What is a wearable cardioverter defibrillator (WCD)? Cardiology Basics

What is a wearable cardioverter defibrillator (WCD)? Cardiology Basics

Wearable cardioverter defibrillator (WCD) is a device which is worn on a vest, monitors the heart rhythm continuously and delivers a defibrillator shock in case of life threatening ventricular arrhythmias. Implantable cardioverter defibrillator (ICD) is a similar device which is implanted subcutaneously for long term use. WCD is meant for temporary use for those who have a high risk of life threatening ventricular arrhythmias but are ineligible for an ICD implantation as per the guidelines.

WCD has patch electrodes for giving the shocks and a monitoring system built-in into a vest while the defibrillator with battery is worn on the belt. WCD can also give audible alerts and does not need the help of a bystander as it works automatically. This in contrast to the automated external defibrillator or AED, which only gives audio prompts and bystander intervention is needed to deliver a shock.

A typical WCD device has a maximum shock output of 150 Joules and could terminate ventricular fibrillation in first attempt with 70 to 100 Joules shock. Still programming at maximum energy output has been suggested for ambulatory patients to ensure good safety margin. WCD has been called as a “Life vest till the life boat arrives”, meaning that it can be considered during the period when the patient is not eligible for an ICD as per guidelines published by medical societies.

These include patients with left ventricular dysfunction, within 40 days of acute myocardial infarction who are at high risk of life threatening ventricular arrhythmias, but still ineligible for ICD as benefits have not been proven. Another, possibly more important reason to use WCD is when an ICD has to explanted due to infection and there is a waiting period before a new one can be implanted, pending the clearance of infection with antibiotic therapy.

As expected, the most important “Achilles heel” of a WCD is compliance. Real world studies have shown average daily usage as low as 17.3 hours, which means that there would be no protection during the remaining period! In a randomized controlled trial involving 2302 patients, of the 48 participants who died, only 12 were wearing the device at the time of death. Another meta-analysis with data on over 32,000 patients showed that mortality while wearing WCD is rare, 0.7 per 100 persons over 3 months.

Like ICDs, WCD can also give inappropriate shocks sometimes, at a rate of 2 per 100 persons over 3 months. This is due to errors in “judgment” by the automatic monitoring software, by which non-life-threatening tachycardias, usually supraventricular arrhythmias are considered as life-threatening and shocks delivered. Errors can be reduced to some extent by meticulous usage technique and programming of the device.

Studies have shown that compliance is more in those who already had a cardiac arrest during previous hospital admission for myocardial infarction as well as in those who had previous heart failure and in those with more WCD alarms. Poor compliance was noted in those who had any WCD shock. When used during the waiting period after ICD explantation for a new ICD implantation, WCD is thought to be a cost effective option.

Usual care typically involves hospital admission for about 21 days while waiting for the new ICD implant till the infection in the previous one has cleared. With the temporary protection given by WCD, if they can go home rather safely and total hospitalization period brought down to about 15 days, a cost saving of 1782 Euros has been estimated in one report.