The PRAETORIAN score: guardian against subcutaneous implantable cardioverter-defibrillator shock failure?

This editorial refers to ‘Implantation technique and optimal subcutaneous defibrillator chest position: a PRAETORIAN score-based study’ by P. Francia et al., on pages 1822--1829. The completely subcutaneous implantable cardioverter-defibrillator (S-ICD) has entered its second decade in everyday clinical practice since the landmark study on its feasibility in the first 50 patients in 2010.1 Several large studies have since been published, that continue to show a high efficacy in converting ventricular arrhythmias with a favourable safety profile of the system, especially with regard to avoiding the vascular space.2,3 Although this boosts confidence in the clinical utility of the S-ICD, there remain some challenges in everyday clinical practice. For one thing, while the SIMPLE trial4 has led to a Class I recommendation that defibrillation testing (DFT) is no longer required for the transvenous implantable cardioverter-defibrillator (ICD), it is still recommended that this should be used for S-ICD5 based on publications reporting difficulties to obtain successful DFT in certain patients with anatomical challenges and obesity. As DFT poses logistical challenges due to the need of anaesthesia under strict monitoring, there are centres and geographies that forego DFT which may pose a hazard to the patient. It would therefore be a major breakthrough if a simple guidance based on reliable parameters could predict successful defibrillation of life-threatening arrhythmias by S-ICD. Recently, the PRAETORIAN score was proposed as a new evaluation method to understand the anatomical landmarks that determine the propensity for successful DFT.6 Clinical experience and computer modelling showed the highest detrimental effects on defibrillation thresholds by incremental fat under the S-ICD lead coil (and to a lesser extent, fat under the S-ICD can) as well as a more anterior chest position of the can.7 These findings were used by the research group at Amsterdam University Medical Centers (AUMC) Amsterdam to device the PRAETORIAN score based on chest X-ray images. By allotting up to 150 points for up to three coil-widths of fat under the lead, multiplied by a maximum factor of 4 for a more anterior S-ICD can chest position, and a factor of 1.5 for sub-can fat, while deducting 40 points for low body mass index (BMI) for those with a score ≥90⁠, a scoring system was developed to predict low DFT failure likelihood (⁠<90 points), medium DFT failure likelihood (⁠≥90 to<150 points)⁠, of high DFT failure likelihood (⁠≥150 points). In this issue of Europace, Francia et al.8 provide more insight into the impact of implant techniques in relation to the more anatomical evaluation on X-rays after an S-ICD implant procedure. They retrospectively differentiated a multicentre cohort of 213 patients based on a subcutaneous (SC) or intermuscular (IM) can position, and by S-ICD lead fixation only at the xiphoid (two-incision) or also at the lead tip (three-incision). In their cohort, an IM pocket was made in 174 patients (81.7%) while the two-incision approach was performed in 199 (93.4%) with different combinations. According to the PRAETORIAN score, the risk of conversion failure was classified as low in 198 patients (93.0%), intermediate in 13 (6.1%), and high in only 2 (0.9%), which mimics the prior findings in the AUMC and Investigational Device Exemption (IDE) cohorts. Patients undergoing the 2-incision and IM technique were more likely to have a low (<90) PRAETORIAN score than those undergoing the three-incision and SC technique (two-incision: 94.0% vs. three-incision: 78.6%; P = 0.004 and IM: 96.0% vs. SC: 79.5%; P = 0.001). The IM plus two-incision technique was associated with low shock impedance and low-risk PRAETORIAN score (hazard ratio 3.76; 95% confidence interval 1.01–14.02; P = 0.04). Of the 187 evaluable patients where DFT was tested, 179 (95.7%) were successfully converted at 65 J or less. Interestingly, the PRAETORIAN score did not predict shock failure at 65 J or less. What should we make of these findings? On the upside, it seems the investigators were able to mirror the distribution of PRAETORIAN scores of the AUMC6 and IDE2 cohorts. Also, by detailing implant characteristics, the investigators provide evidence to support the IM can position with a two-incision lead implant technique as enabling a higher chance of achieving the lower PRAETORIAN score. Such data are not available for comparison in the prior studies that form the basis of the PRAETORIAN score. On the other hand, is there more than one implant technique to achieve low PRAETORIAN scores? To avoid an anterior can position with sub-can fat, the operator simply needs to make an incision at or beyond the anterior mammary line, go down to the fascial muscle plane and prepare the pocket under the fat in a posterior direction. This should not necessarily require an IM position, although inherently low sub-can fat may always be achieved in that way. More importantly, to achieve a low amount of sub-coil fat, the operator simply needs to make the xyphoid incision deep enough to tunnel under the fat keeping the tip just over the sternal surface. This should have nothing to do with employing a two- or three-incision technique. It is therefore intriguing to understand why the two-incision technique would inherently lead to a lower PRAETORIAN score. Since the number of patients with a three-incision technique was only 6.6% one questions whether this provides enough statistical power to discern differences in the PRAETORIAN score or shock conversion efficacy. Indeed, in this cohort, the ability of the PRAETORIAN score to predict shock failure could not be established. Interestingly, in the UNTOUCHED registry,9 69% of 1116 patients had a two-incision technique, while 29% had a three-incision technique, and the number of incisions was also not predictive of DFT failure. The most important driver for DFT failure (both in the current study as well as in UNTOUCHED) was found to be higher BMI, which is more consistent with the amount of fat under lead and/or can than the number of incisions and does not allow for subtracting point for scores >90. Unfortunately, in the UNTOUCHED study the PRAETORIAN score was not part of the data collection, leaving the relation to incision technique obscure. All in all, this does not yet paint a clear picture on the prospective value of the PRAETORIAN score and importance of implant techniques. The development of the scoring system hinges on computer simulations and relatively small cohorts where most patients had a low PRAETORIAN score <90 (AUMC 97%, IDE 87%), and conversion failure was not that common (0.6% AUMC, 7.4% IDE).2,6 While the founder studies for the score suggested that a score <150 points would not require DFT, the data from Francia et al.8 do show conversion failures <150 points despite optimal implants. Patient characteristics such as left ventricular ejection fraction, aetiology of indication, or drug use may also be of importance, as for transvenous ICD. Since moving forward the knowledge leveraged from the PRAETORAN score should be transformed into better implant techniques, a prospective trial is needed to evaluate its predictive power. The PRAETORIAN DFT10 will enroll 965 patients that undergo a de novo S-ICD implantation and randomize them to either standard of care S-ICD implantation with DFT, or S-ICD implantation without DFT but with evaluation of the implant position using the PRAETORIAN score. Patients with a PRAETORIAN score ≥90 in the interventional arm of this study will undergo DFT according to the same DFT protocol as in the control arm. This will tell us more about the ability to perform an optimal implant and how this translates into successful DFT, aiming to avoid DFT altogether. Conflict of interest: L.B. reports being a consultant for Boston Scientific, fees go to the Cardiology Department. The opinions expressed in this article are not necessarily those of the Editors of Europace or of the European Society of Cardiology.