Increasing Data on the Subcutaneous ICD: What Does This Mean for the Athlete?

Editor's Note: Commentary based on Burke MC, Gold MR, Barr CS, et al: Safety and efficacy of the totally subcutaneous implantable defibrillator: 2-year results from a pooled analysis of the IDE Study and EFFORTLESS Registry.

Traditionally, the world of evolving implantable cardioverter defibrillator (ICD) technology and the world of the athlete did not intersect — once an athlete needed an ICD, further athletic participation was prohibited. In the 36th Bethesda Conference in 2005 document about eligibility recommendations for competitive athletes with cardiovascular abnormalities, the authors state that "For athletes with ICDs, all moderate and high-intensity sports are contra-indicated."2 The postulated risks of sports underlying those recommendations included increased frequency of ventricular arrhythmias, potential failure of a shock to convert a life-threatening arrhythmia, damage to the device or lead system, or risk of harm due to momentary loss of control from an arrhythmia, or from the ICD shock itself.2

More recent data, however, suggests that many individuals with ICDs do participate in sports.3,4 In the recent prospective, the ICD Sports Safety Registry,4 372 athletes participating in competitive or dangerous sports, including 60 very high-level athletes (interscholastic high school or college) were followed for a mean of 2.5 years. There were no incidences of failure to defibrillate or arrhythmia- or shock-related injuries,4 suggesting that sports participation may be safer than previously thought. These data do not suggest that any sport is safe for any patient, but do suggest that many athletes can participate in sports without bodily harm, and that the decision to return to play for an athlete who receives an ICD for standard indications should be individualized.

In the ICD Sports Registry, all athletes had transvenous (or a few epicardial) lead systems. However, data regarding the safety and efficacy of the totally subcutaneous ICD (S-ICD), approved by the U.S. Food and Drug Administration in 2012 for the general population of ICD patients, are continuing to increase. In a recent study published in the Journal of the American College of Cardiology, Burke et al,1 combined data from the two largest individual studies of the S-ICD (the EFFORTLESS [Boston Scientific Post Market S-ICD Registry] trial and the IDE [S-ICD System IDE Clinical Investigation]), increased the follow-up time to over two years, increased the patient population to 882 total, and found excellent safety and efficacy of this device. In general, the implant complications, while similar in number to that for the transvenous device, carry lower morbidity (i.e., no risk for pneumothorax or perforation). These data raise important questions: how does the S-ICD compare as a choice for the athlete; what specific factors should be taken into consideration in the choice of device for an athlete who needs an ICD?

Does the Device Work?

The most important consideration behind the restriction of sports participation in the 36th Bethesda Conference document was that the ability of the ICD to terminate ventricular arrhythmias during the metabolic and autonomic changes of exercise was unknown.2 Exercise increases potassium,5 and can induce ischemia, which increase defibrillation thresholds.6,7 Data conflict on whether increased catecholamines, as occur during exercise, increase8 or decrease9 the defibrillation threshold. In the ICD Sports Safety Registry, 22 athletes received 30 appropriate shocks for ventricular arrhythmia occurring during sports participation, all of which successfully terminated the arrhythmia. In six individuals, there were seven episodes in which an arrhythmia occurring during physical activity required more than one shock for termination. These occurred in patients with either coronary disease, idiopathic ventricular fibrillation (VF), or catecholaminergic polymorphic ventricular tachycardia (VT).

Initial reports demonstrated that the S-ICD has excellent efficacy at termination of induced arrhythmias, both at implant and at a follow-up induction study after several months.10 The current report by Burke et al.1 includes the largest series of spontaneous arrhythmias treated by the S-ICD: 111 discrete VT/VF events and 12 VT/VF storms in 104 patients. Of the discrete events, 90% were terminated with the first shock, and 98% within the five programmed shocks. Of the two failures to convert within the programmed shocks, one was due to a sensing issue, and the other did convert after the recording period. This efficacy rate is similar to the transvenous ICD.11 The S-ICD is new and does not have the track record of decades of experience in hundreds of thousands of patients, nor the experience in a smaller group of athletes, the transvenous device has. However, these early data are highly promising that efficacy would be similar in the athlete as well.

How Well Can the Device Differentiate Ventricular Arrhythmias From Sinus or Other Supraventricular Tachycardias (SVTs)?

