(h) the use and interpretation of electrocardiogram (ECG) traces

The idea

An ECG records the electrical activity of the heart as a trace of voltage against time, and its use is to show the rhythm and coordination of the cardiac cycle. A normal trace shows a P wave (atrial depolarisation spreading from the sinoatrial node, causing atrial systole), a QRS complex (ventricular depolarisation, triggering ventricular systole, which hides atrial repolarisation) and a T wave (ventricular repolarisation). Interpretation means reading the trace: counting the number of QRS complexes in a time interval to find heart rate, measuring the distance between successive R waves to judge rhythm regularity, and identifying abnormal patterns such as missing P waves, widened QRS complexes or extra beats.

For example: A trace with no clear P waves and an irregular distance between R waves indicates an abnormal rhythm rather than a normal coordinated cycle.

What earns the marks

• States that an ECG records the electrical activity of the heart plotted as voltage against time.
• Identifies the P wave as atrial depolarisation preceding atrial systole.
• Identifies the QRS complex as ventricular depolarisation triggering ventricular systole.
• Identifies the T wave as ventricular repolarisation.
• Explains that heart rate can be found by counting QRS complexes within a known time interval.
• Explains that rhythm regularity is judged from the spacing between successive R waves.
• Recognises that abnormal traces (for example absent P waves or irregular R–R spacing) indicate disrupted coordination of the cardiac cycle.

Common mistakes and how to put them right

• Confusing the P wave with ventricular activity.

  Put it right: The P wave represents atrial depolarisation, while the QRS complex represents ventricular depolarisation.

• Treating the T wave as ventricular contraction.

  Put it right: It represents ventricular repolarisation, not systole.

• Assuming any ECG trace shows a normal cycle.

  Put it right: Interpretation requires checking wave presence, spacing and rhythm, and abnormal patterns must be recognised.

Exam tips

• Label each wave on a trace before answering, linking it to the electrical event and the mechanical effect.
• When calculating heart rate, state the time interval used and count complete QRS complexes within it.
• For rhythm questions, compare the spacing between successive R waves rather than the height of the peaks.