Electrocardiograms

Electrocardiograms (ECGs)

Electrocardiograms (ECGs) measure and record the heart’s electrical activity. Each heartbeat can be represented in a characteristic trace that is constructed from several key features, each relating to a phase of the heartbeat.

ECG traces

Each heartbeat has a P-wave, QRS-complex and T-wave.
- The P-wave represents the depolarisation of the atria as the impulse passes from the sinoatrial node (SAN) to the atrioventricular node (AVN).
- The QRS-complex represents the depolarisation of the ventricles as the impulse passes through the bundle of His and the Purkinje fibres.
- The T-wave represents the repolarisation of the ventricles.
After depolarisation of the atria, the impulse is held at the AVN before moving down the bundle of His to depolarise the ventricles. This pause is seen on the ECG trace between the P-wave and QRS-complex.

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Abnormal ECG traces

The frequency of each PQRST cycle determines the heart rate (HR).
An ECG trace with PQRST cycles very close together signals a high HR. Sinus arrhythmia is a heartbeat that is too fast or slow.
- Tachycardia is a condition of the heart that involves a resting HR of over 100 beats per minute (bpm).
- Bradycardia is an abnormally slow HR (below 60 bpm) and will be seen in an ECG as the PQRST cycles will be spread out.
- An ectopic heartbeat or arrhythmia is an irregular heartbeat, these ECG traces will be shown by an irregular spread of PQRST cycles.

Abnormal ECG traces 2

Atrial fibrillation is a condition involving a lack of coordinated atrial depolarisation, the ECG trace will represent this condition through the absence of the P-wave.
Ventricular fibrillation is a condition involving a lack of coordinated ventricular depolarisation. The ECG will be very irregular and have no discernable pattern.

1Principles of Science I

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