The term 'heart block' refers to a delay or stoppage at some point of the electrical pathway of the heart, and is a medical diagnosis made on the basis of an electrocardiogram (ECG). Depending on the variety, heart blocks may produce slow heart rates leading to dizziness or blackouts, or may produce no symptoms at all.
The Heart's Conduction System
The heart consists of two separate pumping systems, one on the right for the lungs and one on the left for the rest of the body. Each pumping system consists of two chambers, an atrium and a ventricle. The conduction system of the heart is designed to trigger contraction of the heart muscle in such a way that blood is pushed through from the atria into the ventricles, and then up out of the ventricles into the main arteries. The electrical signal is therefore generated in the right atrium by a natural pacemaker known as the sinoatrial (SA) node. From there, the electrical signal spreads across the atria, causing them to start to contract. In order to stop the signal from spreading across the whole heart at once, the ventricles are electrically insulated from the atria. The signal that has spread across the atria must travel to a checkpoint known as the atrioventricular (AV) node in order to continue on towards the ventricles.
Having started the contraction of the atria, the electrical signal is held up at the AV node for around 0.1 seconds in order to give the atria enough time to do their job. Having passed through the checkpoint, the signal now needs to jump straight to the base of the heart before spreading upwards, as the ventricles need to contract from the bottom upwards in order to squeeze the blood up into the main arteries. The signal does this by travelling in the wall between ventricles along a conductive pathway known as the bundle of His1, which then branches into right and left bundles in order to carry the signal to the bases of the ventricles. The branch heading to the left ventricle then splits further into an anterior and a posterior division. The signal runs up from the base of the ventricles, causing the heart muscle to contract and force blood out into the arteries. Having contracted, the heart muscle then relaxes and the cells reset their electrical potentials ready for the next heart cycle.
- The SA node generates an electrical signal.
- The electrical signal spreads across the atria, causing them to contract.
- The signal is held up at the AV node while the atria finish contracting.
- The signal runs down towards the base of the ventricles via the bundle of His.
- The signal passes along the left and right bundles and then up through the ventricles, causing them to contract.
- The heart muscle relaxes and the cells reset.
Put simply, the ECG is a view of the above electrical activity gained by placing leads on the patient's body. In a regular ECG, the contraction of the atria comes first, represented by a small bump known as a P wave. This is followed by the well-known upward spike that represents the contraction of the ventricles. This is known as the QRS complex, and is followed shortly after by a second small bump known as a T wave, which represents the ventricles resetting.
The gap between the P wave and the QRS complex is known as the PR interval. Most of the PR interval is due to the delay at the AV node, and so a prolonged PR interval usually indicates that there is something amiss with the node.
- The SA node generates an electrical signal.
- The electrical signal spreads across the atria, causing them to contract → P wave.
- The signal is held up at the AV node while the atria finish contracting → PR interval.
- The signal runs down towards the base of the ventricles via the bundle of His → PR interval.
- The signal passes along the left and right bundles and then up through the ventricles, causing them to contract → QRS complex.
- The heart muscle relaxes and the cells reset → T wave.
Atrioventricular (AV) block occurs when there is some delay or disruption of the electrical signal at the point where it passes through the AV node. AV block can be classified according to its severity.
- First degree AV block
- Second degree AV block
In Mobitz type I AV block (aka Wenckebach), the PR interval becomes longer with each beat until the AV node reaches a point where it cannot conduct a signal and the ventricles fail to contract. Thus, the gap between the P wave and the QRS complex lengthens until there is a P wave without a QRS after it. The AV node then recovers and the pattern starts again, with the PR interval once more becoming longer until a signal fails to pass through.
In Mobitz type II AV block, the same pattern occurs, only the PR interval does not lengthen. Mobitz type II tends to be due to a blockade in the bundle of His instead of at the AV node - this explains the lack of lengthening PR interval, as the bundle of His is responsible for only a minority of the PR interval.
In advanced second degree AV block, several signals have to be sent towards the AV node before one manages to make it through and activates the ventricles. This sort of block is classified according to the number of P waves seen before a QRS complex is successfully generated - for instance, two P waves to every one QRS complex is referred to as 2:1 block. Advanced block may be due to a blockade in either the AV node or the bundle of His.
- Third degree AV block
This is the least severe, and is a simple diagnosis made when the PR interval is prolonged to more than 0.22 seconds. Although the signal is delayed, every signal still makes it through and causes the ventricles to contract and the QRS complex still follows every P wave.
Second degree block is more complicated, as it covers all situations where the signal sometimes makes it through and sometimes doesn't. Second degree block may be classified as Mobitz type I, Mobitz type II or advanced.
