Implantable cardioverter defibrillators

 

When the heart is significantly damaged for any reason, there is an increased risk that it can become irritable and go into a fast abnormal rhythm (ventricular tachycardia) which may make the heart so inefficient that it can no longer effectively function as a pump. This may happen out of the blue with no real warning even if the patient has been well. If left unchecked for even a few minutes, this will degrade into a rhythm known as ventricular fibrillation which causes cardiac arrest and sudden death. In this setting, the only thing that may help is immediate cardiac resuscitation with the aim of delivering an electrical shock to the heart as soon as possible. All research indicates that the only intervention that has any chance of reviving the patient is the electrical shock treatment which is usually delivered by a defibrillator machine. A defibrillator machine however may sometimes not be easily accessible. 

Given the importance of early defibrillation in this setting, scientists started working on designing an internal defibrillator which could be inserted in a vulnerable patient and which could automatically detect the onset of ventricular tachycardia or ventricular fibrillation and automatically deliver a shock to the patient thereby saving the patient’s life.

In 1980, the first (rather bulky) implantable internal defibrillator was developed and by 1985, it had received FDA approval. Since then, rapid improvements in design and technology have meant that defibrillators have become much smaller in size and more effective not only in shocking the heart out of ventricular fibrillation but also detecting ventricular tachycardia and preventing it from degrading into ventricular fibrillation. Multiple studies have also confirmed the safety and efficacy of such devices in patients with a high risk of sudden death.

Apart from delivering shocks, defibrillators have other qualities which can be very beneficial.

  1. They are  exceptionally good monitoring devices. Patients with heart disease may also be susceptible to other heart rhythm disturbances such as Afib (which may be ‘silent’ i.e that the patient may not even know about it). In this setting the defibrillator will detect it and when the defibrillator is interrogated, episodes of Afib will be detected. This is important because the presence of any Afib points to the patient being at a higher risk of strokes and therefore the patient would be considered for long term anticoagulation.
  2. They are able to act as pacemakers. Patients with heart disease may be liable to developing very slow heart rates and sometimes the medications that are used to stop episodes of ventricular tachycardia or even stop the heart from weakening further can paradoxically push the heart rate down to very low levels. In this situation, the defibrillator will function as a pacemaker and stop the heart rate falling too low.
  3. They can deliver treatments other than defibrillation. Sometimes when the defibrillator detects that the patient has developed ventricular tachycardia, it tries to treat the ventricular tachycardia before the ventricular tachycardia can degrade into ventricular fibrillation. This is called anti-tachycardia pacing (ATP) and can terminate the ventricular tachycardia painlessly and therefore obviate the need for a (painful) shock.

 

Whilst the idea of an implantable defibrillator sound attractive there are a few things to take into consideration