Inappropriate shocks due to sinus tachycardia, other SVTs, noise, or oversensing, are some of the banes of the ICD. Differentiating sinus tachycardia is particularly important for the young athlete. While recent data has shown that devices can be safely programmed to treat only rhythms faster than 200 bpm,12 a young athlete may get his or her heart rate over 200 bpm with exertion. In the ICD Sports Safety Registry, there were six athletes who received shocks for sinus tachycardia during sports participation (2%), and another nine with inappropriate shocks for other reasons during sports. In the current EFFORTLESS/IDE follow-up,1 the estimated three-year rate of inappropriate shock was 11% in those appropriately programmed to two zones of therapy (i.e., including a lower zone with discrimination criteria). With recent advances in programming, inappropriate shocks with the transvenous system have been lowered to less than 5% in the general population.12 The Subcutaneous versus Transvenous Arrhythmia Recognition Testing (START) study13 compared discrimination of atrial versus ventricular arrhythmias between transvenous ICD systems and the S-ICD in a head-to-head simulation study. Atrial and ventricular arrhythmias were induced in patients undergoing device implantation, recorded from transvenous leads and surface leads set up to mimic the electrodes in the S-ICD, then processed and delivered into each type of device (single-chamber transvenous system, dual-chamber system, and S-ICD). Specificity for supraventricular arrhythmias in the S-ICD, which uses correlation waveform analysis to discriminate atrial from ventricular arrhythmias, was excellent at 98%, which is better than most of the transvenous systems tested.

What Is the Likelihood of Injury to the System From Sports?

Lead malfunction is the other major bane of the ICD. A transvenous lead traverses the venous system and is subject to hundreds of millions of cardiac cycles. Lead survival free of malfunction in unselected populations has varied from 85-98% at five years in multiple studies.14 In the ICD Sports Safety Registry, lead survival free of definite or probable malfunction was 93% at five years. However, whether some sports may increase the likelihood of lead damage is unknown. One well-recognized mechanism of lead damage is entrapment of the lead as it passes between the clavicle and first rib, where the lead can become compressed, termed the "subclavian crush syndrome."15 It has been hypothesized that sports with intensive involvement of the arms, such as swimming or rowing, may increase likelihood of subclavian crush. Longer-term follow-up of the ICD Sports Safety Registry is ongoing, and further analyses may shed light on this question. It is too soon to confirm the long-term lead survival of the S-ICD. However, for an athlete wishing to participate in these types of sports, avoidance of subclavian crush may favor the S-ICD.

Which device may be better for sports with the risk of bodily impact, such as violent contact sports or ball sports, is unknown. In youth baseball, ball velocity is 30-50 miles per hour16; how this impact might affect the subcutaneous lead, outside of the thorax, is unknown.

In summary, the current EFFORTLESS/IDE data describing longer-term follow-up of S-ICD efficacy and safety suggest it may be an important alternative for an athlete requiring an ICD who has made the decision to continue sports. Both transvenous and subcutaneous devices are effective at terminating arrhythmia and differentiating ventricular from supraventricular arrhythmias. Advantages of the S-ICD include less initial procedural risk and likelihood of fewer lead-related issues down the road (and safer revision if needed). The decades-long track record of efficacy, as well as athlete-specific data, is an advantage of the transvenous system. Another advantage is the ability to deliver anti-tachycardia pacing for the subset of patients who have had ventricular tachycardias, which may be amenable to this painless treatment. Data are lacking on the safety of violent contact sports, such as football or hockey, for either device, because very few athletes involved in contact sports were enrolled in the ICD Sports Safety Registry. There may be theoretical concerns with contact sports with each. As with any patient, discussion of the potential advantages of each system is imperative to allow the physician and the athlete to make a shared and informed decision.


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Clinical Topics: Arrhythmias and Clinical EP, Congenital Heart Disease and Pediatric Cardiology, Sports and Exercise Cardiology, Implantable Devices, SCD/Ventricular Arrhythmias, Atrial Fibrillation/Supraventricular Arrhythmias, CHD and Pediatrics and Arrhythmias, CHD and Pediatrics and Quality Improvement, Sports and Exercise and Congenital Heart Disease and Pediatric Cardiology

Keywords: Adolescent, Arrhythmias, Cardiac, Athletes, Baseball, Cardiovascular Abnormalities, Catecholamines, Clavicle, Coronary Disease, Crush Syndrome, Defibrillators, Implantable, Heart Conduction System, Heart Rate, Physical Exertion, Pneumothorax, Potassium, Prospective Studies, Registries, Ribs, Tachycardia, Sinus, Tachycardia, Ventricular, Ventricular Fibrillation

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