For patients getting a subcutaneous implantable cardioverter-defibrillator (S-ICD), routine defibrillation testing could be safely skipped when using a scoring system to evaluate device position and implant technique, according to the PRAETORIAN-DFT randomized trial.

Patients randomized to defibrillation testing only when indicated by the PRAETORIAN score in this setting were numerically less likely to fail their first shock for spontaneous ventricular arrhythmia — a surrogate for ineffective ICD therapy — than those randomized to routine defibrillation testing over a median 41 months of follow-up (1.7% vs 2.3%; P < .001 for noninferiority). 

While mortality and postoperative S-ICD revisions due to inadequate positioning were similar between groups, defibrillation testing led to potentially related complications in 1.7% of patients. 

The findings suggest S-ICDs can follow in transvenous ICDs’ footsteps in shedding defibrillation testing to confirm the devices’ ability to terminate ventricular arrhythmia, which carries procedural burden, costs, need for anesthesia, and risk for serious complications such as stroke, prolonged resuscitation, and death.

“Avoiding defibrillation testing may improve procedural safety and simplify S-ICD implantation,” Reinoud E. Knops, MD, PhD, of Amsterdam University Medical Center in the Netherlands, reported at the Heart Rhythm Society (HRS) 2026 meeting in Chicago. The findings were also published in Circulation.

‘Impedance Will Never Be Enough” 

Defibrillation testing is recommended in consensus documents for S-ICD implantation, but the PRAETORIAN DFT findings should “absolutely” prompt updated guidelines, study discussant Andrea M. Russo, MD, of Cooper Medical School of Rowan University in Camden, New Jersey, said at the late-breaking clinical trial session.

However, she said operators already often do not perform testing and instead guide implantation using fluoroscopy or impedance values, which has potential medical or legal implications.

The PRAETORIAN score, calculated before discharge by the implanting physician, assesses S-ICD positioning on chest x-rays and predicts defibrillation testing success based on sub-coil fat, sub-generator fat, the position of the generator in relation to the mid-axillary line, and BMI. 

Measuring low voltage impedance might improve prediction of efficacy in combination with the score but shouldn’t be used alone, Knops said.

“If you implant the S-ICD very anteriorly close to the lead, you obviously get a very low impedance, but your shock misses the heart because all the energy shunts over the chest wall,” he said. “Step two in the PRAETORIAN score is really important to correct for that. So, only the impedance will never be enough.”

High Complication Rates With Defibrillation Testing 

The trial enrolled 973 adults (965 retained in the modified intent-to-treat population) getting an S-ICD for guideline-concordant indications at 37 centers in in Europe, Israel, and the United States. Participants were randomly assigned to defibrillation testing either routinely or as guided by PRAETORIAN score before discharge. 

Failed defibrillation testing mandated at least device or lead repositioning or additional testing in the routine testing group. In the no routine testing group, scores over the 90-point low-risk threshold on the scale, which goes from 30 to 900 points, were to prompt defibrillation testing at minimum and other actions as deemed necessary.

Three intervention group patients had missing chest x-rays and could not be scored. Six had an S-ICD outside the anatomical zone indicated in the product label. Among the 14 with an intermediate score (90-149 points) and one with a high score (> 150), 14 had the protocol-mandated testing attempted. Two couldn’t be induced, but all induced arrhythmias could be converted without device revisions needed.

The primary endpoint, failed first shock for a spontaneous ventricular arrhythmia during follow-up, was defined as lack of termination of ventricular tachycardia or ventricular fibrillation within 5 seconds of shock delivery. 

In terms of safety, there were no significant differences between the no routine testing and routine testing groups in all-cause mortality (hazard ratio [HR], 0.9; 95% CI, 0.6-1.4) or arrhythmic death (HR, 0.4; 95% CI, 0.04-3.4). None of the patients died as a direct result of S-ICD failure or testing, the researchers noted.

Complications within 24 hours after defibrillation testing occurred only in the routine testing group (1.7%). There were six cases of refractory ventricular fibrillation, with one needing cardiopulmonary resuscitation; one patient with respiratory arrest who required mask ventilation; and one ventricular fibrillation case converted to atrial fibrillation, resulting in an inappropriate shock. Repeated external rescue or S-ICD shocks terminated all of these events.

“The defibrillation testing group also had a significantly higher 30-day complication rate [4.8% vs 1.7%], driven by refractory ventricular fibrillation, unplanned ICU stay, and cardiac decompensation,” the researchers wrote in the full study. “Except for refractory ventricular fibrillation, these events were not predefined as potential defibrillation testing-related in the protocol but may plausibly be attributed to defibrillation testing.”

Limitations 

One limitation was that the use of defibrillation testing was not blinded, and operators more often repositioned the device or lead under fluoroscopy and performed impedance checks during implantation in the no routine testing group, “likely reflecting heightened attention to implant position,” Knops and colleagues wrote. Thus, “although the PRAETORIAN score can only be calculated after the procedure, this did not result in a higher rate of repositions in the No-Defibrillation testing group.”

The highly experienced implanters were likely a factor, Knops said. “But in starting physicians, this is really important and I think it really adds to the learning curve of physicians. It will shorten the learning curve. So I would really emphasize everyone to keep using it.”

Also, the study represented a relatively young population with a lower burden of comorbidities (mean age, 54 years; 24% women; mean left ventricular ejection fraction, 40%). 

“The PRAETORIAN score mostly shows you that the device is optimally implanted … so I think this is translatable also to the more sicker patient,” Knops said. “But I try to teach everyone, also our fellows: Don't be satisfied with a suboptimal score of 60. Go for 30.”

While the proportion of patients with intermediate and high PRAETORIAN scores was too low to calculate predictive values, “defibrillation testing or, preferably, device repositioning should be considered in case of scores ≥ 90, with consideration of re-operation risks,” the researchers said. 

The investigator-initiated study was funded by Boston Scientific. 

Knops disclosed relationships with AtaCor Medical, Kestra, Boston Scientific, Medtronic, Cairdac, and Abbott. 

Russo disclosed relationships with UpToDate, PaceMate, Abbott Medical, Medtronic, Biosense Webster, AtriCure, Bayer Healthcare Pharmaceuticals, Boston Scientific, and Bayer Healthcare Pharmaceuticals. 

Crystal Phend is an award-winning medical journalist with decades of experience reporting on clinical research and healthcare developments across specialties. When not walking the halls at a medical conference, she can be found at a keyboard in upstate New York.