  1. Firstly, the shock that the defibrillator delivers, whilst potentially lifesaving, can also be extremely painful and traumatic for the patient. Many patients describe it like being kicked in the chest by a donkey. In the big scheme of things, it is a small price to pay if it can save the patient’s life. However sometimes defibrillators can get fooled and deliver inappropriate or even unnecessary shocks. As the defibrillator decides on what therapy based on the heart rate (a threshold heart rate is pre-programmed and anything that exceeds that heart rate may make trigger the  defibrillator to shock) and whether the heart rhythm is regular or not. In some situations, a patient may develop fast atrial fibrillation which may exceed the pre-programmed heart rate threshold and because the heart is so fast the defibrillator may mistake the usually irregular heart rhythm of AF to be the regular rhythm of ventricular tachycardia and the defibrillator can deliver an ‘inappropriate’ shock. This, apart from causing the patient great discomfort and trauma, can also significantly deplete the battery life of the device.
  2. The insertion of a defibrillator is not without an albeit small risk. It is an invasive procedure and can expose the patient to the risks of infection as well damage to and formation of blood clots within the blood vessels through which the defibrillator wires are inserted. Once the wires are inserted into the heart, they get embedded within the heart muscle and therefore can not be easily pulled out without risking damage to the heart muscle and therefore once the wires are put in, it is not easy and potentially dangerous to get them out and therefore the device should usually be thought of as a lifelong commitment. In addition, the defibrillator and its wires are a foreign body and can get infected and if infected can pose a real challenge because infection on foreign bodies can be very difficult to clear, even with prolonged intravenous antibiotics (lasting 6 weeks or more), without removing all the foreign material from the chest.
  3. Having a defibrillator may have a significant bearing on the patient’s ability to drive. Patients with a defibrillator are not allowed to have a group 2 driving licence and face prolonged periods of driving bans every-time the defibrillator delivers a treatment. For the current guidelines, please have a look on the infographics page:
  4. Having a defibrillator can have a big psychological impact on the patient and his family members and it is not uncommon for patients and their relatives to develop anxiety. This is why psychological assessment and help should always be considered in patients and their families.
  5. A defibrillator is a costly device and can cost £10,000 for the device alone. In America it can cost upto $38000 for the device itself. These costs do not include the cost of having the procedure and subsequent follow-up care.
  6. A defibrillator is only effective in preventing sudden death from ventricular tachycardia and ventricular fibrillation. Patients can still die of other heart rhythms and even just because the heart is so weak that it starts progressively failing as a pump. In this setting, the defibrillator may just act to delay the inevitable but in so doing can make the patient’s last few days extremely traumatic and uncomfortable. Hence it is is not a good idea in patients who have a terminal condition with a limited life expectancy anyway. When a person with a defibrillator becomes terminally ill, it can be a heart-breaking experience for the patient and the family to ask for the defibrillator to be switched off.

Given all of the above considerations it is therefore really important that the patients who are offered a defibrillator are carefully selected on the basis of the level of risk they are exposed to from their diseased heart and the likelihood of benefit that they may obtain from the device. 

There are 2 types of situations in which a defibrillator should be considered to prevent sudden death

  1. Primary prevention: By this we mean that the patient has all the attributes that make him vulnerable to sudden death but has never experienced a life threatening episode such as a cardiac arrest, ventricular tachycardia or ventricular fibrillation before.  A typical scenario is  when a patient has suffered a big heart attack and the damaged heart fails to strengthen up despite the benefit of time and a prolonged course of optimal medications. In this situation, the risk of sudden death is considered high enough to offer the patient a defibrillator for primary prevention.
  2. Secondary prevention: In this situation, the patient has already experienced an aborted sudden death or an episode of ventricular tachycardia or ventricular fibrillation. A typical scenario is when a person suddenly has a cardiac arrest out of the blue with no obvious remediable cause. A few years ago, a footballer by the name of Fabrice Muamba suddenly suffered a cardiac arrest on the football pitch and was successfully resuscitated and therefore had a defibrillator inserted for secondary prevention.

Defibrillator implantation is usually a very straightforward procedure and usually performed under local anesthetic. The patient is not put to sleep under anaesthesia.  The administration of local anaesthetic however means that apart from the initial sensation of the anesthetic being injected into the skin, there is rarely any pain experienced by the patient. The area of skin just below the collarbone is numbed and a small incision of of 5-8 cm is made into the skin. Through this incision, the operator will insert his fingers to make a little pocket which will house the defibrillator box (which contains the battery). Then the operator will cannulate a vein that lies just below the collarbone and insert the wires into the vein and feed them under x-ray guidance into the right heart. The wires are then connected to the pacemaker box and the wound is stitched up. The procedure may take 30 minutes to an hour. The defibrillator is then tested and the patient can usually go home by the next day. The battery of the pacemaker will usually last 5-8 years after which the pacemaker box can easily be replaced and usually the wires are not changed.

So I hope you found this video useful. In the future I plan a video on living with a defibrillator. I would be grateful if you would consider sharing this with anyone you feel may benefit.

 

About the Author:

Dr Sanjay Gupta
I'm Dr Sanjay Gupta, a Consultant Cardiologist with specialist interest in Cardiac Imaging at York Teaching Hospital in York, UK. I believe that high quality reliable jargon-free information about health should be available at no cost to everyone in the world.

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