Pacemakers and ICD’s – Minimising Ventricular Pacing Part 2 – 1st Degree Heart Block

1st Degree Heart Block

What is First Degree Heart Block and Why it Matters in Minimising Ventricular Pacing.

During one complete heart beat there is a deliberate delay between the Atria (top of the heart) contracting and the Ventricles (bottom of the heart) contracting. This delay is not very long in a healthy adult heart, somewhere between 120 and 200ms.
If the delay between the top of the heart contracting and the bottom of the heart contracting is over 200ms then this is classified as 1st Degree Heart Block. It has not been shown to have any impact on life longevity but has been shown to DOUBLE the risk of Atrial Fibrillation.

How We Measure 1st Degree Heart Block

First Degree Heart Block
P wave
If you remember on an ECG the top of the heart contracting is shown by the P wave, the bottom of the heart contracting is shown by the QRS complex.
The flat line between the two is the delay between the Atria contracting and the ventricles contracting. This is known as the PR interval.
We measure the time period on the ECG, between the start of the P wave and the first deflection of the QRS – if this is over 200ms that person has First Degree HB.
First Degree Heart Block - Right Ventricle Pacing
QRS Complex
Long 1st Degree Heart Block (over 300s) has occasionally been found to cause,  breathlessness, dizziness, palpitations, chest fullness/pain, fatigue and pulsation in the abdomen or neck.
These symptoms and the physiology behind them are similar to pacemaker syndrome, because of this it has been coined pseudo-pacemaker syndrome when these symptoms occur.
The PR Interval
Whilst treating 1st Degree Heart Block (with no other conduction issues) with a pacemaker has not been shown to improve survival rates. Pacemakers have been seen to reduce symptoms and as a result long First Degree Heart Block is a Class IIa indication for a pacemaker.
First Degree Heart Block is an indicator of conductive tissue disease and as a result is associated with a tripled risk of eventually requiring a pacemaker when it is diagnosed.

Why it matters in Minimising Ventricular Pacing?

This is a good question (even though I am biased as I asked it).
Lets think about how dual chamber pacemakers work. They have a lead in the atrium (top of the heart) and a lead in the ventricle (bottom of the heart). In heart blocks the top lead ‘sees’ a heart beat and the bottom lead waits to see if the bottom of the heart responds appropriately. If the ventricles contract (as they should) the pacemaker does nothing, if the signal doesn’t make it through naturally (is blocked) then the pacemaker sends a pulse down the ventricular lead to make it beat.
In first degree heart block there can be a long delay between the top of the heart contracting and the bottom of the heart contracting. So when does the pacemaker kick in? There has to be a cut off point where it says ‘”ok, I have given the ventricles plenty of time to play ball, they aren’t going to contract so I am going to intervene, I am too kind”
The time that the pacemaker waits is called the AV delay (A=Atria, V= Ventricles) this is a programable setting and comes as a preset somewhere between 150-220ms.
Now you have probably figured out already that if a Patient has a First Degree Heart Block of 250ms and the pacemaker has an AV delay of 200ms the the pacemaker will never give the heart enough time to work of its own accord and will ‘pace’ the ventricle when maybe it was not necessary.
See my poor drawing below:

IF a patient with 1st Degree Heart has a pacemaker this can cause the pacemaker to Ventricular pace when it didn’t necessarily need to! Remember we are trying to minimise Right Ventricular Pacing!
As technicians we can extend this delay to allow enough time for the natural heart beat to come through in patients with 1st Degree Heart Block which will minimise Ventricular Pacing. However in a standard DDD mode (dual chamber) we can only extend the AV delay so far and this is not always enough. Some 1st Degree Heart Blocks are over 400ms!!! luckily there are some very clever solutions and I will be explaining those in my next post 🙂
I want to leave you with something to ponder though! 
  • 1st Degree Heart Block has been shown to DOUBLE the risk of Atrial Fibrillation and cause symptoms. 
We have the ability to choose whether a patient has more pacing and no 1st Degree Heart Block OR less pacing and more 1st Degree Heart Block! So which do we chose?
A complete explanation of these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.

Again another good question 🙂
Thanks for Reading
Cardiac Technician
Again your comments below are welcomed!

Image courtesy of photoraidz /

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