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How many patients fulfil the surface electrocardiogram criteria for subcutaneous implantable cardioverter-defibrillator implantation?

Daniel A. Randles, Nathaniel M. Hawkins, Matthew Shaw, Ashish Y. Patwala, Stephen J. Pettit, David J. Wright
DOI: http://dx.doi.org/10.1093/europace/eut370 1015-1021 First published online: 18 December 2013

Abstract

Aims To determine the number of patients with a primary or secondary prevention implantable cardioverter-defibrillator (ICD) indication who are eligible for subcutaneous ICD (S-ICD) implantation according to the S-ICD manufacturer's surface electrocardiogram (ECG) screening template.

Methods and results One hundred and ninety-six ICD patients with a non-paced ventricle were assessed using erect and supine ECG limb lead recordings to simulate the three S-ICD sensing vectors. Each ECG lead was scrutinized by two independent observers. Subcutaneous ICD eligibility required two or more leads to satisfy the S-ICD screening template in both erect and supine positions. Overall, 85.2% of patients [95% confidence interval (CI): 80.2–90.2%] fulfilled surface ECG screening criteria. The proportion of patients with 3, 2, 1, and 0 qualifying leads were 37.2% (95% CI: 30.4–44.0%), 48.0% (95% CI: 41.0–55.0%), 11.2% (95% CI: 6.8–15.6%), and 3.6% (95% CI: 1.0–6.2%). The S-ICD screening template was satisfied more often by Lead III (primary vector, 83.7%, 95% CI: 78.5–88.9%) and Lead II (secondary vector, 82.7%, 95% CI: 77.4–88.0%) compared with Lead I (alternate vector, 52.6%, 95% CI: 45.6–59.6%). A prolonged QRS duration was the only baseline characteristic independently associated with ineligibility for S-ICD implantation. There was 92.9% agreement between the two independent observers in assessment of eligibility using the S-ICD screening template.

Conclusion About 85.2% of patients with an indication for a primary or secondary prevention ICD have a surface ECG that is suitable for S-ICD implantation when assessed with an S-ICD screening template. There is minor inter-observer variation in assessment of eligibility using the S-ICD screening template.

  • Subcutaneous implantable cardioverter-defibrillator
  • Electrocardiogram

What's new?

  • About 85.2% of patients with a primary or secondary prevention implantable cardioverter-defibrillator (ICD) were eligible for subcutaneous ICD (S-ICD) implantation according to the manufacturer's surface electrocardiogram (ECG) screening template.

  • The surface ECG screening template was simple to use and there was 92.9% agreement between observers with regard to S-ICD eligibility.

  • Prolonged QRS duration was the only baseline characteristic independently associated with S-ICD ineligibility according to the surface ECG screening template.

Introduction

Implantable cardioverter-defibrillators (ICDs) reduce the risk of sudden death in individuals at high risk of ventricular arrhythmias.1,2 Conventional ICD systems use a transvenous lead for sensing of ventricular arrhythmias and delivery of ICD therapy. Transvenous ICD leads are associated with complications such as lead fracture, infection, and venous obstruction.35 Removal of failed or infected transvenous leads is associated with considerable morbidity and mortality.6 An entirely subcutaneous ICD (S-ICD, Cameron Health) was recently approved by the US Food and Drug Administration agency and is now commercially available.7 The S-ICD comprises the SQ-RX pulse generator, positioned over the left sixth rib between the mid and anterior axillary lines, connected to the Q-TRAK subcutaneous lead which consists of a left parasternal coil electrode flanked by distal and proximal sensing electrodes at the level of the manubriosternal junction and xiphoid process.

