Successful defibrillation testing in patients undergoing elective subcutaneous implantable cardioverter-defibrillator generator replacement

Abstract

Aims

After implantation of a subcutaneous implantable cardioverter-defibrillator (S-ICD), a defibrillation test (DFT) is performed to ensure that the device can effectively detect and terminate the induced ventricular arrhythmia. Data on DFT efficacy at generator replacement are scarce with a limited number of patients and conflicting results. This study evaluates conversion efficacy during DFT at elective S-ICD generator replacement in a large cohort from our tertiary centre.

Methods and results

Retrospective data of patients who underwent an S-ICD generator replacement for battery depletion with subsequent DFT between February 2015 and June 2022 were collected. Defibrillation test data were collected from both implant and replacement procedures. PRAETORIAN scores at implant were calculated. Defibrillation test was defined unsuccessful when two conversions at 65 J failed. A total of 121 patients were included. The defibrillation test was successful in 95% after the first and 98% after two consecutive tests. This was comparable with success rates at implant, despite a significant rise in shock impedance (73 ± 23 vs. 83 ± 24 Ω, P < 0.001). Both patients with an unsuccessful DFT at 65 J successfully converted with 80 J.

Conclusion

This study shows a high DFT conversion rate at elective S-ICD generator replacement, which is comparable to conversion rates at implant, despite a rise in shock impedance. Evaluating device position before generator replacement may be recommended to optimize defibrillation success at generator replacement.

Graphical Abstract

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Introduction

Over the past decades, the use of implantable cardioverter-defibrillators (ICDs) has significantly improved the life expectancy of patients at risk for sudden cardiac death.^1,2 For the conventional transvenous device, the lead remains the Achilles heel, due to the risk of serious device-related complications.³ The subcutaneous implantable cardioverter-defibrillator (S-ICD) was developed to overcome these complications and is an effective therapy for the termination of ventricular arrhythmias.^4,5 A defibrillation test (DFT) is recommended at implant and/or generator replacement to determine whether the S-ICD is implanted correctly and can detect and terminate a ventricular arrhythmia effectively.⁶ Previous trials in patients with a transvenous ICD (TV-ICD) have provided evidence that performing a DFT does not reduce the incidence of failed first shock in spontaneous arrhythmias. As a consequence, routine DFT is no longer recommended after de novo implantation of a TV-ICD.^7,8 During S-ICD implantation, anatomical landmarks are used to establish correct positioning of the device. As the S-ICD is completely extravascular, positioning of the lead and generator is important for successful shock delivery. Hence, there is still a Class I recommendation for DFT after S-ICD implantation.⁶ However, defibrillation testing can cause uncommon but serious complications, with a complication rate of 0.2% reported by earlier studies.⁹ The most common complication is prolonged resuscitation and the most severe complication is DFT-related death. In addition, patients are at risk for DFT-related stroke and complications related to the required anaesthetics to perform a DFT. Finally, as a DFT is a probabilistic test, the value of performing one can be debated.^10,11

The PRAETORIAN score is a tool that is developed as an alternative for DFT to predict the risk of conversion failure in S-ICD recipients based on implant position of the device, using posteroanterior and lateral chest radiographs. This score is based on three implant characteristics that affect the defibrillation threshold. This includes the amount of fat between the coil and sternum and between the generator and the thoracic wall, which can both increase the impedance and the position of the generator in relation to the midline of the lateral side of the thoracic wall to confirm that the shock vector will reach the ventricles and cause an effective defibrillation. The score results in risk categories indicating a low (<90 points), intermediate (90–150 points), or high (≥150 points) risk of conversion failure. It is expected that patients with an intermediate risk have a first shock efficacy below 90% but are able to convert with multiple shocks. The conversion success rate in patients with a high risk of conversion failure is around 50%. A more detailed description of the score is published elsewhere.¹²

The ongoing PRAETORIAN-DFT trial aspires to investigate whether routine DFT after de novo S-ICD implantation can be replaced with the PRAETORIAN score.¹³ However, only limited studies are available on DFT after S-ICD generator replacement, and the available studies are performed with a limited number of patients and show conflicting results.^14,15 This retrospective data collection is an extension of our previous study and aims to analyse the conversion efficacy during defibrillation testing after elective S-ICD generator replacement.¹⁵

Methods

Study design

Data were retrospectively retrieved from electronic patient records. Data collection of these patients was approved by our local Medical Ethics Committee, and the need for informed consent was waived. This study was conducted in accordance with the Declaration of Helsinki.

