This episode highlights 2 characteristics commonly found in presence of T wave oversensing : 1) it often occurs in presence of a low-amplitude R wave, as the gain adjusts the sensitivity automatically, based on the amplitude of the sensed R-wave, and 2) it often occurs during exercise, when effort is associated with a decrease in the R wave and an increase in the T-wave amplitude. The speed of the signal is also increased, which modifies the slew-rate, bringing the T wave into a bandwidth where it is sensed as an R wave, causing the incorrect diagnosis of VF.
This episode is associated with a charge of the capacitors without shock delivery. This must be diagnosed promptly to minimize the battery drain. More importantly, the device must be reprogrammed to eliminate the delivery of inappropriate shocks. Remote monitoring allows the early identification of this type of asymptomatic episode. One means of correcting this T wave oversensing is to lower the ventricular sensitivity, though this is associated with a risk of VF underdetection, particularly in presence of low-amplitude R waves. The SmartShock technology incorporated in the new Medtronic ICD models may solve this problem, by analyzing differences in amplitude, rate and pattern to distinguish R from T waves. The results in its first recipients seem promising.