Subcutaneous ICD utilize one of three subcutaneous electrograms recorded between two sensing electrodes and the pulse generator for ventricular sensing (Figure 1). In certain patients, subcutaneous electrograms are inadequate for sensing. During early experience, S-ICD implantation was occasionally aborted mid-procedure due to inadequate R-wave amplitude for sensing. In real-world patients with S-ICD systems, T-wave oversensing is a frequent cause of inappropriate shocks.810 To avoid S-ICD implantation in patients who are likely to experience sensing problems, the manufacturer has developed a pre-implant surface electrocardiogram (ECG) screening template to identify patients that are likely to have unsuitable subcutaneous ECG. Surface and subcutaneous ECG are highly correlated with similar R-wave amplitudes and signal-to-noise ratios.1113 However, surface ECG exhibit postural and beat-to-beat variations in depolarization and repolarization amplitudes relating to changes in cardiac axis and autonomic tone.13,14 As such, erect and supine surface ECG must be assessed with the screening template. It is uncertain how many patients with an ICD indication are eligible for S-ICD according to this surface ECG screening template.

Figure 1

Subcutaneous ICD components, sensing vectors, and surface ECG lead positions. RA, right arm; LA, left arm; LL, left leg; N, neutral.

In this study, we assessed an unselected series of patients with a conventional ICD to determine the number of patients that fulfil surface ECG screening criteria for S-ICD implantation, to compare the three available subcutaneous electrogram vectors, and to examine predictors of eligibility for S-ICD use.

Methods

Study population

Liverpool Heart and Chest Hospital is a tertiary centre serving a population of 2.2 million people. All patients with a single-chamber or dual-chamber transvenous ICD that attended routine follow-up at ICD clinic between February 2012 and October 2012 were screened for inclusion. An ECG was performed at ICD clinic. We excluded patients with an S-ICD, patients with a paced QRS complex, and patients who were unable to stand for the time required to record an erect ECG. Use of routinely collected anonymous data was approved by the local ethics committee and deemed not to require formal patient consent.

Electrocardiogram collection

Additional ECG recordings were obtained using limb lead electrodes to simulate the S-ICD sensing vectors (Figure 1). The left arm (LA) and right arm electrodes were positioned 1 cm lateral to the left sternal border, and, respectively, 1 cm above the xiphoid process and no more than 14 cm superior to the LA electrode. The left leg electrode was positioned in the fifth intercostal space on the mid-axillary line, and the neutral electrode on the lower torso to ensure other lead positions were undisturbed. The resulting augmented leads (Leads I, II, and III) correspond to the alternate, secondary, and primary vectors of the S-ICD, respectively. Two page rhythm strips were recorded using these three leads, at two gains (5 and 10 mm/mV) both erect and supine with a further 20 mm/mV recording made in each posture if the smallest R-wave amplitude seen in any channel of the supine 10 mm/mV recording did not reach 0.5 mV.

Electrocardiogram analysis

Use of the surface ECG screening template is described in Figure 2. The template consists of six coloured shapes that correspond to the six S-ICD automatic gain settings. Each shape has two constant threshold periods and a subsequent exponential decay curve. The screening template was placed over a single QRS-T complex. The horizontal line on the template was aligned with the isoelectric line and the left edge of the template was aligned with the onset of the QRS complex. To satisfy the template, no part of the QRS-T complex may lie outside coloured shape and the maximum amplitude of the QRS complex must cross the dashed line. To be eligible for S-ICD implantation, at least two ECG leads (I, II, or III) must satisfy the template (at any gain) in both erect and supine postures. All ECG were analysed by two independent blinded observers and their assessment of S-ICD eligibility was compared. When there was disagreement, ECG for that patient were adjudicated by a third independent observer. All three observers received brief training before using the surface ECG screening template.

Figure 2

Flowchart showing the surface ECG screening template and process used to determine eligibility for S-ICD implantation.

Sample size estimation

The sample size was calculated using the standard confidence interval (CI) approach to a proportion. Prior observations suggested over 90% of patients matched the template.15 We conservatively estimated the true population proportion to be 85%. A sample size of 196 patients is required to provide 95% confidence that the true population proportion lies within the range 85 ± 5% (α = 0.05).

