Introduction

An electrocardiogram (ECG) is a test that records the electrical activity of the heart. ECGs are commonly used to diagnose heart problems, such as arrhythmias, heart attacks, and other cardiac conditions. As a junior doctor, it is important to have a basic understanding of how to interpret ECGs.

ECG Components

There are several key components of an ECG that you should be familiar with:

• P wave: Represents atrial depolarization (contraction).
• QRS complex: Represents ventricular depolarization (contraction).
• T wave: Represents ventricular repolarization (relaxation).
• ST segment: Represents the time between ventricular depolarization and repolarization.

By understanding these components, you can begin to interpret ECG tracings.

Interpreting ECGs

When interpreting ECGs, there are several key steps to follow:

1. Check the patient’s details (name, age, sex) to ensure you have the correct ECG.
2. Check the ECG quality – calibration, gain (usually 10 mm/mV), paper speed (usually 25 mm/s), and artefact (muscle tremor, electrical interference).
3. Identify the heart rate – for a standard 25 mm/s ECG, you can count the QRS complexes in a 10-second strip and multiply by 6, or use the “300 / large squares” method.
4. Assess the rhythm – are there P waves? Do they precede each QRS? Is the rhythm regular or irregular? Decide if it is sinus rhythm or another rhythm (e.g. atrial fibrillation).
5. Look at intervals and morphology – PR interval, QRS duration, QT interval, axis, and then ST segments and T waves for ischaemia or injury.

Conclusion

ECG interpretation is an important skill for junior doctors to develop. By understanding the key components of an ECG and following a systematic approach to interpretation, you can begin to diagnose and manage a range of cardiac conditions and know when to seek senior or cardiology input.