from january 2012 until may 2015 , other than the period from march to october 2013 ( because of sicd unavailability in the united states ) , hcm patients presenting to our institution , identified at risk for scd based on the 2011 american college of cardiology foundation / american heart association guidelines,4 and without a pacing indication were considered for sicd implantation .
those patients were screened for potential twave oversensing utilizing a manufacturerprovided tool that analyzes surface ecg recordings taken during lying , standing , and immediately after running in place .
surface ecg recordings are obtained from 3 vectors that mimic the sensing vectors of the sicd .
the  primary  sensing vector is between the electrode at the xyphoid position and the midaxillary sicd generator ; the  secondary  vector is between the superior parasternal electrode and the generator and the  alternate  vector is between the 2 parasternal electrodes .
screening was not performed in any other alternative electrode position other than the standard 3 configurations available in the sicd .
screening was deemed successful if a single ecg vector passed while lying , standing and after exercise .
patients underwent standard informed consent including for step down dft testing , which is routine in our laboratory for hcm patients . at implant ,
the sicd assesses the vector for sensing , and the vector with the greatest distinction between the qrs and the twave is chosen .
if vf was successfully terminated , vf induction was performed again ( after 5  minutes ) and a 50  j shock was delivered by the sicd . if successful , vf induction was repeated again after 5  minutes and treated with a 35  j shock from the sicd .
if an sicd shock failed , the patient was externally defibrillated and no further dft testing was performed .
approval for this retrospective analysis was obtained from the institutional review board of tufts medical center ( boston , ma ) .
a comparison was made in baseline factors between the group of patients who were successfully screened for the sicd and those who failed screening .
age , body mass index ( bmi ) , left ventricular ejection fraction ( lvef ) , and maximal wall thickness were reported as median ( interquartile range ; iqr ) and compared using the wilcoxon ranksum test .
the presence of a bundle branch block was compared using the fisher 's exact test .
a similar comparison of dft success or failure at 50  j was made , and baseline data are reported as median ( iqr ) and compared using the wilcoxon rank sum test .
twentyseven hcm patients at risk for scd were considered potential candidates for the sicd . after screening for oversensing ,
23 patients ( 85% ) remained eligible . in one of the patients who failed screening , the potential for twave oversensing was only apparent after exercise in the only sensing vector that passed screening under resting conditions ( figure  2 ) .
patients who failed screening were more likely to have a bundle branch block on ecg ( 4 of 4 vs 4 of 23 ; p=0.004 ) and had a trend toward a higher bmi ( 33 [ 27.636.9 ] vs 27.4 [ 20.837.3 ] ; p=0.07 ; table ) .
age , lvef , and maximum wall thickness were not associated with potential for oversensing .
of 23 patients who screened successfully , 7 did not proceed with sicd implantation either because of insurance denial ( 2 patients ) or patient decision to have a transvenous device ( 3 patients ) .
, 42 hcm patients underwent implantation of a transvenous icd , none of which were screened and were thus not included in the current study .
twentyfour patients had a pacing indication , 11 of which had complete heart block after invasive septal reduction therapy .
in addition , 10 patients without a pacing indication were not screened and underwent transvenous icd implant during a time period of 8  months when the sicd was unavailable . the perceived need for antitachycardia pacing
. single lead iii electrocardiogram ( ecg ) preexercise in the upright position ( top panel ) and postexercise ( lower panel ) showing change in twave amplitude .
failure of screening is indicated by a portion of the twave outside the shaded area on the superimposed screening tool .
comparison of baseline characteristics of patients who passed and failed screening for sicd bmi indicates body mass index ; iqr , interquartile range ; lbbb , left bundle branch block ; lvef , left ventricular ejection fraction ; rbbb , right bundle branch block . among the 16 patients who had an sicd implanted ,
the indication for sicd implant was secondary prevention in 3 patients and primary prevention , based on the presence of 1 of the conventional risk factors , in 13 patients .
mean maximal left ventricular ( lv ) wall thickness was 18.45.4  mm ( range 1130 ) and none of the patients had left ventricular lv outflow tract obstruction > 30  mm  hg under basal conditions , including 2 patients who had previous successful invasive septal reduction therapy .
