Bundle branch block

What is bundle branch block?

Throughout the heart there's a system of nerves that conduct electrical signals telling the heart muscle to contract. Think of it like the heart's electrical wiring system.

As signals travel through the heart, they tell different bits of heart muscle to contract in a specific order, to push blood through.

1. The first phase is a resting phase, where the heart fills with blood.

2. In the second phase, an electrical signal begins in a spot high up in the atria or small upper chambers, called the sino-atrial node. This signal spreads across the muscles of these chambers, making them contract, squeezing blood into the ventricles or lower chambers.

3. Finally, waves of electrical activity wash through the ventricles, making them contract and pump blood into the lungs and into the large arteries of the body.

So electrical signals start in the node and spread out through a network of conducting fibres across the heart.

Heart block is like a fault in one of the wires, which interrupts the passage of the nerve signal. This means one patch of muscle doesn't receive the signal to contract, or receives it late once the signal has travelled in a roundabout way to avoid the broken circuit.

How much this signal blockage affects the heart depends on where it is.

If it's close to the node, then the signal to contract never reaches the ventricles and they end up beating totally out of time with the atria.

The ventricles usually end up beating much more slowly, about 40 times a minute. It's a bit like a fault in the main electricity cable coming into your home - most of the house is affected.

This is called complete heart block. It can cause problems, such as low blood pressure or inadequate blood supply to vital organs, because the heart isn't working efficiently as a pump and the chambers aren't filling very full of blood during each heart beat.

More importantly, the ventricles don't respond in the normal way to exercise by beating more quickly, instead they just carry on at about 40 beats a minute and this can severely limit a person's ability to exercise.

Bundle branch block, however, is a bit like having a faulty wire to your spare bedroom.

The break is on one of the more minor branches of the wiring and, consequently, usually causes fewer problems.

Symptoms

In its mildest form, there's just a slight delay between different chambers of the heart contracting, which can result in occasional missed heart beats. Sometimes it can cause frequent missed beats and this may lead to symptoms such as heart failure, chest pains, dizziness, faints or blackouts and shortness of breath.

Causes and risk factors

Bundle branch block is a common condition and in about half of cases there's no obvious cause. Some of the known causes are coronary artery disease, inflammation of the heart muscle and an overdose of digitalis-type drugs.

If the block's in the right side of the heart (known as right bundle branch block) it's likely to be a variation of normal heart functioning.

If it's in the left side (left bundle branch block) there's a greater chance of some heart problem associated with it.

Treatment and recovery

If you have no problems with your heart block, then it's best left alone. Many people with bundle branch block never know they have the condition until it's found by accident and then never have any problems from it.

If it starts to cause symptoms, however, then the treatment usually consists of putting in an artificial pacemaker, which takes over the faulty electrical system in the heart.