Which patients are not suitable for a subcutaneous ICD: incidence and predictors of failed QRS-T-wave morphology screening

BACKGROUND: The subcutaneous cardioverter-defibrillator (S-ICD) relies on a pre-implantation QRS-T morphology screening (TMS) of the ECG to assure that it reliably detects the QRS complexes and T waves. The prevalence and clinical characteristics of the patients who fail this TMS is unknown.

METHODS AND RESULTS: QRS-TMS was done in 230 consecutive ICD outpatients (75% male, age 57 ± 15 years) without an indication for cardiac pacing, using an ECG simulating the 3 sensing vectors of the S-ICD (TMS-ECG). Patients were defined suitable when at least 1 sensing vector was considered appropriate in both supine and standing position. In total, 7.4% of patients, who were all male, were considered not suitable for a S-ICD according to the TMS-ECG. Independent predictors for TMS failure were hypertrophic cardiomyopathy (HCM; odds ratio [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).

CONCLUSION: In patients without an indication for pacing, 7.4% would have been not suitable for a S-ICD according to the TMS. HCM, a heavy weight, a prolonged QRS duration and a R:T ratio <3 in the ECG lead with the largest T wave were independently associated with TMS failure. These data might alert physicians that selection of patients for a S-ICD should be considered with special caution in certain patient groups, because they may not satisfy ECG criteria for adequate sensing.