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; 64:256-264). 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; 32:211-224). “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