Study subjects

Data from all patients undergoing elective S-ICD generator replacement between February 2015 and June 2022 in our tertiary centre, were collected. Patients undergoing their first generator replacement for battery depletion, with subsequent DFT were included. Patients <18 years old, patients in whom induction of ventricular fibrillation was not successful, in whom no second DFT was performed after conversion failure in the first DFT due to instability of the patient, and patients in whom the lead was simultaneously replaced or repositioned were excluded from this study. Participation in other S-ICD studies was no exclusion criterion.

Replacement procedure

Replacement procedures were performed by experienced S-ICD implanters, using local and general anaesthesia per physician discretion. Prior to the procedure, the chest radiograph that was made after the implant procedure was checked, after which the physician could decide on generator repositioning during replacement. Other reasons for device repositioning could be clinical events or patient’s request in case of discomfort caused by the generator. The incision made at the initial implant was used to open the pocket, after which the old generator was disconnected from the lead, and a new device was implanted. Any fibrous capsule surrounding the generator was left intact. The implanter solely used anatomical landmarks during replacement and no additional fluoroscopy was used to determine the generator position. After partial or complete closure of the pocket, defibrillation testing was performed. Ventricular fibrillation (VF) was induced by a single 50 Hz alternating current burst. Subsequently, the S-ICD delivered a shock at a maximum of 65 J in standard or reversed polarity, depending on the settings of the device during standard care. If the first shock failed to terminate the arrhythmia, external defibrillation was used and an additional DFT was performed, generally in the opposite polarity. After conversion failure in the first DFT at 65 J and a successful conversion in a second test, the test was considered successful due to the probabilistic nature of the test. A successful defibrillation with any energy lower than 65 J was considered a successful first defibrillation at 65 J. In case of ≥2 failed conversions at 65 J, the test was declared unsuccessful. Actions upon DFT failure were per physician discretion.

PRAETORIAN score

For this analysis, PRAETORIAN scores were calculated using chest radiographs at implant. Radiographs were rated by two independent physicians blinded to patient characteristics—except body mass index (BMI)—and DFT results. In case of disagreement in risk category between the two raters, a third physician rated the chest X-ray to reach agreement.

Statistical analysis

Continuous data are presented as means with standard deviation or medians with interquartile range (IQR). Dichotomous data are presented as proportions and percentages. Differences in characteristics between implant and replacement were calculated using a paired samples t-test or Wilcoxon signed-rank test. Differences in characteristics between patients with a successful and unsuccessful first conversion were calculated with a Mann–Whitney U test and presented as median with range, or calculated using Fisher’s exact test for categorical data, due to the small sample size in one group. A two-sided P-value <0.05 was considered statistically significant. Statistics were performed using IBM SPSS Statistics 28.

Results

Patient characteristics

In total, 121 patients were included with a mean age of 50 ± 16 years. The majority of the population was male (65%, sex assigned at birth), had a primary prevention indication for ICD implantation (61%), and were implanted using the two-incision technique (85%). Forty per cent of patients had a genetic arrhythmia syndrome as underlying diagnosis. The median left ventricular ejection fraction at implant was 49% (IQR 34–57). The mean time from implant to replacement was 5.4 ± 0.9 years. Mean BMI at time of generator replacement was 26.1 ± 4.4 kg/m² which was significantly higher compared with implant (25.5 ± 4.5 kg/m², P = 0.006). An overview of the patient characteristics is shown in Table 1.

