WI pacing mode

The WI mode is characterized by sensing and pacing in the ventricle so that pacing will occur when no intrinsic ventricular activity has occurred within a specified interval defined by the LRL. There are four intervals in the WI pacing mode VV, VR, RV, and RR where R is an intrinsically sensed R wave and V is a ventricular-paced event. VVI pacemakers have three basic timing cycles that must be considered LRL, hysteresis rate, and the ventricular refractory period. 

The DDD, AAI, and WI pacing modes are equally effective in preventing syncope owing to bradycardia in patients with sinus node dysfunction (54). However, as outhned in the following section, different pacing modes are associated with unique chnical outcomes (Table 9.4). 

In general, patients with AV conduction abnormalities and organized atrial activity should be programmed to the DDD mode. Patients with chronic atrial fibrillation are the best candidates for WI or VVIR pacing modes. 

WI pacemaker with another WI or VVIR unit is probably reasonable in truly asymptomatic patients. At the time of pacemaker replacement, such patients should be carefully evaluated to determine whether they might benefit from an upgrading procedure to a more physiologic pacing mode. Single-lead ventricular pacing may also be appropriate in patients who are incapacitated and inactive as well as those with other medical problems associated with short-life expectancy. 

The pacing mode may temporarily switch to a WI or DDI back-up mode when a tachycardia has been detected. 

Fig. 3.2 Normal function in the WI mode. R refers to a sensed ventricular event. V refers to a paced ventricular event. The two successive events may be expressed as the following intervals VV, RR, RV, and VR. The LRL (also called the low rate timer) in the example shown is 60bpm. Thus, the VV and RV intervals are 1,000ms. A sensed ventricular event (first QRS complex) resets the low rate timer. The low rate timer expires without a sensed ventricular event and a ventricular stimulus is provided. After two successive ventricular-paced events (second and third QRS complexes), sinus rhythm at a rate of 75 hpm (800ms) with normal AV conduction inhibits pacing. In the VVI mode, AV synchrony is not maintained during pacing.

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.

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