Heart Block Explained
The Conduction System
The conduction system is the network of specialist cells responsible for starting this electrical signal and facilitating its movement through the entire heart. For the heart to beat healthily (and sometimes at all) it is vital for the electrical signal to make it all the way through this specialist network. If the electrical signal is disrupted between the top and bottom chambers of the heart, this is known as heart block.
A normal heartbeat on an ECG as the electrical signal passes through the heart.
|The electrical impulse started at the S.A. node passes through the atria (top chambers of the heart) making them contract. This can be seen on an ECG as the P wave circled above.|
|The PR segment circled above, represents the delay as the electrical signal passes through the A.V. node.|
|As the electrical signal passes into the ventricles (bottom chambers of the heart) it makes them contract. The ventricles contracting are seen as the QRS Complex circled above.|
|The T Wave represents the ventricles ‘recharging’ (preparing to contract again). Today we will be ignoring the T Wave for explanation purposes.|
First Degree Heart Block
First degree heart block is a condition where the electrical signal takes longer than usual to pass through the A.V. node, from the atria to the ventricles. On an ECG, a healthy delay between the beginning of the P wave and the start of the QRS complex is between 120ms – 200ms (PR Interval). 1st degree heart block is any PR Interval that exceeds 200ms.
PR Interval is between 120-200ms
|1st Degree HB
PR Interval is longer than 200ms
Most people with first degree heart block rarely feel any symptoms as their heart rate will not be affected (heart rate is defined by ventricular contractions).
Isolated 1st degree heart block is not an indication for a permanent pacemaker implant.
2nd Degree Heart Block
2nd degree heart block refers to conduction system disorders where some, but not all electrical signals fail to conduct through the AV node. Without the electrical stimulus reaching the ventricles they will fail to contract and heart rate will be inappropriately low. As a result people with 2nd degree heart block may experience light-headedness, mild to severe dizziness and sometimes fainting.
2nd Degree Heart Block Mobitz Type I (Wenckebach)
Mobitz Type I (also called Wenckebach) is usually considered the ‘milder’ type of 2nd degree heart block. It describes a pattern where the delay between the atria contracting and the ventricles contracting lengthens until eventually one signal fails to conduct through altogether.
These ECG’s of a normal heart beat and Mobitz I will help explain this.
Normal Heart Rhythm
|The PR segment remains constant throughout and each atrial contraction is followed by a ventricular contraction.|
Mobitz I (Wenckebach)
|The PR segments lengthen until one is completely blocked and a ventricular contraction fails to occur. Afterwards the PR segment returns to normal duration.|
Mobitz I is usually a benign rhythm because the majority of signals do make it through to the ventricles. As a result not too many heart beats are ‘missed’ and blood supply to the body is not too impeded.
It is very unusual for isolated Mobitz I (Wenckebach) to require the implantation of a permanent pacemaker.
2nd Degree Heart Block Mobitz Type II
Mobitz Type II is a type of 2nd degree heart block where the delay between the atria contracting and the ventricles contracting stays constant but every now and again one signal is blocked and fails to conduct through to the ventricles.
|PR segments remain constant however atrial contractions are not always followed by ventricular contractions as the electrical impulse is blocked. (In Mobitz II the electrical signal is actually usually blocked just below the AV Node.)|
There can be a pattern to the block or it can be sporadic. When there is a pattern the heart block is often referred to with a ratio.
2:1 Heart Block (two to one heart block)
Two atrial contractions to every one ventricular contraction.
|Every second electrical signal is blocked before it reaches the ventricles. So only the atria are seen contracting on the ECG. 2 P Waves to every 1 QRS.|
3:1 Heart Block (three to one heart block)
Three atrial contractions to every one ventricular contraction.
If 2:1 heart block occurred in a heart wanting to beat at 60bpm, the actual heart rate would be 30 bpm.Patients with this type of heart block are much more likely to be symptomatic and a permanent pacemaker is the treatment of choice to alleviate these symptoms.These people are also much more likely to develop complete heart block.
3rd Degree Heart Block (Complete Heart Block)
If you have read through this post then you are probably assuming that complete heart block is where no electrical signals make it from the atria to the ventricles… and you would be right.
This may also leave you thinking that complete heart block would mean your ventricles do not contract, your heart rate would be 0bpm and you would die. Luckily the heart has a safety mechanism called an escape rhythm that prevents this.
You can read a full explanation of escape beats here, but essentially escape beats are heartbeats that originate from an alternative part of the heart to the S.A. node. Escape beats originate from a region of conductive tissue that is below that of the block.
Because the escape rhythm is independent from the atrial rhythm there is no relationship between the two on the ECG.
Complete Heart Block on an ECG
|Complete heart block there is no relationship between the atrial and ventricular events. All electrical signals are blocked before they reach the ventricles, instead an escape rhyhm has kicked in.|
|The same rhythm strip as above but this time the atrial contractions are drawn in black and the escape rhythm overlaid in red. This should make it clearer that the two rhythms have no relationship.|
The further down the conduction system that the escape rhythm occurs the slower it is. Patients in complete heart block can present to hospitals with heart rates in the 20’s!
Needless to say that pacemakers are necessary to treat complete heart block.
Thanks for reading,