one implanted patient did not undergo dft testing because of the presence of a left atrial thrombus . in the others , vf was terminated by a 65j shock from the sicd in all patients and a 50j shock was successful in 12 of 15 .
a 35j shock terminated vf in 10 of 12 patients ( figure  1 ) .
vf was detected appropriately by the sicd after all inductions , and in no instance was there significantly delay or lack of detection necessitating external defibrillation .
sicd defibrillation failure at 50  j was associated with a higher bmi ( 26.55 [ 21.5033.2 ] vs 33.7 [ 32.2037.30 ] ; p=0.025 ) .
no other baseline patient characteristic showed any association with shock failure at 50  j , including bundle branch block , lvef , maximum wall thickness , or implant indication ( primary vs secondary ) .
the 3 patients with the greatest maximum lv wall thickness of 30 , 26 and 24  mm all had an adequate dft ( 35 , 35 and 50  j , respectively ) .
followup data after a median of 17.5 ( 335 ) months was available for all 16 patients ; there were no appropriate shocks , and 1 patient received an inappropriate shock attributable to a temporary reduction in qrs amplitude in the device chosen sensing vector , which resulted in oversensing of other ecg elements ( figure  3 ) .
interrogation of that device after the inappropriate shock showed normal sensing without any oversensing in the same vector ( alternate ) that the device had automatically chosen for sensing in both an upright and bendingover position .
the sensing channel was then manually set to a different vector , and the patient has not received any further shocks . stored electrogram of alternate vector recording from an sicd .
this patient who passed screening for an sicd , recieved a shock resulting from oversensing on nonqrs elements of the recording .
twentyseven hcm patients at risk for scd were considered potential candidates for the sicd . after screening for oversensing ,
23 patients ( 85% ) remained eligible . in one of the patients who failed screening , the potential for twave oversensing was only apparent after exercise in the only sensing vector that passed screening under resting conditions ( figure  2 ) .
patients who failed screening were more likely to have a bundle branch block on ecg ( 4 of 4 vs 4 of 23 ; p=0.004 ) and had a trend toward a higher bmi ( 33 [ 27.636.9 ] vs 27.4 [ 20.837.3 ] ; p=0.07 ; table ) .
age , lvef , and maximum wall thickness were not associated with potential for oversensing .
of 23 patients who screened successfully , 7 did not proceed with sicd implantation either because of insurance denial ( 2 patients ) or patient decision to have a transvenous device ( 3 patients ) .
, 42 hcm patients underwent implantation of a transvenous icd , none of which were screened and were thus not included in the current study .
twentyfour patients had a pacing indication , 11 of which had complete heart block after invasive septal reduction therapy .
in addition , 10 patients without a pacing indication were not screened and underwent transvenous icd implant during a time period of 8  months when the sicd was unavailable . the perceived need for antitachycardia pacing
. single lead iii electrocardiogram ( ecg ) preexercise in the upright position ( top panel ) and postexercise ( lower panel ) showing change in twave amplitude .
failure of screening is indicated by a portion of the twave outside the shaded area on the superimposed screening tool .
comparison of baseline characteristics of patients who passed and failed screening for sicd bmi indicates body mass index ; iqr , interquartile range ; lbbb , left bundle branch block ; lvef , left ventricular ejection fraction ; rbbb , right bundle branch block .
among the 16 patients who had an sicd implanted , mean age was 39.611.0  years ( range , 2157 ) .
the indication for sicd implant was secondary prevention in 3 patients and primary prevention , based on the presence of 1 of the conventional risk factors , in 13 patients .
mean maximal left ventricular ( lv ) wall thickness was 18.45.4  mm ( range 1130 ) and none of the patients had left ventricular lv outflow tract obstruction > 30  mm  hg under basal conditions , including 2 patients who had previous successful invasive septal reduction therapy .
one implanted patient did not undergo dft testing because of the presence of a left atrial thrombus . in the others , vf was terminated by a 65j shock from the sicd in all patients and a 50j shock was successful in 12 of 15 .
a 35j shock terminated vf in 10 of 12 patients ( figure  1 ) .
vf was detected appropriately by the sicd after all inductions , and in no instance was there significantly delay or lack of detection necessitating external defibrillation .