Statistical analysis

The proportion of patients fulfilling ECG criteria are presented as percentages with 95%CI.16 Baseline characteristics are reported as means with standard deviations for continuous variables and by frequencies and percents for categorical variables. The data were assessed for normality using the Shapiro–Wilk's test. The characteristics of patients fulfilling or failing the ECG criteria were examined. Means were compared using the Wilcoxon rank sum test or Student's t-test (with Levene's test for equality of variances) depending on the distribution of the data. Proportions were compared using the χ2 test or Fisher's exact test when the observed frequencies were <5. Logistic regression was used to examine the effect of baseline characteristics on S-ICD eligibility according to the surface ECG screening template.17 A two-tailed P value of <0.05 was considered statistically significant. Statistical analyses were performed using SAS for Windows Version 9.2 (SAS Institute).

Results

Study population

A total of 196 patients with a single-chamber or dual-chamber satisfied the inclusion and exclusion criteria. The population was predominantly male (80.1%) with a mean age of 66 years. The indication for ICD implantation was primary prevention in 40.3% and secondary prevention in 59.7% of patients. Baseline characteristics are presented in Table 1.

View this table:
Table 1

Baseline characteristics stratified by eligibility

CharacteristicAll patients
(n = 196)
Eligible
(n = 167)
Ineligible
(n = 29)
P value
Male (%)157 (80.1)130 (77.8)27 (93.1)0.06
Age (years)66 ± 1265 ± 1267 ± 130.61
Height (cm)172.1 ± 8.4172 ± 8.5172.8 ± 7.60.73
Weight (kg)85.9 ± 1786.4 ± 17.583.3 ± 14.10.47
Body mass index (kg/m2)29 ± 5.329.2 ± 5.427.9 ± 4.60.22
Indication
 Primary79 (40.3)68 (40.7)11 (37.9)0.78
 Secondary117 (59.7)99 (59.3)18 (62.1)
 Ischaemic heart disease115 (58.7)95 (56.9)20 (69.0)0.22
 Arrhythmogenic right ventricular cardiomyopathy12 (6.1)12 (7.2)0 (0.0)0.22
 Long QT syndrome4 (2.0)3 (1.8)1 (3.4)0.48
 Dilated cardiomyopathy25 (12.8)21 (12.6)4 (13.8)0.77
 Hypertrophic cardiomyopathy25 (12.8)22 (13.2)3 (10.3)>0.99
 Brugada syndrome3 (1.5)3 (1.8)0 (0.0)>0.99
 Tetralogy of Fallot3 (1.5)1 (0.6)2 (6.9)0.06
Medical history and previous intervention
 Left ventricular systolic dysfunction
  None39 (19.9)32 (19.2)7 (24.1)0.54
  Mild27 (13.8)21 (12.6)6 (20.7)0.25
  Moderate40 (20.4)36 (21.6)4 (13.8)0.34
  Severe90 (45.6)78 (46.7)12 (41.8)0.60
 Myocardial infarction108 (55.1)93 (55.7)15 (51.7)0.69
 Atrial fibrillation37 (18.9)33 (19.8)4 (13.8)0.45
 Diabetes mellitus34 (17.3)33 (19.8)1 (3.4)0.03
 COPD23 (11.7)17 (10.2)6 (20.7)0.12
 External defibrillation112 (57.1)95 (56.9)17 (58.6)0.86
 Ventricular tachycardia ablation9 (4.6)8 (4.8)1 (3.4)>0.99
 Device/lead extraction17 (8.7)15 (9.0)2 (6.9)>0.99
 Device therapy—ATP48 (24.4)41 (24.6)7 (24.1)>0.99
 Device therapy—shock51 (26.0)44 (26.3)7 (24.1)0.80
Medication
 Beta-blockers164 (83.7)139 (83.2)25 (86.2)>0.99
 Amiodarone43 (22.0)37 (22.2)6 (20.7)0.86
 Flecainide1 (0.5)1 (0.6)0 (0)>0.99
Device location and type
 Left183 (93.3)157 (94.0)26 (89.7)0.42
 Right13 (6.6)10 (6.0)3 (10.3)
 Single chamber98 (50.0)81 (48.5)17 (58.6)0.31
 Dual chamber98 (50.0)86 (51.5)12 (41.4)
Baseline ECG characteristics
 QRS duration (ms)114.9 ± 26.8112.3 ± 24.6130 ± 340.008
  <120 ms123 (62.8)112 (67.1)11 (37.9)0.003
  120–149 ms47 (24.0)38 (22.8)9 (31.0)0.34
  >149 ms26 (13.3)17 (10.2)9 (31.0)0.005
 QRS morphology
  Normal132 (67.3)119 (71.3)13 (44.8)0.005
  Left bundle branch block27 (13.8)20 (10.2)7 (24.1)0.09
  Right bundle branch block20 (10.2)14 (8.4)6 (20.7)0.09
  Intra-ventricular conduction delay17 (8.7)14 (8.4)3 (10.3)0.72
 Corrected QT interval (ms)442.1 ± 33.8441 ± 32.1447 ± 430.63
  • Continuous variables given as mean ± standard deviation; categorical variables given as number (percentage).

