Ventricular double counting

Simplified Interpretation ofICD Electrograms Ventricular Double Counting... 

A Atrial tachycardia above upper rate that leads to ventricular double counting... 

The phenomenon of transitioning from atrial synchronous ventricular pacing to atrio-ventricular sensing due to upper rate limit violation is sometimes called "pre-empted Wenckebach" upper rate response. Pre-empted Wenckebach becomes troublesome in the patient with a biventricular ICD because of the possibility of ventricular double counting. 

Ventricular double counting may occur in a patient with a. system and significant. ... 

There are biventricular ICD models that utilize sensing from both the right and left ventricular leads for tachyarrhythmia detection. In such an ICD, an R wave may be sensed as two separate ventricular events ("double counted") because the native conduction delay between the ventricular leads is so significant. Not all patients with such a device will double count. In those that do, double counting each R wave can lead to an inappropriate detection in a tachyarrhythmia zone of the ICD and delivery of therapy. For a dual chamber pacing mode this can occur for any atrial rhythm that conducts to the ventricles above the upper rate limit. 

In a VVIR mode double counting each R wave could theoretically occur when the sensor indicated pacing rate is surpassed by ventricular conduction. Therefore, it is extremely important to perform all maneuvers that minimize the likelihood of losing ventricular capture over the expected range of normal heart rates. Again, this includes the use of drugs to control the native ventricular response in AF, and potentially the use of AV node ablation when drugs fail to accomplish the desired effect. 

The newer generation of Medtronic and Guidant CRT ICDs that are currently approved for clinical use do not utilize sensing from the left ventricular lead for tachyarrhythmia detection, and are therefore immune to double counting because of sensing by two separate ventricular leads. 

Both leads can deliver pacing due to the "D" in the DVIR mode. Only the right ventricular lead can sense given the "V" in the second position. Thus, biventricular pacing is effectively accomplished with the short AV interval, the risk of double counting is eliminated as there is no sensing from the LV lead, and the cost of the standard dual chamber ICD is significantly lowered from that of a traditional biventricular heart failure device. 

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. 

As discussed earlier, some biventricular ICDs utilize both right and left ventricular lead sensing for tachyarrhythmia detection. The mechanism facilitating the double counting here, as opposed to earlier cases, was... 

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. 

It is unusual to see QRS double counting by a single ventricular ICD lead. It was theorized that this patient s native bundle branch block was severe enough, and the far-held signal from the delayed ventricle large enough to result in the double counting. 

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. 

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