The Conduction System

Heart Block Explained Simply

Heart Block Explained

Having written many posts on pacemakers, it suddenly dawned on me that I had not fully covered heart block. This seems crazy now as people with heart block make up huge percentage of those with pacemakers.
During each heartbeat an electrical signal moves like a wave through the heart’s cells causing the heart muscle cells to contract (shorten). It is this mass synchronised contraction of cells that causes the heart to beat.

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.

The conduction system can be seen here in green, firstly as part of the heart and then as a simplified version.
Cardiac Heart Conduction System
Red Dot = Sinoatrial Node (S.A. Node or Sinus Node)
Top Set of Green Tracks = Specialist Conductive Tissue Running Through the Atria
Blue Dot = Atrioventricular Node (A.V. Node)
Bottom Set of Green Tracks = Specialist Conductive Tissue Running Through the Ventricles


The electrical impulse starts at the S.A. node and moves through the atria before reaching the A.V. node. The A.V. node acts as a gateway passing on the electrical signals to the ventricles after a short and deliberate delay.
Please Note: This delay enables the atria to contract and fill the ventricles with extra blood before they then contract pumping blood around the body.  Click here for a full explanation of how the heart beats.

Heart Block

The term heart block is generally used to describe a range of conditions where the electrical signal is slowed or even blocked at the A.V. node, before it reaches the ventricles. This is usually as a result of disease, medication, congenital heart defects, vagal tone or age.
The best way to explain or help people understand heart block is by showing how they present themselves on an ECG (rhythm strip). Please do not stop reading I promise this will be simple.

A normal heartbeat on an ECG as the electrical signal passes through the heart.

Heart Block 1st Degree, 2nd Degree, Complete
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.
Heart Block 1st Degree, 2nd Degree, Complete
The PR segment circled above, represents the delay as the electrical signal passes through the A.V. node. 
Heart Block 1st Degree, 2nd Degree, Complete
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. 
Heart Block 1st Degree, 2nd Degree, Complete
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.

Heart Block

Shorter PR Segment

PR Interval is between 120-200ms
Heart Block 1st Degree, 2nd Degree, Complete

An elongated PR Segment

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).

The Framingham Heart Study did show that 1st degree heart block doubles a person’s risk of developing atrial fibrillation and triples their risk of requiring a pacemaker.

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

Heart Block
The PR segment remains constant throughout and each atrial contraction is followed by a ventricular contraction.

Mobitz I (Wenckebach)

Heart Block
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.

Heart Block
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.

Heart Block
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.

Heart Block
Only every 3rd electrical signal conducts through to the ventricles. 3 P Waves to every 1 QRS.

Mobitz Type II can be a much more clinically significant finding, with more blocked electrical signals likely, the heart rate can drop significantly and lead to severe bradycardia (slow heart rate).
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

Heart Block
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.
Heart Block
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.

If you are interested in a full introduction to pacemakers then my book Pacemakers Made Easy is available electronically on Ibooks or in hard copy.

Thanks for reading,



Comments 9

  1. Profile photo of Glenda Howie

    I have purchased and read ‘Pacemakers Made Easy’ – and it’s a great book – but reading through this again was great also. I have total heart block and it’s wonderful to have it explained so well. Thanks,Carl.

    1. Profile photo of Kristian Webb Post

      Hi Glenda and thank you so much for your feedback! If you do have any questions in the future you can pop them on the forum and eventually one of the ‘experts’ on the site will get around to answering you… or attempting to answer you 😉


  2. Hi Carl,
    I must admit to some confusion. What you refer to as the PR interval, I believe is actually the PR segment. The PR interval begins at the beginning of, not the end of the P wave and ends at the beginning of the QRS complex. Apart from this (and please elaborate if you believe I am incorrect here), I have always found your postings and exlanations incredibly helpful and frequently refer both my students and patients to your site. Congratulations on a fantastic new website and great new format by the way!!!
    With kindest regards,

    1. Profile photo of Kristian Webb Post

      Hi Antony!
      You are exactly right in all that you say… I have described the PR interval but then drawn on the end of the P Wave to the first QRS deflection. So to all that are confused the PR interval starts at the beginning of the P Wave to the first deflection of the QRS. I have highlighted the isoelectric point between the end of the P Wave and beginning of the QRS on my illustrations. This is not the PR interval but I guess could be referred to as the PR segment in much the same way an ST segment describes the time between the end of the QRS and the start of the T Wave. Really appreciate the heads up Antony!

      1. Profile photo of Kristian Webb Post

        I write quite a lot about cardiology and much of it ends up online. Human nature means I will of course make mistakes – something that I believe prevents other healthcare professionals from being active on the internet. I am always suprised and delighted that mistakes I have made are politely pointed out and I genuinely appreciate the feed – you are essentially my copywriters and you are all extremely good at it! 🙂

  3. Hi again Carl,
    I have recently come across a couple of articles that suggest septal pacing (in temporary transvenous pacing) is more beneficial for the patient than floating a temporary wire into the RV apex. The readings mentioned benefit in the ability to capture and pace the bundle branches potentiating a more physiological conduction and even biventricular pacing if the electrode can capture the bundle branches. We actually floated a bipolar temporary wire in an emergent situation a few months ago and I was surprised to see a rather narrow QRS complex. Chest X-ray showed the tip of the wire rather high against the septum in the RV. I’d be very very interested to hear your thoughts on this, and whether you know of any studies which compare apical vs septal pacing.

  4. Profile photo of Maggie

    This has explained so much!! I have complete heart block and when I was shown my ECG from my Loop recorder I just looked at it and nodded…..making a strange agreement of understanding sound… however; I think i get it!
    So the ‘long pauses’ I was told about/shown are the escape rhythm without the atrial rhythm?
    Good work, I think my cardiac team should download your books, they will be my Christmas presents to them amd my family. Saves me being a broken record 🙂


  5. Profile photo of Meri

    Hey Maggie , I’ve read that you have complete heart block, and I have too. I was diagnosed with congenital heart block 3rd degree, but I still don not have a pacemaker. Here, in my country Macedonia, the doctors said there is no need of a pacemaker, because I don’t have problems at all. I was diagnosed at 21 years, now I am 30 and still the same situation, doctos say the same and have no problems. The last time I was asking about pregnancy, what If I get pregnant?! And I’ve got the same answers. Could you all say a word to me, what do you think??????

  6. Hi, I’m a 36 year old female and train (exercise) alot and always have done. I do mainly cv work for about 40 -45 mins 5 or 6 days a week at a very high intensity. I have had a few 24 hr holters for skipped beats and they have found that I sometimes (but not all the time) get 1st degree heart block during the day and get episodes of 2nd degree type 1 (wenkebach) during my sleep at night. My cardiologists are not concerned and haven’t recommended any treatment, they say it is due to my “increased vagal tone” as I am physically fit but they haven’t explained what that means so I am worried as I don’t really understand what is causing it or how I can stop it from occurring. I generally feel fine training but can sometimes get light headed if I stand up quickly and i often feel extremely tired and a bit breathless. Can anyone help as I can’t find any info on the net about vagal tone and what it is? Thanks, Lucy

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