The Heart's Electrical System

A heartbeat depends on both muscle and electricity. The heart can generate its own electrical impulses, while nerves and hormones adjust the rate. Following the normal conduction pathway helps explain what an ECG records and what it does not show directly.

The heart generates its own electrical impulses Specialised heart cells can depolarise without a separate nerve impulse starting every beat. In normal sinus rhythm , the electrical impulse usually begins in the sinoatrial node , or SA node, high in the right atrium. The SA node is therefore called the heart's natural pacemaker. The autonomic nervous system and hormones can speed or slow its activity, so the heart is not electrically isolated from the rest of the body. The impulse spreads through the atria From the SA node, electrical activity spreads through the atrial muscle. This produces atrial depolarisation , followed by atrial contraction. Most ventricular filling happens before the atria contract. Atrial contraction adds a final contribution before the ventricles begin their own contraction. The AV node creates a brief delay The impulse reaches the atrioventricular node , or AV node. Conduction slows here for a short time. This delay helps the atria complete their contribution to ventricular filling before the ventricles are activated. It also helps organise the sequence from atrial to ventricular contraction. The ventricular conduction system spreads the impulse After the AV node, the impulse passes through the atrioventricular bundle, often called the bundle of His . It then travels down the right and left bundle branches and through Purkinje fibres. This rapidly distributes electrical activation through the ventricles. Ventricular depolarisation is followed by ventricular contraction, which drives blood into the pulmonary and systemic circulations. An ECG records electrical activity from the body surface An electrocardiogram , or ECG, records voltage changes produced by the heart's electrical activity using electrodes on the skin. It does not show the heart muscle contracting directly. The P wave represents atrial depolarisation. The QRS complex represents ventricular depolarisation. The T wave represents ventricular repolarisation, the electrical recovery that prepares ventricular cells for another cycle. Mechanical contraction follows electrical activation, so electrical and mechanical events are closely linked. They are not the same measurement. The activity of the SA node and AV node is not usually seen as a separate labelled wave on a standard surface ECG. Clinicians infer conduction through those areas from the timing and pattern of the recorded atrial and ventricular activity. What rhythm assessment can show A pulse can provide information about rate and regularity. An ECG adds direct information about the heart's electrical activity. An irregular pulse can raise suspicion of an arrhythmia, but it does not identify the rhythm by itself. Atrial fibrillation, for example, requires ECG evidence for diagnosis. Some rhythm problems are intermittent. A short ECG can therefore be normal between episodes, and longer monitoring may be useful when the clinical pattern justifies it.

In normal sinus rhythm, the SA node initiates atrial electrical activity, the AV node delays conduction briefly, and the ventricular conduction system spreads the impulse through the ventricles. An ECG records electrical activity, not contraction itself.