ICD Threshold Testing: Time to Give Practice a Jolt?

To test or not to test? It’s a challenging question. With implantable cardioverter-defibrillator (ICD) technology now providing more efficacious and frequent shocks, some electrophysiologists have begun to question whether standard threshold testing is necessary for all implantations.

A house divided

When defibrillation testing is used, the patient’s heart is placed into ventricular fibrillation and the device shocks the heart back into rhythm. This establishes the energy threshold at which normal rhythms re-engage. It assures the physician that when the patient needs the device, the heart responds to the shock the device provides.

Threshold testing ensures that devices are functional, leads have been correctly placed and if a patient has more than one implanted cardiac device, testing reveals possible problems with the way devices interact. It also ensures a sufficiently broad safety margin exists between the output of the device and the threshold. Guidelines currently recommend that to have an adequate safety margin, the device must be set to deliver a shock 10 joules or more beyond the patient’s threshold (J Am Coll Cardiol 2014; 64[3]:256-264). When patients have low defibrillation thresholds, a wide safety margin is possible, even without setting a device to maximum output.

While some cardiologists say testing is still necessary for assuring future patient safety, others contend that it is no longer necessary in all cases. Among the reasons given for not testing are that newer models produce multiple and more effective shocks than older models. They suggest that as long as the device is set to a maximum threshold, the safety margin should be sufficient and testing isn’t needed. Newer devices have a higher maximum output, generally providing a large safety margin and multiple defibrillations may succeed where earlier devices would not have.

Others take a neutral stance, stating that for some patients it is more important to determine where that threshold and safety zone are. They want to test those patients who need it most and pass on those who might not require adjustment; however, the question becomes how best to determine which patients fall into which category.

“Standard of care until recently is to perform defibrillation threshold testing in the lab when the device is initially implanted or before the patient goes home from the hospital,” says Andrea M. Russo, MD, a cardiologist with Cooper University Hospital in Camden, N.J. “And that is thought to potentially predict whether or not the device will work on its own when it’s needed out in the clinical arena.” She says that part of the controversy about defibrillation threshold testing is “whether those two things [threshold testing and real-world function] really correlate because there are other things that can happen.”

A number of conditions could change a patient’s defibrillation threshold, even when it has been tested and adjusted for. “[Patients] could have heart failure that may increase the amount of energy required to defibrillate,” Russo says. “They may have angina and ischemia and that may increase the energy requirement. They may have started on certain drugs, including antiarrhythmic drugs like amiodarone that may also increase the energy requirement.”

She argues that in those circumstances the device may not work as expected. The converse can be true, too. “Patents who, when they’re tested in the lab, may have a high-energy requirement but the device still works when you send them home. There’s been a lot of controversy in that area.”

Russo and her colleague, Mina K. Chung, MD, from the Cleveland Clinic, published a review of defibrillation testing points-of-view, looking at it from both sides (Cardiol Clin 2014; 32[2]:211-224). “It’s one of the most common debates we have,” says Chung.

They determined that between 2.2 percent to 12 percent of implants need adjustment following defibrillation testing. In new, high-output devices, however, the need for system modification is reduced by half following the test.

Physicians implant devices to prevent sudden cardiac death in at-risk patients, but many patients never need their ICDs to provide the appropriate shock. Only between 5 and 10 percent of appropriate ICD shocks occur each year (J Am Coll Cardiol 2014; 64[3]:256-264). 

Also, with antitachycardia pacing, improved arrhythmia detection capabilities and contemporary device programming, the risk of unnecessary shocks is far more reduced than in earlier generation models.

An arresting alternative

When a patient’s heart and the ICD are put through their paces during threshold testing, physicians also obtain other data on the device and its functionality. “Opponents of defibrillation testing [say] that connection integrity can be evaluated without defibrillation testing by looking at lower voltage pulses,” Chung says. “There are indirect ways to look at system integrity and we test pacing and sensing of the leads, but very rarely there may be a problem that’s detected [with defibrillation threshold testing] that can’t be detected without delivering a shock.”

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While not frequently used in the U.S., upper limit of vulnerability testing can show defibrillation thresholds without putting a patient into full arrest. “Most of the time you can get pretty close,” Russo says of the method. “You can identify what that defibrillation threshold would be while not really having to induce [arrhythmia] much. Give a little shock on the t-wave and you can, most of the time, get a feeling for what the threshold would potentially be.”

