TWO Much T-Wave Oversensing

A limitation of implantable defibrillators (ICDs) is T-wave oversensing (TWO). Modern ICDs use filtering techniques to avoid sensing the T-wave. However, the T-wave can be large relative to the QRS in some patients and lead to double-counting by the ICD and sometimes shocks. For these patients, one can usually manage the problem by making adjustments to the ventricular sensitivity or to other programmable portions of the sensing algorithm.

The totally subcutaneous ICD (S-ICD) is now available for patients who have an indication for an ICD and have no need for pacing. There are several advantages to the S-ICD including the avoidance of the complications related to intravascular lead placement. Another advantage of the S-ICD is that, unlike a transvenous ICD, it can sense from three different vectors. Despite this versatility, however, the S-ICD is also vulnerable to TWO. 

How often does TWO lead to innappropriate shocks by the S-ICD? In the U.S. IDE S-ICD trial, 330 patients who were enrolled.¹ A shock due to a non-VT/VF event occurred in 38 patients. The inappropriate shock was due to oversensing in 24 patients (7.4% of the patients who received the device). After institution of dual zone programming with a conditional shock zone that uses discriminators, there was a 54% reduction in innappropriate shocks from TWO.

What can be done to avoid implantation of an S-ICD in a patient who is likely to have TWO? A simple screening tool is available to help identify patients who are likely to have TWO by the S-ICD device. Basically, a 10-second rhythm strip is recorded using the Boston Scientific programmer at voltage gains of 5, 10, and 20 mV, along the same three chest vectors that the device would use for detection. These rhythm strips are analyzed by applying a clear plastic screening template to the QRS and T-waves. To determine that the T-wave is not too big or too delayed, relative to QRS complex, the T-wave must not be not visible outside the window chosen on the basis of the QRS amplitude. Patients are considered candidates for the S-ICD if they pass the template in any same vector in both the supine and standing positions, at any gain. This can also be done with exercise to further identify patients who might be prone to TWO. The advantage of the screening test is that it appears to be very sensitive and can successfully screen out patients who should not get the device. The disadvantages are that it is an extra step that must be taken in clinic, and requires time and training to accomplish. It can be difficult for a patient who has heard about the pros and cons of the S-ICD relative to a transvenous system to then discover that he or she is not a candidate for the S-ICD after the ECG screening.

How often do patients fail the ECG template screening test for the S-ICD? Because screening logs were not kept for the S-ICD IDE trial, it has not been known exactly how often patients are ineligible for the S-ICD because of failure of the ECG screening template. A recent paper published online in the Journal of Cardiovascular Electrophysiology, Olde Nordkamp et al applied the ECG screening template in both supine and standing position to 230 outpatients who had an ICD and no indication for pacing.² In total, 7.4% of patients failed the screening test. Independent predictors for failure were hypertrophic cardiomyopathy (OR 12.6), a heavy weight (OR 1.5), a prolonged QRS duration (OR 1.5) and a R:T ratio <3 in the lead with the largest T-wave on a standard 12-lead surface ECG (OR 14.6).

T-wave oversensing was not one of the limitations of transvenous ICDs that the S-ICD effectively addressed. More work is needed to improve the sensing algorithm in the S-ICD to avoid TWO so that more patients can take advantage of the technology.

References

1. Weiss R, et al. Safety and efficacy of a subcutaneous implantable defibrillator. Circulation. 2013;128:944-53.
2. Olde Nordkamp LRA, et al. Which Patients are Not Suitable for a Subcutaneous ICD: Incidence and Predictors of Failed QRS-T-Wave Morphology Screening. J Cardiovasc Electrophysiol. 2013 Dec 9. [Epub ahead of print]