Medtronic aCRT – Minimising RV Pacing in BiV Pacing!

Biventricular Pacing
Pacing Leads in a Biventricular Pacemaker

Medtronic aCRT – Minimising RV Pacing in BiV Pacing!

Biventricular Pacing (CRT)

In previous posts I have explained CRT (Cardiac Resynchronisation Therapy) otherwise known as Biventricular Pacing. Biventricular Pacing is all about timing! Controlling when the Ventricles contract in relation to the Atrium and when the Left and Right Ventricle contract in relation to one another. It is this control that allows us to improve the pumping ability of a persons heart
An example of this would be making the Left Ventricle contract at the same time as the Right Ventricle in a heart where this no longer happens naturally.

The AV Delay!

When a heart beats, it begins in the top of the heart (Atria) then there is a small delay as it passes through the AV Node before reaching the bottom of the Heart (Ventricles). This delay is very important in Biventricular Pacing.

Why is this Delay Important?

Biventricular Pacing
The AV Delay – Time between the Atrial Contraction and
the larger Ventricular Contraction

As explained earlier we want to take control of how the bottom part of the heart contracts. We do this in the population of patients who’s natural Ventricular contraction is abnormal, substandard and out of sync! So when we are Biventricular Pacing we have to start the device controlled contraction BEFORE the natural one kicks in.
By doing this we NEVER allow any natural conduction of a Heart Beat and we ALWAYS ‘Pace’ the Right Ventricle and the Left Ventricle.
We do this by KNOWING the delay in milliseconds between the top of the heart beating and the bottom of the heart following suit (AV Delay). The device cuts in just before the natural Ventricular contraction occurs.
This works pretty well in improving the efficiency of an individual heart beat and in turn increasing Cardiac Output (the amount of blood being pumped per beat). In patients with Biventricular pacemakers any extra oomph given to each heart beat is a bonus – and is what makes the patient feel better.

So it works this is great… what is the issue?

Unfortunately with this therapy, it is necessary to always pace the Right Ventricle. This does come with some side affects;- An increase risk of Heart Failure and an Increased risk of Atrial fibrillation.
These devices also require ‘optimisation’ to maximise benefits of having this type of pacemaker. Optimisation is the process of finding out the best sequence to make the Left Ventricle and Right Ventricle contract in relation to one another to get the best results. This can often be time consuming and is not an exact science.

Medtronic Viva XT
Viva XT from Medtronic is
Armed with aCRT

This all sound pretty glum 🙁 what is the solution?

There are Algorithms (programming options) with in some newer Pacemakers that show promise in solving or minimising this problem!
One such Algorithm is Medtronics aCRT.

What is aCRT?

aCRT stands for Adaptive Cardiac Resynchronisation Therapy. It is a clever program comprised of TWO solutions that are really very clever!

Adaptive LV Pacing

One solution called AdaptiveLV Pacing looks to give RESYNCHRONISATION but without having to pace the Right Ventricle. The way it does this is brilliantly simple.
Firstly it is worth mentioning that this type of Pacemaker and Algorithm is recommended in patients with Left Bundle Branch Block. This is a condition affecting how the heart beat moves through the heart, without getting technical all you need to know is this:-
In Patients with Left Bundle Branch Block, the Right Ventricle contracts ‘early’ in comparison to the LEFT Ventricle… The Right Ventricle contracts first!
The AdaptiveLV Pacing software does the following.
  1. Intermittently watches a natural heart beat to measure the natural AV Delay.
  2. It calculates that the LV should be stimulated about 70% of the AV delay to synchronise the contraction of the LV with the RV.
  3. Puts this in to practice and paces the LV after 70% of the natural AV Delay. 

Thus just LV pacing but at a time where it gives us a truly synchronised heart beat.

Medtronic aCRT Adaptive LV Pacing

Those clever cloggs out there might be asking the following question. “Why not just pace the Left Ventricle as soon as the Pacemaker ‘Sees’ the Right Ventricle Contract?’ Answer:- this is too late. By the time the Contraction in the Right Ventricle has been detected, the device has figured this out and then the Left Ventricle has been stimulated – an efficient heart beat has not been achieved!
This part of the Algorithm is only suitable during the following conditions:- 
  • Normal AV Conduction (upto 250ms in paced events) 
  • Heart Rates under 100 BPM
The reason behind this is the importance of Atrial and Ventricles contracting pretty close together. If the Ventricles contract too long after the Atria have contracted the heart beat is less efficient so the benefits would be nullified.
This part of the Algorithm is GREAT at reducing RV Pacing – because under the right circumstances there is no Right Ventricular Pacing required!

Adaptive BiV Pacing

The second prong of aCRT is Adaptive BiV Pacing. 
Adaptive BiV Pacing is useful and works when Adaptive LV Pacing is not.  So it works when:-
  • A patient has a LONG natural delay between the Atria and Ventricles contracting (1st Degree HB)
  • When a person with LV Adaptive Pacing has their heart rate go above 100BPM
What is Adaptive BiV Pacing?
Very similar to traditional BiV Pacing in that the device paces both the Right Ventricle and the Left Ventricle. The only difference is that it changes the exact timing of when the pacemaker does this. The timing of the BiV Pacing in relation to the Atrial contraction is vital. The device has a quick gander and watches a natural heart beat every minute. Once it has this information it initiates its BiVentricular Pacing 30 ms after the end of the Atrial contraction but at least 50 ms before the onset of the natural Ventricular contraction. This is what the device and studies have suggested is a good idea 🙂
Another clever part of the algorithm makes tiny changes to the timing of the LV and RV Pacing in relation to the Intrinsic QRS duration. You do not need to know the maths for this but just know that its clever stuff!
Medtronic aCRT Adaptive BiV Pacing
This Part of the Algorithm is GREAT at reducing Heart Failure Symptoms!

I know you guys like evidence that this stuff works, So here is a little taster!

AdaptivCRT Long Term Outcomes presented at HFSA Late Breaking Clinical Trials on Monday, Sept 23
Data presented by Dr. David Martin, MD: Cleveland Clinic and represents longer term follow up than previously published (average of 20 months vs 6 months).
AdaptivCRTReduced AF Risk by 46%
As compared to patients receiving echo optimized CRT
aCRT Evidence Medtronic
HF Hospitalisation – aCRT vs Control
Normal PR Interval sub-group

aCRT Evidence Medtronic

As you can see from the article, this Algorithm depends on a Stable and Predictable AV Delay – In Atrial Fibrillation this does not exist and for this reason this software is not suitable for patients with Permanent AF.

Further explanation around these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.

Thanks for Reading

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Cardiac Technician

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