The electrocardiogram (ECG) assesses cardiac rhythm and conduction and assists in diagnosing myocardial ischaemia and infarction.
Recording and activation sequence
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Body-surface electrodes detect electrical depolarisation of myocardial tissue produced by a small dipole current. The signals are amplified and printed or displayed.
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During sinus rhythm, the SA node triggers atrial depolarisation, producing the P wave. Depolarisation then proceeds slowly through the atrioventricular (AV) node, which is too small to generate a wave detectable at the body surface.
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Activation of the bundle of His, bundle branches and Purkinje system initiates ventricular myocardial depolarisation, producing the QRS complex.
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The QRS complex is larger than the P wave because ventricular muscle mass is much greater than atrial muscle mass.
Conduction intervals and injury
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The PR interval runs from the onset of the P wave to the onset of the QRS complex and largely reflects AV nodal conduction time.
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Injury to either bundle branch delays ventricular depolarisation and widens the QRS complex. Selective injury to a left fascicle, termed hemiblock, affects the electrical axis.
Repolarisation
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Repolarisation is slower and spreads from the epicardium to the endocardium.
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Atrial repolarisation does not produce a detectable signal, whereas ventricular repolarisation produces the T wave.
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The QT interval represents the total duration of ventricular depolarisation and repolarisation.