Jeffrey S. Healey, MD, of the Population Health Research Institute at McMaster University in Hamilton, Ontario, emphasizes that testing has its limits. “There’s a certain probabilistic nature of these successes in people with a normally functioning device,” he says. “You put them into cardiac arrest a dozen times, the first shocks work most of the time, but it will not work all of the time.”

A not-so-SIMPLE question

This debate has led to wide variability in the frequency of testing. “In many jurisdictions, the rate of testing was anywhere between 25 to 75 percent,” Healey says. Healey is the lead researcher on the SIMPLE (Shockless IMPLant Evaluation) trial, a large-scale assessment of defibrillation testing. “Any time you have that much discrepancy on what people do on an important issue like this one, that’s a good sign that trials are needed.” 

Healey and colleagues performed SIMPLE with an eye toward determining rates of successful resuscitation following a cardiac arrest, regardless of whether shock testing had occurred at device implantation. They also wanted to determine the rate of perioperative complications in both groups.

Healey notes that defibrillation testing increases the time patients are in the surgical suite, costs and the risk for rare or serious side effects and perioperative infections.

Russo says part of what helps support the idea of not testing is that devices being implanted now are not monophasic, as they were in the past. “Now, devices all have biphasic wave form instead of monophasic wave form, which increases defibrillation efficacy and makes it more likely that the devices will work at a lower output.” She adds that while a patient’s defibrillation threshold may be 10 or 15 joules, the device may deliver 35 joules, providing a larger safety margin.

Chung notes that along with other advances to ICD technology, biphasic wave forms decreased defibrillation thresholds, which has allowed more patients to be implanted with defibrillators than before.

The technology has changed in the last 20 years, Russo adds. “What was good in the 1980s, what applied then doesn’t really apply now with our modern technology because the devices are so good they’re going to work almost all of the time.” She asks why patients should be put at additional risk from testing when there may not be much benefit.

Like many cardiologists in the field, too, they’re waiting for the hard data from SIMPLE and other upcoming trials to determine whether patients will be better off with or without defibrillation testing. Regardless of the findings of studies like SIMPLE, Chung adds, “I think that some people will continue to test and I think what you’re going to see is that it’s safe and outcomes are equivalent. We’ll have to wait and see.”

Patient-centric testing

There is no disagreement that defibrillation threshold testing is, at this point, an evidence-based standard of care. While the field has evolved, the reasons for testing have not. However, determining which patients need defibrillation testing could further improve care. If electrophysiologists could determine which patients were more likely to need threshold testing, they could add the extra focus to the implantations for those patients while saving time with others.

“Not every patient has a low defibrillation threshold,” says John Hsu, MD, MAS, assistant clinical professor for cardiac electrophysiology at the University of California, San Diego. “In fact, often times, they have a high threshold such that even by setting the device to the maximum output, it is still not high enough or strong enough to convert ventricular arrhythmia or ventricular tachycardia to normal rhythm.”

Hsu and colleagues are working on understanding how best to identify patients who would ultimately have inadequate defibrillation safety margins based on clinical predictors (J Am Coll Cardiol 2014;64:256–264). Without clinical predictors, testing every patient for defibrillation thresholds allows the physician to be certain that problems with thresholds are found early and consider alternate configurations and settings.

A major concern for physicians who test for thresholds is the lack of generally accepted metrics to define which patients may be more at risk for high thresholds. Without testing, physicians can’t be sure that the configurations and settings they used will be effective. “If the threshold is high, even with modern day devices that have outputs of up to 40 to 41 [joules], you still may not be able to have a safety margin that’s adequate because of the threshold of the patient,” says Hsu.

While their research is not intended to look at whether to test or not to test, Hsu says their findings could inform physician decision-making, allowing operators to “use that information to identify those who would be at a higher risk for inadequate defibrillation safety margin.”

Identifying patients who may be at risk prepares the electrophysiologist for the possibility that defibrillation testing could reveal the need for “additional therapies and maneuvers with the device and leads to obtain an adequate safety margin,” Hsu says. With that in mind, risk scores could be a reliable tool to ensure that those who need to be tested are, even if others are not.

Hsu adds, “I really do believe that there are certain patients who do need to be tested, and potentially the paper that we published showing patients at risk of inadequate defibrillation safety margins could be used in the future in conjunction with the findings from SIMPLE to help operators really identify those patients who we do want to test to discover whether they might need further therapy.”