  • COPD, chronic obstructive pulmonary disease; ATP, anti-tachycardia pacing.

How many patients are eligible for subcutaneous implantable cardioverter-defibrillator according to the surface electrocardiogram screening template?

The number of patients with zero, one, two, or three ECG leads that satisfied the surface ECG screening template is summarized in Figure 3. For a patient to be eligible for an S-ICD, at least two ECG leads had to satisfy the screening template. Overall, 167 (85.2%, 95% CI: 80.2–90.2%) patients were eligible for S-ICD implantation. The eligible group comprised 73 (37.2%, 95% CI: 30.4–44.0%) patients with three qualifying ECG leads and 94 (48.0%, 95% CI: 41.0–55.0%) patients with two qualifying ECG leads. Conversely, 29 (14.8%, 95% CI: 9.8–19.8%) patients were ineligible for S-ICD implantation. The ineligible group comprised 22 (11.2%, 95% CI: 6.8–15.6%) patients with only one qualifying ECG lead and 7 (3.6%, 95% CI: 1.0–6.2%) patients with no qualifying ECG leads.

Figure 3

Proportion of patients with zero, one, two, and three ECG leads that satisfy the surface ECG screening template. The errors bars represent 95% CIs.

How frequently does each surface electrocardiogram lead satisfy the screening template?

The number of times that each lead satisfied the surface ECG screening template is described in Table 2. In order for a lead to qualify, that lead had to satisfy the screening template (at any gain) in both the erect and supine position. Lead III (primary vector) had an overall qualification rate of 83.7% (95% CI: 78.5–88.9%). Lead II (secondary vector) had an overall qualification rate of 82.7% (95% CI: 77.4–88.0%). Lead I (alternate vector) had an overall qualification rate of 52.6% (95% CI: 45.6–59.6%). The overall qualification rate for Leads III and II were significantly greater than Lead I (P < 0.05).

View this table:
Table 2

Proportion of patients complying with template for each vector

Lead
S-ICD vector
Lead I
S-ICD ‘Alternate’
Distal–proximal
Lead II
S-ICD ‘Secondary’
Distal–can
Lead III
S-ICD ‘Primary’
Proximal–can
Supine
 5 mm/mV58 (29.6%)2,3121 (61.7%)1136 (69.4%)1
 10 mm/mV108 (55.4%)111 (56.6%)89 (45.4%)
 20 mm/mVa29 (14.7%)323 (11.7%)14 (7.1%)1
Erect
 5 mm/mV38 (19.4%)2,3118 (60.2%)1132 (67.3%)1
 10 mm/mV93 (47.4%)110 (56.1%)110 (56.1%)
 20 mm/mVa30 (15.3%)22 (11.2%)19 (9.7%)
Overall
 Lead eligibility103 (52.6%)2,3162 (82.7%)1164 (83.7%)1
  • 1P < 0.05 vs. Lead I, 2P < 0.05 vs. Lead II, 3P < 0.05 vs. Lead III, using Bonferroni's correction.

