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A non-invasive investigation that records the electrical activity of the heart. An ECG machine is an electrocardiogram, the ECG trace is an electrocardiograph. Electrocardiography is generally carried out in order to:

  • investigate chest pain
  • investigate suspected heart disease or defects
  • monitor drug treatment
  • screen for cardiac arrhythmias

Nurse/HCA training is usually in-house. Many ECG monitors are connected to the practice computer, with tracings displayed on screen and stored in the patient’s notes. Local cardiac rehabilitation units may run courses.

The basic ECG waveform represents the electrical activity of the heart muscle as it goes through a cycle of contraction and relaxation. The atria contract first (Fig. 1). Contraction is triggered by an impulse from the sinoatrial (SA) node. This impulse spreads through the atria to the atrioventricular (AV) node, and then passes quickly along fibres (the bundle of His) in the interventricular septum and thence into the ventricular walls.

The P wave represents contraction of the atria. Atrial muscle mass is small, hence the voltage generated is low, and the peak on the trace is small.

The QRS complex represents ventricular contraction. The ventricular mass is larger, so contraction generates a higher peak. Normal width of the QRS complex is up to 3 small squares on the ECG recording paper, representing c. 0.12 s, ie, the time for excitation to spread through the ventricles; this can be longer if the heart is abnormal.

The T wave represents relaxation of the heart.

The PR interval is the delay while excitation spread from the SA node, through the atrial muscle and AV node down the bundle of His and into the ventricular muscle.

The ECG made easy JR Hampton, 7th Edition Churchill Livingstone, 2008. Kindle edition available at

ECG Library

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