Electrical Safety of Implantable Cardioverter Defibrillators

Patients at risk of developing life-threatening heart rhythms may require the implantation of a small device called a cardioverter-defibrillator (ICD), which constantly monitors the heart rhythm and delivers an electrical shock to the heart when indicated, in order to return the heart back to a normal rhythm. Many thousands of these devices have been implanted and are electrically active in patients who collapse and need resuscitation. When a patient with an ICD collapses, the device may discharge without warning while a rescuer is performing external chest compressions (cardiac massage). Conventional ICDs placed below the left collar bone typically deliver 35-50 J energy when they discharge, but newer ICDs placed under the skin (S-ICD) alongside the breastbone deliver a larger energy when discharging; typically 50-80J energy. Rescuers performing external chest compressions on a patient during conventional ICD discharge have reported the sensation of a painful electrical shock and permanent nerve damage. In these situations, rescuers appear to have been exposed to electrical current from the ICD considerably in excess of that which is considered a safe threshold. Studies of surface current resulting from discharge of conventional ICDs have been reported in excess of 100 mA which is far in excess of the safe 1 mA limit, and puts the rescuer at considerable risk of tissue damage and possible dangerous heart rhythms. The newer S-ICDs deliver approximately 50% more energy and have the potential to result in exposure of a rescuer to even higher currents. With increasing numbers of the S-ICDs being implanted, and the inevitability that rescuers will soon find themselves exposed to leakage current from these devices, there is a need to examine the leakage currents arising from these devices and assess any subsequent risk to a rescuer performing external chest compressions.