  • aOnly 69 of the 196 patients were screened at the 20 mm/mV gain setting as described in the Methods section. These percentages were still calculated from the total sample size as enough information was derived from the compulsory gain settings in the remaining 127 patients to formulate an outcome and allow comparison.

There were minor differences in lead qualification rate between the erect and supine positions. For Lead I (alternate vector) at 5 mm/mV, the qualification rate was higher in the supine position, compared with the erect position (P = 0.02). For Lead III (primary vector) at 10 mm/mV, the qualification rate was higher in the erect position, compared with the supine position (P = 0.01). For all other leads at all other gains, there was no significant difference in qualification rate between erect and supine postures. The colour template satisfied by each lead is described in the Supplementary material online, Table.

Inter-observer agreement and adjudication

Agreement between the two independent blinded observers is summarized in Table 3. Both observers agreed on the overall eligibility for S-ICD implantation in 182 (92.9%) of patients and disagreed on 14 (7.1%) of patients. Where the two observers disagreed, ECG were adjudicated by a third independent observer. After adjudication, 12 patients were deemed eligible and 2 patients were deemed ineligible. In addition to the overall eligibility for S-ICD implantation, the two observers disagreed about the number of qualifying leads in 32 patients, but these disagreements did not alter the overall eligibility. Finally, the two observers frequently disagreed with regard to the colour template that was satisfied by a lead, but once more, these disagreements did not alter the overall eligibility.

View this table:
Table 3

Inter-observer agreement for overall S-ICD eligibility

 Observer 2
EligibleIneligible
Observer 1Eligible1559
Ineligible527

Baseline characteristics associated with the eligibility for subcutaneous implantable cardioverter-defibrillator implantation

All baseline characteristics, stratified by S-ICD eligibility, are described in Table 1. There were no differences in demographics, ICD indication, underlying aetiology, and co-morbidities between patients who were eligible for S-ICD implantation and patients who were ineligible for S-ICD implantation. However, patients who were ineligible for S-ICD implantation had a significantly longer QRS duration compared with patients who were eligible for S-ICD implantation (P = 0.008). This difference was particularly marked for those with severe QRS prolongation (>149 ms), of whom only 65% were eligible for S-ICD implantation. Correspondingly, a greater proportion of patients who were eligible for S-ICD implantation had normal QRS morphology, compared with those patients who were ineligible for S-ICD implantation (P = 0.005).

Predictors of eligibility

All baseline characteristics were assessed in a multivariable logistic regression model to identify independent predictors of eligibility. The only baseline characteristic that was associated with eligibility for S-ICD implantation was QRS duration. When QRS duration was analysed as a continuous variable, the odds ratio was 0.977 (95% CI: 0.964–0.991) and the area under the receiver-operating curve (ROC) was 0.66. When QRS duration was analysed as a categorical variable (>149 ms), the odds ratio was 0.25 (95% CI: 0.1–0.63) and the area under the ROC was 0.61.

Discussion

This is the first study to describe the proportion of patients with an ICD that would be eligible for S-ICD implantation according to surface ECG screening criteria. In a series of unselected patients with an indication for a primary or secondary prevention ICD, 85.2% (95% CI: 80.2–90.2%) of patients had a surface ECG that satisfied the pre-implantation screening template in at least two leads (vectors) and these patients would therefore be eligible for S-ICD implantation. Our results are similar to unpublished observations of a single German centre, who screened 58 patients using an identical screening tool, and reported that 88% of patients had a satisfactory surface ECG for S-ICD implantation.15

