Ventricular sensing zone

VF Zone Fast VT Zone Slow VT Zone Ventricular Sensing Zone... 

Thus as time passes a sensed heartbeat may fall into (i) a VF zone and be counted as a "VF" event, (ii) a VT zone, if present, and be counted as a "VT" event, or (iii) a ventricular sensing zone and be counted as a "VS" event. Once a beat is classified this process is repeated for each subsequent sensed ventricular heartbeat. Classification in a St Jude ICD also takes into account an average of the time interval of the three prior heartbeats in order to assign a ventricular heartbeat into a VF, VT, or VS zone. If enough beats are classified over time in a VF and/or VT zone, then the initial criterion for tachycardia detection may be satisfied. What constitutes "enough beats" is determined by the manner that the ICD counts and how the ICD has been programmed. 

Underdetection Ventricular tachycardia will not be detected in some cases, even in the presence of adequate sensing. One troublesome cause of underdetection is slow ventricular tachycardia with rates that are lower than the programmed tachycardia detection rate. Patients who receive concomitant antiarrhythmic drugs may be predisposed to this problem owing to slowing of ventricular tachycardia. In one study, 8% of patients who received ICDs developed ventricular tachycardia with rates that had significant overlap with spontaneous sinus rates (89). Similarly, irregularity of the tachycardia, which causes sensed rates to straddle two zones (between sinus rhythm and ventricular tachycardia or between ventricular tachycardia and ventricular fibrillation). 

As with pacemaker analysis an electrocardiogram (EGG) cable can be attached to the patient to provide EGG rhythm strips. They can aid in assessing the pacing and sensing functions of the IGD. "Marker chaimel" annotations are also available. They vary from those for pacemakers in that additional annotations are used to delineate ventricular events that occur within the various IGD tachycardia treatment zones (i.e. "VT" for VT zone or "VE" for VF zone for the below example). These tachycardia annotations differ between IGD manufacturers, but tend to convey similar information. 

Each sensed ventricular heartbeat undergoes a classification determined by its relationship in time to the previous sensed/paced ventricular beat, and the number of zones of detection that have been programmed. "Time zero" below marks the point of initial pacing or sensing. An absolute refractory period ( ) serves to prevent this beat from being sensed should it be a ventricular paced complex, or from being sensed a second time should the complex be a particularly wide native beat. 

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. 

The most obvious finding is the low amplitude signals being sensed in the VF and VT-1 zones, and that the ventricular escape is barely 20 b.p.m. with complete heart block. The low amplitude signals are caused by a loose set screw in the pacing/sensing lead port. These potentials result because the loose set screw intermittently makes and breaks contact with the lead. These potentials also inhibit pacing. Note the lack of these potentials on the shock EGM. 

First, the patient s AF is undersensed by the pacemaker as evidenced by the presence of atrial pacing. The first, second, third, and sixth atrial pacing spikes from the permanent pacemaker are oversensed as ventricular events by the ICD. This causes each of the subsequent paced QRS complexes from the pacemaker to fall into the ICD s VF detection zone. The first QRS is also double detected (sensed by the RV and LV leads at a different time). 

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