Defibrillation test at replacement

The first DFT after replacement resulted in successful conversion in 115/121 (95%) patients. After a second test was performed in patients with a failed conversion in the first test, successful conversion was reached in 119/121 (98%) patients (Figure 1). At implant, a DFT was performed in 114/121 patients (94%), with a conversion rate of 95% after one DFT and 98% after two DFTs. Mean impedance at replacement was 83 ± 24 Ω, which was significantly higher compared with implant (73 ± 23 Ω, P < 0.001). Differences between implant and replacement are shown in Table 2. In 17 patients (14%), it was reported that the generator was repositioned per physician discretion during replacement, before the DFT (Table 3).

PRAETORIAN scores

Eligible chest radiographs for calculation of the PRAETORIAN score were available in 118/121 cases (98%). A total 101/118 (86%) had a score <90, 10/118 (9%) had a score of 90–150, and 7/118 (6%) had a score of ≥150 at implant. Two out of these seven patients (29%) with a PRAETORIAN score ≥150 had at least one unsuccessful conversion at replacement.

Clinical characteristics associated with an unsuccessful first conversion

Body mass index was significantly higher in patients with an unsuccessful first conversion compared with patients with a successful first conversion [28.6 kg/m² (range 25.3–40.6) vs. 25.9 kg/m² (range 16.9–43.6) P = 0.031]. Similarly, impedance was significantly higher in patients with an unsuccessful first conversion [107 (range 88–131) vs. 77 (range 43–182 Ω) P = 0.002]. There was a 29% correlation between impedance and BMI (R² statistic of 0.29, P < 0.001). Significantly more patients with an unsuccessful first conversion had a PRAETORIAN score ≥90 (3/6 vs. 14/112, P = 0.038) (Table 4).

Action after two unsuccessful conversions

This dataset included two patients with two consecutive unsuccessful conversions at 65 J. Both patients were male, had a BMI >30 and an impedance ≥94 Ω. Patient 1 had a low PRAETORIAN score, and Patient 2 had a high score. After failure of both 65 J tests, the patients underwent two consecutive DFTs at 80 J (standard and reversed polarity). In Patient 1, these were both successful, and this was accepted. Patient 2 had a successful conversion during the first DFT at 80 J, but unsuccessful conversion in the second DFT, where after the test was repeated. This final test resulted in a successful conversion at 80 J. In this case, the treating physician recommended to reposition the lead and/or generator during a future procedure.

Discussion

This is the largest cohort of S-ICD patients who underwent elective generator replacement that is described in current literature. The data presented in this study show that the vast majority of patients undergoing elective S-ICD generator replacement have a successful DFT. Despite the significant rise in impedance and BMI compared with implant, the induced ventricular arrhythmia was terminated in 98% of patients.

It has been reported that the risk for defibrillation failure increases with an increase in BMI and that within BMI categories, patients with an unsuccessful DFT have a higher impedance.¹⁶ In our population, patients with one unsuccessful conversion had both significantly higher BMI and higher impedance. Fat between the generator and the thoracic wall, or the lead and sternum, causes a higher resistance and therefore a higher impedance. The risk of sub-generator or sub-coil fat is higher in obese patients. We found a 29% correlation between BMI and impedance, which points out that only a part of the impedance value is explained by BMI.

Correct implantation of the S-ICD with the lead on the sternum and the generator intermuscular without sub-generator or sub-coil fat in overweight and obese patients may result in a resistance and defibrillation success that is equal to patients with a healthy BMI. In our study, BMI of patients with a successful first conversion ranges from 16.9 to 43.6. This confirms that with correct positioning, the S-ICD can also defibrillate successfully in overweight and obese patients.

It was hypothesized that a rise in impedance is the result of device encapsulation with fibrotic tissue and that this may contribute to chronic conversion failure. Our study showed a significant rise in impedance between implant and replacement, whereas DFT efficacy remained comparable. Therefore, although encapsulation may have increased impedance, this did not affect the DFT success during the first elective generator replacement in our cohort.