We found that the primary vector (Lead III) and the secondary vector (Lead II) satisfied the surface ECG screening template more frequently than the alternate vector (Lead I). The S-ICD automatically selects the optimal sensing vector during implantation, although the operator may select a different vector if desired.18 Previous studies have not described which sensing vector is selected by the S-ICD.8,9,1921 In theory, the primary or secondary sensing vectors are most attractive as they encompass the heart within the sensing field between the sensing electrode on the lead and the generator. The sensing field of the alternate vector is between two sensing electrodes on the parasternal portion of the QTRAK subcutaneous lead. Interestingly, there is a case report of a patient in which it was desirable to select the alternate vector and exclude the generator from the sensing field when it had been positioned more inferiorly than usually recommended.22

Previous studies have reported significant differences in R-wave amplitude between erect and supine postures.14 There is consequent concern that postural changes may result in oversensing or undersensing by S-ICD systems. In the present study, we did not see any significant difference between erect and supine ECG in terms of the overall eligibility according to the surface ECG screening template. The only difference in lead qualification rates between erect and supine posture was for the alternate vector (Lead I) at the 5 mm/mV gain setting and primary vector (Lead III) at the 10 mm/mV gain setting. There was no difference at higher gain settings for either lead. As such, it would appear that postural changes in R-wave or T-wave amplitude that might have an adverse effect on S-ICD sensing are likely to be accommodated by the ability of the S-ICD to automatically adjust gain.

Use of the pre-implantation screening template was straightforward with minimal requirement for training. All three observers were able to use the template on an independent basis after 5min of tuition. We demonstrated 92.9% agreement between two independent observers when using the surface ECG screening template to determine the overall eligibility for S-ICD implantation. This observation suggests that use of the surface ECG screening template is relatively reproducible and should be useful in daily clinical practice. During subsequent discussion between the three observers, it appeared that most discrepancies in lead qualification according to the screening template were the result of minor beat-to-beat variation in the isoelectric baseline and R-wave amplitude.

Finally, our study has demonstrated that eligibility or ineligibility for S-ICD implantation according to the surface ECG screening template was not associated with age, gender, height, weight, primary or secondary prevention indication, or underlying aetiology of heart disease. The only baseline characteristic associated with ineligibility for S-ICD implantation was an abnormal QRS complex on the baseline ECG, such as left or right bundle branch block. Although S-ICD ineligibility was more frequent in patients with severe prolongation of the QRS complex (>149 ms), 65% of these patients remained eligible for S-ICD implantation and an area under the ROC of 0.61 implies that QRS prolongation is not a useful predictor of eligibility in clinical practice. In addition, it is uncertain whether QRS abnormalities limit real-world use of S-ICD systems. Irrespective of surface ECG eligibility, severe QRS prolongation would frequently lead to selection of a transvenous device rather than an S-ICD, either for delivery of cardiac resynchronization therapy or bradycardia pacing.

Limitations

First, the external validity of our study is limited to the included patient group. Due to the organization of cardiac services in our region, we did not include any patients aged <19 years and only 1.5% of patients included had adult congenital heart disease. These patient groups are important potential S-ICD recipients. Secondly, our study only determined S-ICD eligibility according to the surface ECG screening template. We did not examine other reasons why a patient might be ineligible for an S-ICD. In a Dutch study of 463 patients with a transvenous ICD, 45% of patients would have become ‘ineligible’ for S-ICD over 5 years of follow-up because of an indication for atrial or ventricular pacing, delivery of successful anti-tachycardia pacing without a subsequent shock, or upgrade to cardiac resynchronization therapy.23 Finally, an even larger sample size might have provided greater insight into the influence of other baseline characteristics on S-ICD eligibility.

Conclusion

About 85.2% of patients with an indication for a primary or secondary prevention ICD had a surface ECG that was suitable for S-ICD implantation when assessed with a surface ECG screening template. The screening template was simple to use, with 92.9% inter-observer agreement. No baseline characteristics other than QRS duration predicted eligibility, suggesting that the S-ICD may have broad clinical applicability.

Supplementary material

Supplementary material is available at Europace online.

Conflict of interest: none declared.

References

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