Pacemakers and Complete Heart Block.
Treating Complete Heart Block (CHB) with a Cardiac Pacemaker
is probably one of the easier concepts for me to explain!
Before we continue we need to understand Complete Heart Block.
The S.A. Node (Sinoatrial) is the hearts natural pacemaker it lives in the top of the heart (top of the Right Atrium). In a healthy heart it is the musical conductor of rate and rhythm. Starting each heart beat with a self generated electrical signal that travels through the other heart cells like a Mexican wave. This electrical potential causes each cell to contract (get smaller) as passes by.
It is this synchronised contraction of cells that causes the heart to contract pumping blood around the body.
|Signal Travels through the Atrium and into the
Ventricles via the AV Node.
The Electrical Signal does not just propagate over the entire heart! it is far cleverer than this. It moves through the heart with specific timing and with specific order.
1)Through top of the heart (Atrium)
3)Through the bottom of the heart (Ventricles)
|Efficient order of
It does this to maximise its pumping efficiency (please read how the heart beats for a detailed explanation) but in short from the top contracts first pushing more blood into the Ventricles (bottom of the heart) and then the Ventricles are able to contract with more force and volume pumping blood up around the body.
|Complete Heart Block – Electrical Signal
that causes the Heart to beat does not pass
through the AV Node into the Ventricles.
Complete heart block is when the signals from the top chambers of the heart fail to reach the chambers in the bottom of the heart. They do not pass through the AV Node – In most cases because the AV Node has become diseased and lost its conductive properties. Other Heart Blocks exist like 2-1 where every second Atrial Contraction is ‘blocked’ at the AV Node. Complete Heart Block is when all Atrial Signals are blocked for a period of time (sometimes permanently).
Remember the bottom of the heart does MOST the work and is therefore the most important.
So the signal telling the bottom of the heart to beat isn’t reaching it?!? We all know that the bottom of the heart is doing most of the work keeping you alive!!! So why don’t we drop down dead when we go into complete heart block?!?
I have done a comprehensive explanation of escape beats
so please read it but the salient facts are thus:- The SA Node is not the only group group if cells that can initiate an electronic signal. There are other groups of cells that have this property… But generate an electrical charge of their own far slower! When a normal heart beat occurs these cells never get the opportunity to self initiate (they are reset by each natural heart beat)
In an unhealthy heart where the signals are disrupted – escape beats
occur as a safety mechanism.
So when the signal does not pass through the AV Node after a short time the heart will think… oh wow I need to do something and contract anyway.
|Schematic of the Hearts Escape Beats and their Average Beats Per Minute
They generally take longer the lower down the conductive circuit you go! and these can be seen in the picture above. If you are beating along at 30 BPM the chances is are you will feel unwell your slow heart rate will be picked up on and you will end up having a pacemaker.
|The Pacemaker sits outside the Heart with the Leads Inside.
A pacemaker comes into play by seeing that the top of the heart has contracted and delivering that signal to the bottom.
As the electrical signal passes through the Atrium it also passes up the pacemakers
Atrial lead and is ‘seen’ by the pacemaker. The pacemaker now knows that the bottom of the heart should follow suit shortly after.
Two things can then happen.
The heart works normally with the signal passing through the AV Node and into the Ventricle. The signal passing through the Ventricles also passes up the Ventricular Pacemaker Lead and is ‘seen’ by the pacemaker. The pacemaker knows everything is fine and does nothing.
Scenario 2. Complete Heart Block Occurs – The Signal is Blocked at the AV Node (Heart Block) and the Ventricles don’t contract in turn as they should. The pacemaker does not ‘see’ a contraction so instead sends an electrical impulse down the lead that instigates a Ventricular contraction – pacing the heart.
To explain some pacing percentages consider this….
If you are in Permanent Complete Heart Block then this will happen 100% of the time. We would then expect to see that you are 100% A Sense (seen the Atrial Activity) and 100% V Paced (Paced in the Ventricle). If you were in and out of Complete Heart Block 50% of the time we would expect to see you are 100% A Sense and 50% V Paced.
Further explanation around these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.
I hope this explains how a pacemaker treats AV Block and Complete Heart Block.
Thanks for Reading
Time for a Nandos