What is an ICD (Implantable Cardioverter Defibrillator) and ICD Therapy! – Simplified

What is an ICD Pacemaker?

Not a lot of the general public know exactly what a pacemaker is, even less know what an ICD (Implantable Cardioverter Defibrillator) is. Firstly I want to let two video clips demonstrate what an ICD is all about.
Both clips essentially show the same thing – two footballers having fatal arrhythmias (cardiac arrests) and collapsing during a game. In this first video the footballer does not have an ICD implanted at the time but as a result he now does have one implanted. Please do not watch the clips if you will find them distressing. Both players survive.

Fabrice Muamba’s heart stopped for over 70 Minutes he was lucky enough to have survived the ordeal following successful CPR and because by chance there was a Consultant Cardiologist in the crowd watching the football match. Now compare this to the next video in which the footballer has a cardiac arrest just like Fabrice Muamba but he had an ICD implanted previous to the collapse.

You can see his body jolt as the ICD delivers its therapy! Such is the wonder of an ICD that the player in question tried to continue playing in the football match but his medical team wouldn’t let him.

So what is an ICD and what is happening….

An ICD does three things; 

– recognise when the heart goes in to a dangerous arrhythmia 
– treat such dangerous arrhythmias

I will focus on how an ICD recognises fatal arrhythmias in my next post. All you need to know for now is that an ICD can categorise an arrhythmia and decide on the most appropriate action, of which there are three broad options.

1) Monitor.

The ICD can make a recording of how an arrhythmias started, how long it lasted and how it ended. This is then downloadable from the device. You may wonder why I have put this in a treatment option. Really I guess the pacemaker is not treating anything immediately. This information though is integral in the decision making in treatment options moving forward. Do we need to increase/decrease medications and do we need to make the ICD more or less aggressive in ATP or Defibrillation Therapy?

2) ATP. (Anti-Tachycardia Pacing)

Sometimes referred to as overdrive pacing. ATP is very successful in treating Atrial and Ventricular Tachycardias and does not ‘shock’ the patient so is pain free. In fact ATP terminates 85-90% of Ventricular Tachycardias and only accelerates 1-5% of ventricular arrhythmias.
Just as an arrhythmia works by interrupting and hijacking the normal conduction system, ATP works by interrupting and hijacking the abnormal arrhythmias conduction system.
This is possible thanks to refractory periods (which will get their own post soon). The key point of refractory periods is that once a cell has contracted, it needs time to reset before it can conduct (pass on a signal) again.

So let us look at the image of a Ventricular Tachycardia. We know that at the ‘heart’ of this arrhythmia (excuse the pun) is a re-entrant circuit… ATP works by breaking this circuit. 


Re-Entrant Circuit
Re-entrant Circuit
Ventricular Tachycardia V.T.
Ventricular Tachycardia

If we manage to make some of the cells involved in the re-entrant circuit contract at the right time, the circuit will be broken. ATP is a series of multiple impulses delivered very quickly which hopefully land at the right time to make some of the cells in the re-entrant circuit ‘refractory’ i.e. unable to continue carrying the re-entrant signal.

The series of pictures shows this much better than my ramblings!!

Subsequent ‘pulses’ being delivered by the ICD device start to hijack the myocardium (heart muscle) until it breaks the re-entrant circuit. Once the re-entrant circuit is broken the Sino-Atrial Node (Hearts Natural Pacemaker) is able to take control once more.

Anti-Tachycardia Pacing (ATP)
1. Start of ATP The First Impulse Only Captures
A Small Amount of Tissue.
Anti-Tachycardia Pacing (ATP)
2. Next Impulse Captures More Myocardium
Anti-Tachycardia Pacing (ATP)
3. Further Impulses Hijack even More Myocardium
Anti-Tachycardia Pacing (ATP)
4. Enough Impulses are Delivered at the Right Time
Which breaks/interrupts the re-entrant circuit.
Normal Cardiac Conduction
5. Circuit Broken, Normal Ventricular Conduction Prevails.
As mentioned earlier ATP can sometimes accelerate an Arrhythmia (probably making it more dangerous) in 1-5% of cases. This is because the the ATP can potentially change the dynamic of a re-entrant tachycardia or start a new one elsewhere! In these case the device will probably up its level urgency and break out the big guns…

3. Defibrillation

Defibrillation normally comes into place when ATP has failed or when the arrhythmia has been classified as VF (Ventricular Fibrillation).
Defibrillation is a BIG SHOCK, the patient feels it! 
Up to 41J of Energy is delivered in one swift blow. The mechanism is simple; It makes all cells refractory (unable to continue the chaotic mess of signals we see in VF). All the cells then re-polarise (get themselves back to a state where they can contract again) and await instruction to contract again. The Sino Atrial Node is usually the first to initiate a contraction and therefore normal conduction returns.

I think we need another series of pictures….

Ventricular Fibrillation - ICD - Shock
1. Ventricular Fibrillation is Running Wild
The ICD Recognises This
Ventricular Fibrillation - ICD - Shock
2. The ICD Delivers its Defibrillation Shock
Ventricular Fibrillation - ICD - Shock
3. All Cells are Reset (Made Refractory
and then Repolarising)
Ventricular Fibrillation - ICD - Shock
4. Having Been Reset – Normal Ventricular
Conduction Returns

There we have it, the three main treatment options that an ICD has. My next post will be on how the ICD recognises different Arrhythmias.

 Thanks for Reading Time for the 2nd Half of the Football!! (Ironically)

 Cardiac Technician

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