Third degree block represents a complete blockade between the atria and the ventricles such that the signal from the SA node cannot pass through to the ventricles. The ventricles still continue to contract, but are driven by their own 'escape' rhythm, an electrical signal produced downstream from the block which acts as a failsafe to keep the heart beating. These signals are always present in the heart, but occur at slower rates than that produced by the SA node and are thus drowned out by the normal signal in healthy individuals.
In the case of a complete blockade at the AV node, the bundle of His can often produce this escape rhythm signal, which then travels down to the ventricles as per normal. This is seen on the ECG as a normal-looking 'narrow' QRS complex, and as such the escape rhythm is described as being a 'narrow complex' one. Narrow complex escape rhythms providing something reasonably close to normality, with the heart rate being maintained at around 50 beats per minute.
On the other hand, a complete blockade in the bundle of His can force the ventricles to provide their own escape rhythm. As the signal does not originate from the usual pathway, it is forced to travel slowly from one side of the ventricles to the other, and thus produces an abnormal-looking, 'broad' complex on the ECG. Furthermore, this 'broad complex' escape rhythm occurs at a slower rate than those produced by the bundle of His and the heart rate tends to fall to 15-40 beats per minute. Broad complex escape rhythms are unreliable and dangerously slow, leading to low blood pressure, poor circulation and a risk of collapse.
Causes of AV Blocks
An AV block may be precipitated by drugs designed to control the heart's rhythm, but can also be due to damage to the heart's electrical pathways caused by a number of factors:
Congenital malformations of the heart. Infections such as infective endocarditis, Chagas disease or Lyme disease. Damage following heart surgery. Heart disease such as a heart attack or cardiomyopathy. Connective tissue disease such as lupus or rheumatoid arthritis. Neuromuscular disease such as Duchenne muscular dystrophy. Infiltration of the heart by cancerous cells or due to sarcoidosis.
Two specific syndromes known as Lev's disease and Lenegre's syndrome can also lead to an AV block. Lev's disease causes fibrosis and calcification of the conductive pathways of the heart in elderly patients, destroying the pathways to the point that the patient has a third degree block with a broad complex escape rhythm. Lenegre's syndrome is an inflammatory disease that causes much the same effect in younger patients, though the cause of either disease is currently unknown.
Treatment of AV Blocks
While first degree AV block requires little treatment other than stopping any offending drugs and correcting any imbalance in the ions in the patient's blood, second and third degree blocks are more serious and thus require an investigation into the underlying cause.
Out of the different types of second degree AV block, Mobitz type I is the least serious and only warrants a permanent pacemaker if the patient is experiencing symptoms of dizziness or collapse. Mobitz type I block rarely progresses to third degree heart block, and tends to produce a reliable escape rhythm if it does, as the cause of the block is usually in the AV node and thus the bundle of His is still able to produce a reliable escape rhythm for the ventricles.
Mobitz type II block represents a greater threat to the functioning of the heart and will usually require a permanent pacemaker. This is because type II blocks are liable to progress to third degree blocks and, as they usually result from a blockade in the bundle of His, are liable to produce an excessively slow and unstable escape rhythm if the block does progress to third degree.
As well as treatment of the cause, third degree AV blocks often require electric pacing. Narrow complex escape rhythm blocks only require a permanent pacemaker if the patient experiences symptoms of dizziness or collapse, or if the block is associated with heart disease. If the cause is a heart attack then, depending on the part of the heart affected, a third degree block can either be transient (as in the case of an inferior infarct, which requires temporary pacing only) or permanent (such as with an anterior infarct, thus requiring a permanent pacemaker). Broad complex rhythm blocks often provoke blackouts known as Stokes-Adams attacks, and are thus treated with a permanent pacemaker or an implantable cardioverter defibrillator2.
Bundle Branch Block
As mentioned, the bundle of His splits into left and right bundles, with the left bundle branching further into an anterior and a posterior division. A blockade of the right bundle is known as right bundle branch block; a blockade of the left bundle is known as left bundle branch block. A blockade of one of the divisions of the left bundle is known as a hemiblock - thus, left anterior hemiblock and left posterior hemiblock. A blockade of the right bundle plus one of the left divisions, or of both left divisions, is known as a bifascicular block. A block of the right bundle plus both left divisions is equivalent to an AV block.
Bundle branch blocks can occur alone or as part of a heart condition (such as a heart attack, congenital defect or cardiomyopathy) or a lung condition (such as a pulmonary embolism). In the latter cases, the underlying cause is treated. Bundle branch blocks tend not to produce symptoms, although some may cause dizziness and collapse due to either an intermittent third degree AV block or a fast heart rhythm resulting from the bundle branch block. In these cases, ECG monitoring can be used to investigate the cause of collapse. The signs produced on an ECG by a bundle branch block are more complicated than those for AV blocks and will not be described here.