In order to avoid VF induction for defibrillation testing, physicians sometimes use shock impedance as an alternative to estimate defibrillation success. However, our data indicate that impedance values should be used carefully as it is difficult to determine a proper cut-off at which defibrillation success is likely. Impedance in patients with a successful first conversion ranged from 43 to 182 Ω. This shows that even with high impedance values, the S-ICD is able to convert induced arrhythmias, whereas patients with a fairly normal impedance did not have a successful first conversion. In addition, it is important to realize that the shock efficacy is dependent on the amount of cardiac tissue that is defibrillated. The impedance can be low, but if the heart is not positioned between the coil and generator, for instance in case of anterior positioning of the can, this can still result in an unsuccessful shock.¹⁷ Furthermore, the impedance may be impacted by transient factors such as air entrapment in the pocket.¹⁸ Therefore, whereas impedance can give additional information on the defibrillation success, it remains important to first consider the position of the device.

In our cohort, 98% of the patients had successful DFT after generator replacement, which is in contrast with an earlier observational study by Rudic et al.,¹⁴ where an unsuccessful DFT after generator replacement was reported in 5 of 25 patients. Defibrillation success is dependent on device positioning, as proper positioning decreases the energy needed to defibrillate.^19,20 Our data showed that patients with an unsuccessful first conversion significantly more often had a PRAETORIAN score ≥90 (P = 0.038). As Rudic et al. reported a PRAETORIAN score with an IQR of 30–90, at least 25% of their population had a PRAETORIAN score ≥90, making their population more prone to an unsuccessful DFT. Whereas in that study no significant difference in PRAETORIAN scores between patients with a successful and unsuccessful DFT were found, this is likely due to the fact that a statistical test for continuous data was used, whereas the PRAETORIAN score is a categorical variable.

In this study, one patient with a low PRAETORIAN score had two unsuccessful conversions at 65 J. This patient had a BMI of 40.6. Epicardial adipose tissue (EAT) may be present in this patient. Epicardial adipose tissue is fat tissue between the myocardium and pericardium and the deposition of EAT is potentially associated with obesity.^21,22 The presence of EAT may reduce the conduction of shock current from the ICD through the heart. Future studies on the presence of EAT in patients with defibrillation failure are needed to support this hypothesis.

Repositioning before defibrillation test

A replacement procedure is a natural moment to correct suboptimal placement of devices. In our centre, the implanting physician determines whether optimization of the generator location is possible to improve shock efficacy, based on the chest radiograph post-implant. In our dataset, for 17 patients it was reported that the generator was repositioned during the replacement procedure, but unreported minor changes cannot be excluded. As generator positioning is only part of the PRAETORIAN score, a suboptimal placement of the generator does not always lead to a score ≥90 but can still be worthwhile to be corrected. As repositioning was done as part of the standard care prior to DFT, it is unknown what the conversion efficacy would have been without repositioning. However, it can be concluded that with the indicated strategy, defibrillation success after generator replacements is similar to defibrillation success after implant. We therefore recommend to always evaluate the implant position before generator replacement, to achieve optimal conversion rates. Additionally, as an intermuscular pocket is associated with a lower PRAETORIAN score, we suggest to consider creating a new intermuscular pocket during generator replacement, in case of an originally subcutaneous pocket.²³

Study limitations

The single-centre and retrospective design of this study, executed in a highly experienced centre comes with inherent study limitations. The vast majority of our population had a low PRAETORIAN score. Less experienced implanters might have a higher rate of intermediate or high PRAETORIAN scores, which could result in a higher overall chance of an unsuccessful DFT. As it is not standard practice in our hospital to perform a chest radiograph after generator replacements, only PRAETORIAN scores after implant were available. Finally, as the proportion of patients with an unsuccessful DFT was low, risk factors for an unsuccessful DFT are hard to discriminate from our results, although several trends for an unsuccessful first conversion have been observed such as high BMI, high impedance, and high PRAETORIAN score.

Conclusions

This study shows a high rate of successful defibrillation testing at elective S-ICD generator replacement, which is comparable to implant, despite an increase in shock impedance. It may be recommended to evaluate the implant position before generator replacement, to achieve optimal conversion rates.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethics approval statement and informed consent

The need for informed consent was waived by our local medical ethics committee.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

References

Author notes