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As with pacemaker patients, medications that regulate cardiac rhythm are not always appropriate for long-term treatment of recurring tachycardias, as they may trigger additional rhythm disturbances that can further predispose the patient to a potential cardiac arrest.

Strong electrical pulses from a defibrillator offer the best prospects for eliminating life-threatening ventricular rhythm disturbances (ventricular flutter or ventricular fibrillation). External defibrillators, which are familiar to most people as standard equipment in intensive care units and ambulances, are now showing up more frequently in public areas such as airports. 

Implantable cardioverter-defibrillators (ICDs)

Since the early 1980s, implantable defibrillators have been small enough to be inserted in a patient’s chest. This advancement has significantly improved the survival chances of people with recurring tachycardias.
ICDs provide seamless monitoring of cardiac rhythm and, in an emergency, can terminate life-threatening ventricular fibrillation anytime and anywhere using a high-energy shock (defibrillation). By delivering a shock, it temporarily halts cardiac activity and returns the heart to the normal sinus rhythm. The heart then beats at a normal pace again. 

ICDs: Harmonizing abnormal cardiac rhythms

Terminating rapid cardiac rhythm disturbances requires a significantly higher energy output (current output) than pacing the heart using a pacemaker. It therefore requires a much more powerful battery. The battery thus occupies the largest portion of the implanted ICD device.

In addition to the long-lasting battery, and a capacitor for charging the shock, the ICD also contains a minicomputer (circuitry). The housing of the ICD is about as large as a matchbox and is made of tissue-compatible titanium.

The cable connections for the leads that connect the implanted device to the heart are located on the top of the titanium housing (header). The leads consist of precious metal wires such as silver, platinum or iridium and are insulated with tissue-compatible silicon or polyurethane. The ends of the leads contain fine sensors that transmit signals from the heart to the ICD and back from the ICD to the heart.

Delivering very strong electrical pulses to the heart requires a special defibrillation lead, which is anchored in the right ventricle. Through the lead in the ventricle, any irregularity in the cardiac rhythm can be detected and treated using electrical pulses.

Most implanted devices from BIOTRONIK are equipped with an additional transmitter that lets the physician treat patients at home via telemedicine. This lets physicians stay up to date on the status of patients’ hearts and devices between follow-up appointments.

What types of ICDs are available?

Single-chamber defibrillators use only one lead (defibrillation lead), which is anchored in the cardiac apex in the right ventricle. Patients with a single-chamber ICD are usually not dependent on the pacemaker function, so pacing is only used when the heart rate is, for example, too slow (<40 beats/minute) after shock delivery. 

Dual-chamber defibrillators have a second lead for the right atrium and are usually used when there is a conduction system disorder 
 (an indication to use a pacemaker). Dual-chamber ICDs are also implanted when the electrical signals from the atrium need to be recorded to differentiate rhythm disturbances (e.g., slow ventricular tachycardia, polymorphic VTs [ventricular tachycardias], paroxysmal atrial fibrillation). Many dual-chamber ICDs integrate additional pacemaker features to reduce unnecessary right ventricular pacing. Unnecessary administration of right ventricular pacemaker stimulations can promote the development of congestive heart failure and trigger or intensify atrial fibrillation.

What does an ICD look like? How does it work?

At the top of an ICD are connections for the leads that are fed through a vein and into the right side of the heart. The leads are made of silver, platinum or iridium and are insulated with biocompatible silicone. Sensors at the ends of the leads continually transmit the cardiac signals to the ICD microcomputer.

If necessary, the ICD transmits electrical signals through the leads and to the heart. In case of an emergency, a defibrillation electrode transfers an electroshock to the ventricle.

To customize the device to meet an implant patient’s individual medical requirements, the physician uses a programming device to set the ICD parameters. The programming device is composed of a programming unit and a wireless data transfer unit that the physician places on the skin, on top of the implant.

Therapy options

Each implantable cardioverter-defibrillator (ICD) can be programmed from the outside and can be tested and optimized to the patient’s status as part of a follow-up. Using its leads, the defibrillator (ICD) can monitor the patient’s rhythm around the clock and detect when the heart loses rhythm.

In response to heart rate abnormalities, the device can automatically deliver increasingly more aggressive therapy to stop the disturbance:

  • Level 1: Antitachycardia and antibradycardia stimulation
    The heart often races just before fluttering or fibrillating. In this case, the ICD delivers weak electrical signals in quick succession. In 60% to 80% of all cases, this causes the heart to stop racing before ventricular flutter or fibrillation begins. The ICD also stimulates the heart when it beats too slowly (bradycardia). This is the same type of stimulation a pacemaker offers.
  • Level 2: Cardioversion
    Another therapy level is possible if the heart does not stop racing in spite of the stimulation, or if the ventricles start to fibrillate. Cardioversion (which means converting the heart’s rhythm) is a weak electroshock that stresses the patient less than a defibrillation shock would.
  • Level 3: Defibrillation
    The ICD delivers a strong electroshock in case of ventricular fibrillation, or if the other therapy levels remained unsuccessful. This usually ends the tachyarrhythmia reliably. The energy for this strong shock is ready in less than ten seconds after ventricular fibrillation has begun. Should the life-threatening situation continue after the first shock, the ICD delivers additional shocks. Before delivering a therapy, the ICD always checks whether the tachyarrhythmia still exists.

Would I feel the shocks?

Patients can barely feel antitachycardia and antibradycardia stimulation (level 1), but notice that their heart rhythm stabilizes if this therapy was successful.

Cardioversion and defibrillation (levels 2 and 3) are briefly painful. Due to the ventricular fibrillation, however, many patients lose consciousness before the shock is delivered, so they do not feel anything at all. Those who are conscious as the electroshock is delivered find it to be like a strong blow to the chest, but the pain immediately subsides. The chest and arm muscles on the side of the body where the implant is located tense up, so the patient has muscle aches for one to two days after the episode.

Statistically speaking, the strong electroshocks are most frequently delivered in the first few months after implantation. After this period, many ICD patients live without any cardioversion or defibrillation for years to come.



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