sicd defibrillation failure at 50  j was associated with a higher bmi ( 26.55 [ 21.5033.2 ] vs 33.7 [ 32.2037.30 ] ; p=0.025 ) .
no other baseline patient characteristic showed any association with shock failure at 50  j , including bundle branch block , lvef , maximum wall thickness , or implant indication ( primary vs secondary ) .
the 3 patients with the greatest maximum lv wall thickness of 30 , 26 and 24  mm all had an adequate dft ( 35 , 35 and 50  j , respectively ) .
followup data after a median of 17.5 ( 335 ) months was available for all 16 patients ; there were no appropriate shocks , and 1 patient received an inappropriate shock attributable to a temporary reduction in qrs amplitude in the device chosen sensing vector , which resulted in oversensing of other ecg elements ( figure  3 ) .
interrogation of that device after the inappropriate shock showed normal sensing without any oversensing in the same vector ( alternate ) that the device had automatically chosen for sensing in both an upright and bendingover position .
the sensing channel was then manually set to a different vector , and the patient has not received any further shocks . stored electrogram of alternate vector recording from an sicd .
this patient who passed screening for an sicd , recieved a shock resulting from oversensing on nonqrs elements of the recording .
these data suggest that the majority of hcm patients who are at risk of scd and do not have a pacing indication are eligible for sicd implantation , and that the device appears effective at recognizing and terminating induced vf at implant . screening these patients
high bmi and presence of a bundle branch block were associated with failure of screening .
although no past study has specifically looked at sicd use in this patient population , 2 previous studies on the subject have included hcm patients . in 1 study , 2 of 18 ( 11% ) patients with hcm failed screening,10 4 of 18 ( 22% ) patients in a similar study,11 whereas 15% patients failed in our study .
the overall failure rate of screening in all patients , with varying icd indications in various studies is 7.4% to 14.5%.10 , 11 , 12 in the study by oldenordkamp et  al .
, screening failure among the larger group of patients was independently associated with hcm , elevated bmi and prolonged qrs duration , which is similar to our findings . based on our experience , in addition to the manufacturerrecommended screening supine and standing , performance of screening after exercise
should be considered , given that twave amplitude and morphology can change dramatically and introduce the potential for inappropriate shocks . despite successful preimplant screening , inappropriate shocks for oversensing
defibrillation was achieved with a wider safety margin than has been deemed to be adequate in the largest sicd registry to date , which included patients with other types of heart disease .
these results were present across a diverse phenotypic expression of hcm including a subset with substantial lv wall thickening .
although dft testing has fallen out of favor in the majority of patients undergoing icd implantation , hcm remains a unique condition in which dft testing should  still be strongly considered for both transvenous and sicds.13 the dft with transvenous icds in hcm , in general , can be expected to be higher than in other cardiomyopathies , thus reducing the margin between the maximum output and the dft.8 in addition , hcm is a progressive process with increasing lv wall thickness and mass , which can potentially adversely affect the dft over time .
an adequate safety margin for defibrillation is especially relevant for patients with hcm given that many patients will live for decades with the need for scd protection .
it is reassuring to find that , based on our data , there is a wider dft safety margin at implant than had been previously documented in a large sicd registry where testing was only performed at 65  j. though potentially important , the actual clinical significance of the width of the dft safety margin for an individual patient is unclear and could only be answered by longer followup in a large cohort of hcm patients . in this series
, there was significant association between a higher bmi and sicd defibrillation failure at 50  j. the vector of an sicd shock is between the midaxillary , apical subcutaneous defibrillator and the parasternal lead coil .
it is possible that the presence of a greater amount of body fat between those 2 positions could be responsible for less energy delivery to the myocardium .
this would be in contrast to a transvenous icd , which typically rests over the pectoralis muscle and shocks by a vector between that position and an endocardial right ventricular coil and , in some cases , a superior vena cava coil as well .
this finding would be similar to data showing body weight predicting failure of external cardioversion of atrial fibrillation.14 this finding is likely not unique to hcm , and should be validated in a larger group of patients .
longterm studies of transvenous icd leads show that lead failure is an important issue , particularly for young individuals . in a pediatric hcm registry , lead failure occurred in 15 of 224 patients ( 6.6% ) in a 4.3year average followup.3 seven of these individuals had inappropriate shocks attributable to lead failure . in an early hcm icd series ,
12 of 128 ( 9.4% ) individuals followed for an average of 3.1  years had a lead fracture.2 nine of these had an inappropriate shock .
