Ventricular undersensing

Fig. 3.13 Undersensing of an intrinsic R wave in the WI mode. In this example, the lower rate interval is 1,200ms. The first intrinsic R wave is sensed, initiating the lower rate interval timing. However, because the second intrinsic R wave is not sensed, a paced ventricular event occurs shortly after it. The interval between the ventricnlar-paced events (W) is 1,200ms.

Fig. 3.14 Undersensing of a premature ventricular contraction. In the example, surface ECG, refractory periods and intracardiac electrograms (EGM) are shown. The electrograms generated by native QRS complexes are 7 to 8 mV and are appropriately sensed (because the sensitivity is set to 5 mV). The electrogram from the premature ventricular contraction is only 4 mV and is not sensed by the pacemaker. The problem can be corrected by reducing the sensitivity value to 3 mV (making the pacemaker more sensitive).

Undersensing. Inappropriate delivery of pacing stimuli when the pacemaker system fails to sense P-waves or QRS complexes defines undersensing. Delivery of stimuli can be harmful if they occur during the atrial and ventricular relative refractory periods that are predisposed to tachyarrhythmia induction. Of particular concern is the induction of ventricular tachycardia or fibrillation when ventricular pacing occurs on the terminal portion of the T-wave ( R-on-T ), especially in the critical care setting where concomitant ischemia, metabolic and electrolyte abnormalities are frequently present. 

Similarly, some systems identify an event sensed on the ventricular channel of the pacemaker which is not preceded by an atrial event, either paced or sensed, as a premature ventricular contraction (PVC). PVC s have a very definite implication for the clinician. The pacemaker s definition is far more specific as the pacemaker cannot analyze the morphology of the complex. Hence, nonphysiologic make-break electrical potentials associated with an internal insulation failure or conductor fracture will also be identified as PVCs as will accelerated junctional rhythms or episodes of atrial undersensing but with intact AV nodal conduction. Some systems may also identify runs of ventricu-... 

The least sensitive setting caused numerous ventricular "drop-out" beats (undersensed beats) to occur. At the end of the upper strip the ICD found enough "slow" ventricular beats (below rate cutoff), due to drop-out, to think that the arrhythmia had terminated. Thus it diverted the shock. 

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). 

This phenomenon represents one of the other significant pacemaker/ICD interactions that can occur besides undersensing of VF. In fact if the atrial and ventricular pacing spikes are sensed at a fast enough rate the ICD can detect what it thinks is a tachyarrhythmia and deliver treatment. 

---