Implantable Cardioverter Defibrillators

An implantable cardioverter defibrillator (ICD) is a type of cardiovascular medical device that is surgically implanted under the skin of people who face the risk of sudden death caused by ventricular fibrillation. Ventricular fibrillation is a medical emergency that is caused when the ventricles of the heart lack coordination and contract in rapid, unsynchronized rhythms (cardiac dysrhythmia) that typically culminates in the heart’s inability to pump blood (heart failure).

Although ICD devices are primarily designed for the detection of ventricular fibrillation, technological advancements have allowed for their use to be expanded to include the treatment of:

  • Atrial arrhythmias: Abnormal heart rhythm of the atria.
  • Ventricular arrhythmias: Abnormal heart rhythm of the ventricle.
  • Pacing of bradycardia: Heart rate slower than 60 beats per minute; biventricular pacing.

ICDs and Heart Rhythm

Implantable cardioverter defibrillators are designed to deliver 15- to 20-seconds of electro-countershock treatment to people suffering from ventricular fibrillation. The device monitors heart rhythm and can therefore detect the onset of cardiac dysrhythmia.

ICDs are programmed to deliver shock therapy whenever the ventricular heart rate exceeds a certain programmed level that is deemed to be the safety “ceiling” for a given patient; this “ceiling” is programmed by doctors on a case-by-case basis. ICDs use a combination of methods to determine whether or not a patient’s heart rhythm is functioning normally, some of which include:

  • Rate discrimination: By monitoring the rate of the ventricles and comparing it to the atria, an ICD can determine whether or not an abnormal rhythm is ventricular in its origin.
  • Rhythm discrimination: By monitoring the regularity of a faster than normal heart rate, an ICD can determine whether the rhythm is normal or irregular. Occurrence of an irregular rhythm during ventricular tachycardia typically originates in the atria (like atrial fibrillation).
  • Morphology discrimination: By monitoring the morphology of each and every ventricular heartbeat, the ICD can compare the results with the “normal” ventricular impulse stored in the ICD memory. A discrepancy in the beating patterns could signal ventricular fibrillation.

Medtronic & Guidant ICDs

Medtronic Inc. and the Guidant Corporation are two of the industry’s largest developers/manufacturers of cardiovascular medical devices. Both companies offer a wide variety of implantable cardioverter defibrillators, artificial cardiac pacemakers and cardiac resynchronization therapy (CRT) devices. Both companies have made headlines recently because of FDA recalls targeting some of their products that have proven to be defective, putting patients’ lives at risk. In some cases, both Medtronic Inc. and the Guidant Corporation have been found liable for personal injuries or deaths caused as a result of their defective medical products.

In March of 2007, a safety advisory update was issued by the UK Medicines and Healthcare Products Regulatory Agency with regards to a defect affecting some implantable heart devices’ ability to function properly. The exact nature of the defect relates to the devices’ failure to administer necessary shocks in a timely manner. Though there have been no injuries reported as a result of this most recent safety advisory, it certainly stokes the fears of consumers still wary of the threats posed by certain pacemakers and ICDs.

ICDs and Artificial Cardiac Pacemakers

Implantable cardioverter defibrillators are somewhat like artificial cardiac pacemakers in that they both incorporate a wire that runs through the right chambers of the heart, ultimately ending up in the highest point of the right ventricle. Both devices monitor heart rhythm and intervene whenever a patient suffers from an abnormal beating pattern. The similarities between ICDs and pacemakers are such that the respective devices are often confused with one another.

Artificial pacemakers are designed to help regulate the beating of the heart by monitoring heart rhythm and applying a set amount of stimulation if the device doesn’t sense the appropriate amount of electrical energy.

The underlying difference between ICDs and pacemakers is with the respective devices’ response to atrial fibrillation. When atrial fibrillation is detected by a pacemaker, the device sends electronic signals to the heart in order to regulate blood flow. When atrial fibrillation is detected by an ICD, the device can respond in one of two ways:

  • ICDs can mimic pacemaker functionality if the heart is beating too slowly, sending electronic signals to the heart to regulate blood flow.
  • If the heart is beating too quickly, the ICD can deliver the appropriate therapy required to slow its rhythm down.

Basically, ICD cardiovascular devices can provide treatment for patients suffering from tachycardia (faster-than-normal heart rate) and bradycardia (slower-than-normal heart rate), while pacemakers can only provide treatment for patients suffering from bradycardia.