Pacemakers and ICD’s – Minimising Ventricular Pacing Part 3 – AAI<=>DDD
So far in this series on Minimising Ventricular Pacing – we have discussed WHY we want to minimise Right Ventricular Pacing and also discussed 1st Degree Heart Block and its interactions with minimising RV Pacing. This post is going to explain AAI<=>DDD Pacemaker Mode and how this minimises Ventricular Pacing.
Before reading this post I recommend reading my Introduction to Pacemaker Modes AAI,VVI and DDD
In the previous post on minimising RV pacing we uncovered a problem. In some people (especially at night) there is a long delay between the top of the heart (Atria) contracting and the bottom of the heart contracting. So we can tell a DDD pacemaker to give the bottom part of the heart a longer time to sort itself out and contract before the pacemaker jumps in and paces the right ventricle unnecessarily.
However – due to some intricate algorithms and timing cycles that I will undoubtedly go through with you one day. It can only extend to a certain length. What happens when the delay between the Atria and the Ventricles is longer than this? The pacemaker cannot adapt and will pace the ventricle when you might not necessarily need it!
Hold on a Minute! I can hear a few of you raising a very good point… would you really want a huge delay between the top of the heart and the bottom of the heart contracting? Surely thats bad for you?
You are exactly right, in fact a long delay can lead to mitral regurgitation and pacemaker syndrome – so it is a balancing act. There are instances where we a longer gap between the top and the bottom of the heart contracting are not really a concern and even where we can let one contraction of the heart not be followed by a contraction of the bottom of the heart. These things commonly occur at night in healthy hearts and AAI<=>DDD is particularly helpful in avoiding unnecessary Right Ventricular Pacing in these instances…. and in theory will reduce the incidence of heart failure and atrial fibrillation in patients with pacemakers 🙂
Therefor we do occasionally want a bit of leeway so the solution is AAI<=>DDD Pacing
How it Works?
Well essentially you get the best of two Modes AAI and DDD 🙂 that was pretty obvious right? OK so I am going to give you a very common example of a patient and how this Algorithm (clever bit of programming) fits in!
So we have a patient and they wear a 24 Hour Heart Monitor !
At night there is a long delay between the Atria and the Ventricles contracting (PR Interval) and also every now and again the top of the heart contracts and then the bottom of the heart doesn’t. This only happens once and again and not very often.
In the day between 3pm and 4.30pm the patient went into 2:1 Heart Block – Every second contraction of the top of the heart was not followed by a ventricular contraction. The patients heart rate went down to 30BPM and they felt dizzy and generally unwell.
Now, Observation 1 is often seen in healthy hearts where as observation 2 definitely is not! So a pacemaker was put in.
If the pacemaker is programmed DDD then this will solve the problem of 2:1 Heart Block and stop the patient having these episodes of 30BPM, however as a byproduct the pacemaker will also treat Observation 1, and the patient will be having right ventricular pacing at night when really they do not need it!
When programmed AAI<=>DDD a pacemaker sits happily in AAI Mode (as discussed a lovely pacing mode) at the same time it keeps and eye on the bottom of the heart. It uses a bit of common sense and savvy to let certain things slide that aren’t necessarily dangerous (these vary slightly between pacemaker brands). If it sees something that isn’t OK and decides that action needs to be taken, then the pacemaker switches to DDD a mode where there will likely be more Ventricular pacing! So when reading these remember the pacemaker wants to stay in AAI when it can! When a pacemaker changes mode automatically we call this a Mode Switch (obvious i know).
1. Long PR Intervals (the gap between the top heart beating and bottom of the heart following as planned) – In AAI the pacemaker can instead say “as long as there is a contraction of the bottom of the heart between two contractions of the top of the heart” Then that is ok 🙂 No need to pace here!
|P Waves Represent the top of the Heart and QRS the Bottom of the heart.
So as long as there is a QRS between 2 P Waves then the pacemaker is
Happy and stays in AAI!
In some pacemakers you can tell it that we don’t mind a very long PR interval as shown above but if it keeps happening and the interval is toooo long then enough is enough and we will swap to DDD where we will pace the Ventricle a bit closer to the contraction of the atria.
2. A Contraction of the top of the heart P Wave not followed by a Ventricular Contraction. So as mentioned earlier there are occasions when one ‘skipped’ beat is well worth letting slide and the device will stay in AAI… 2 skipped beats on the other hand… then the pacemaker will swap to DDD and pace the Ventricle.
|One P wave without a QRS – We can let that Slide – Stay AAI
2 P Waves without a QRS? Wow thats Complete Heart Block
Time to switch to DDD and start Pacing the Ventricle
3. Regular P waves without a QRS. In the example above we accept that one P wave without a QRS is ok and we stay in AAI – If this happens too regularly – for example 3 out of the last 12 QRS’s are missing… then we will witch to DDD also!
Now what I have Explained here is very basic, I have also taken it upon myself to explain a proper real life AAI<=>DDD Algorithm used by Sorin – I haven’t simplified it deliberately – I will post it in the next day or so – Give it a read and see how you get along 🙂
A complete explanation of these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.
Thanks for Reading
Now to finish the Newsletter!
Image courtesy of samarttiw/ FreeDigitalPhotos.net