ICD’s Discriminators Part 1 – How ICD’s Recognise Heart Rhythms.

ICD Discriminators – Pacemakers and ICD’s Simplified.

ICD's Discriminators and Pacemakers
If you have read my earlier post on how ICD’s work you will have noticed that I mention that an ICD can categorise an arrhythmia and decide on the most appropriate action.
This is important because some Arrhythmias like VT and VF are dangerous where as others like Atrial Fibrillation are not life threatening. So the ICD needs to be able to say – ok this rhythm is not dangerous lets just keep watching OR wowsers this is the real deal I am going to treat this dangerous arrhythmia with either ATP or Defibrillation and save the patients life.
It is able to do this because different arrhythmias have different properties/characteristics – Some people have blonde hair, some are brunette. 

The device asks questions of the arrhythmia and the answers determine the treatment decision. The questions that the ICD asks are called Discriminators. Multiple discriminators are often used to find out exactly what arrhythmia is occurring. We program how the ICD treats each arrhythmia so all the device has to do is figure out what the rhythm is!

Some Common Types of ICD Discriminator (FAQ’s):-

Narrow/Broad Complex?
When a heart beat comes from the top of the heart it follows a normal conduction pathway – which is fast! This means that all the electrical energy released by a heart beat occurs rapidly and is over quickly.
We call this narrow complex.
If a heart beat is originating from in the Ventricle the electrical energy is released more slowly as the signal is passed on cell by cell and this appears wider, we call this a broad complex.

Narrow Complex QRS and Broad Complex QRS ECG made Easy

Some discriminators look at what the complex ‘looks’ like to decide where about the origin is. Narrow complex? probably originating in the top of the heart we call this a Supraventricular Tachycardia (Translation: Originating above the Ventricles and Fast >100bpm).


What rate is the heart beating? Many arrhythmias have a typical rate bracket… VT is over 100bpm and commonly found to be between 150-200+ beats per minute. So the rate of any arrhythmia helps us to decide on what treatment action to take. Ventricular Fibrillation is upwards of 300 bpm!
A’s and V’s?
One of the key questions a device will ask of an Arrhythmias is the relationship between activity in the Atria (top of the heart) and the activity in the Ventricles (bottom of the heart). In Pacemakers and ICD’s wherever we have a lead in the heart we receive information on electrical activity at the end of that lead – we call this information an Egram (short for electrogram).
Atrial and Ventricular Egram
Atrial and Ventricular Egram
In the image above we can see TWO Egrams from about 6s of heart activity. The top one is the Egram from the lead in the Atria and the bottom one is the Ventricular Egram. The ‘Spikes’ on the top tracing show Atrial activity and the ‘Spikes’ on the bottom tracing showVentricular activity. 
I have circled two events just as an example, if you think of a natural heart beat – the Atria contract there is a delay and the Ventricles contract. This can be seen here, top activity (light blue) a delay and then bottom activity (red). This is probably a natural beat.
When we are categorising arrhythmias the timing and number of Atrial activity (A’s) in relation to the timing and number of Ventricular activity (V’s).
For Example Atrial Fibrillation (below) has more A’s than V’s, remember all the Atrial Activity in AF!!.
In this example the device uses a (P) to mark Atrial activity (A’s) these markers often vary and make life a little confusing – thank you manufacturers 😉
Egram of Atrial Fibrillation
Image Credit: cogprints.org
You can see the device registering the activity the P’s are Atrial markers and the V’s Ventricular makers.  In an Arrhythmia, are there more Atrial events than Ventricular events? Yes well its more likely to be Atrial Fibrillation.
How did the rhythm start? If you run up and down a hill your heart can easily reach rates of 150bpm and more! If we were using the RATE? discriminator (mentioned earlier) then the ICD may start to think that the heart rate is fast enough to be a dangerous arrhythmia…. 
Many arrhythmias, and i am sure some readers will be able to confirm this, come on all of a sudden. Exercise generally raises the heart rate gradually.  The Onset discriminator helps categorise the rhythm by looking at how rapidly the heart rate increased.
Exercise Vs Sudden Arrhythmia
Exercise Gradual Increase of Heart Rate Vs Arrhythmia Sudden Onset
Spikes indicate heart beats!… Rubbish Image but you get the Idea 🙂
Quite simply, is the arrhythmia regular or irregular? For example Ventricular Tachycardia is regular and Ventricular Fibrillation is irregular. We are referring to the interval between one sensed event and the next. Are all intervals the same? or are they irregular. 
VT Ventricular Tachycardia
VF Ventricular Fibrillation
So there you have some examples of discriminators…
Using a combination of these discriminators (questions) the ICD can categorise an arrhythmia and then the treat the different arrhythmias as it has been programmed! Got it? Good!
In my next post on ICD’s I will detail some arrhythmia characteristics and there will be a video clip in there too to make it come to life!
Let me know if you have any comments or feedback… Use the comments box below!!!!
Time to do some washing!
Thanks for reading
Cardiac Technician

Image courtesy of ddpavumba FreeDigitalPhotos.net

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