the medtronic fidelis ( medtronic inc , minneapolis , mn ) lead has failed at a rate of 2.8% a year , with a higher incidence in the younger individual.15 in a canadian series the failure rate for the fidelis lead at 5  years was 16.8%.16 all transvenous leads are subjected to repetitive shoulder and cardiac motion .
thus , it is generally accepted that no transvenous lead will last the lifetime of a young individual .
the sicd lead , by contrast , is not subject to the same stresses as a transvenous lead .
in addition , the absence of a lumen in the sicd lead should theoretically reduce the risk of lead failure .
therefore , survival of an sicd lead can be expected to be longer than a transvenous lead , making it a particularly good option for the young individual . moreover , the consequences of lead failure or infection are far less serious with an sicd compared to a transvenous system , because removal of an sicd lead is of much lower risk compared to a transvenous lead .
in addition , there is no antitachycardia pacing ( atp ) available in the sicd .
patients who have a pacing indication at the time of icd implant should not be considered for the sicd and they were not included in this series .
patients who are being considered for an icd for secondary prevention who have had multiple past episodes of sustained monomorphic ventricular tachycardia that may be amenable to atp may be best served by a transvenous icd . otherwise ,
the lack of atp , in our opinion , should not be an argument for favoring a transvenous device .
atp has never been shown , in a prospective , randomized trial , to affect mortality , and its absence is far outweighed by the much larger potential benefit of avoiding transvenous leads and the associated potential future morbidity and mortality in this typically younger population .
our approach is to consider any patient with hcm in need of sudden cardiac death protection who does not have a current pacing indication as a candidate for an sicd .
the majority of hcm patients undergoing icd implant are young , and the younger the patient , the stronger our preference for the sicd .
further longitudinal studies with larger cohorts and longer followup are necessary to determine the efficacy of the sicd in treating lifethreatening ventricular arrhythmias in hcm .
further longitudinal studies with larger cohorts and longer followup are necessary to determine the efficacy of the sicd in treating lifethreatening ventricular arrhythmias in hcm .
in this singlecenter series of sicd in patients with hcm , the sicd appropriately recognized and terminated induced vf at implant testing , with a wide safety margin .
longerterm experience is necessary for further confidence in this system , but the current shortterm results are promising .
 backgroundthe subcutaneous implantable cardioverter defibrillator ( sicd ) has been developed to avert risks associated with transvenous defibrillator leads . 
 the technology is attractive for younger patients , such as those with hypertrophic cardiomyopathy ( hcm ) . 
 however , there are limited data on sicd use in hcm.methods and results 
 hcm patients identified at risk for sudden death were considered for sicd implantation . 
 patients were screened for potential oversensing by surface electrocardiography ( ecg ) . at implant , defibrillation threshold ( dft ) testing 
 was performed at 65 , 50 , and 35 joules ( j ) . 
 twentyseven patients were considered for sicd implantation , and after screening , 23 ( 85% ) remained eligible . 
 the presence of a bundle branch block was associated with screening failure , whereas elevated body mass index ( bmi ) showed a trend toward association . 
 one patient passed screening at rest , but failed with an ecg obtained after exercise . at implant , 
 the sicd terminated ventricular fibrillation ( vf ) with a 65j shock in all 15 implanted patients and a 50j shock was successful in 12 of 15 . 
 a 35j shock terminated vf in 10 of 12 patients . 
 dft failure at 50  j was associated with a higher bmi . 
 there were no appropriate shocks after a median followup of 17.5 ( 335 ) months , and 1 patient received an inappropriate shock attributable to a temporary reduction in qrs amplitude while bending forward , resulting in oversensing , despite successful screening.conclusionsin a highrisk hcm cohort without a pacing indication referred for consideration of an icd , the majority were eligible for sicd . 
 the sicd is effective at recognizing and terminating vf at implant with a wide safety margin . 
Methods
Results
Patient Selection and Screening
Baseline Demographics
DFT Testing
Followup
Discussion
Limitations
Conclusion
Disclosures