without compromising VF detection, was programmed. 

Fig. 3.12 Oversensing of the T wave in the VVI mode. In this example, the lower rate interval is 1,000ms. The T waves following the intrinsic QRS complex and the paced complex are sensed, resulting in a longer than programmed escape interval. The problem would be solved by programming a longer VERP.

The T waves with ventricular pacing are oversensed. Each time this occurs the subsequent P wave is sensed, but ventricular pacing not delivered 90 ms following (AV interval) as this violates the upper rate. The native ventricular QRS complexes are double counted. The ICD then thinks it has detected an arrhythmia and changes the pacing mode to a back-up of VVI. This then allows continuous double counting of each QRS complex and the subsequent shock. 

The initial rhythm is P wave tracking. Next, a T wave is oversensed. Ventricular pacing does not then occur, as this would have violated the upper tracking limit. This results in double counting of each QRS, detection in the VF zone, and finally a high energy shock. 

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