Principles of operation for obtaining the ECG waveform; explanation of the…

The idea

An ECG records the heart's electrical activity using electrodes on the skin. Depolarisation of cardiac muscle creates potential differences of about 1 mV that a differential amplifier detects, filters and displays against time. The P wave is atrial depolarisation; the QRS complex is ventricular depolarisation, with the large R spike because ventricular muscle mass is greater; the T wave is ventricular repolarisation. The PR interval reflects conduction through the atrioventricular node, and the QRS width reflects ventricular conduction time. A typical trace shows a small rounded P wave, a sharp QRS complex and a broader T wave, repeating at roughly 0.8 s per beat. Comparing a patient's trace with these features helps identify arrhythmias, so the machine must reject common-mode noise and have a high input impedance.

What earns the marks

• Electrodes on the skin detect small potential differences, of order 1 mV, produced by depolarisation and repolarisation of cardiac muscle.
• A differential amplifier amplifies the voltage between pairs of electrodes while rejecting common-mode interference such as mains hum.
• The time base and display produce a voltage–time trace, allowing intervals and wave amplitudes to be measured.
• The P wave represents atrial depolarisation spreading from the sinoatrial node.
• The QRS complex represents ventricular depolarisation; the R peak is large because ventricular muscle mass is greater.
• The T wave represents ventricular repolarisation, and the PR interval reflects conduction delay at the atrioventricular node.
• A normal trace repeats regularly, with identifiable P, QRS and T features in each cardiac cycle.

Common mistakes and how to put them right

• Thinking the ECG measures the heart's mechanical contraction directly.

  Put it right by stressing that it records electrical activity that precedes contraction.

• Labelling the T wave as atrial repolarisation.

  Put it right by stating that atrial repolarisation is hidden within the QRS complex and the T wave is ventricular repolarisation.

• Assuming the QRS complex represents a single electrical event in one direction.

  Put it right by explaining that it is the combined depolarisation of both ventricles.

• Treating the trace as a direct measurement of blood pressure.

  Put it right by noting that pressure is inferred separately and the ECG is an electrical record.

Exam tips

• Sketch a labelled trace and annotate each wave with the electrical event it represents.
• Use comparative language: state which chamber is depolarising or repolarising and why the R peak is the largest feature.
• Link each machine stage to its purpose, such as amplification, noise rejection and time-base display, rather than listing components.
• When asked about abnormalities, compare timing or shape with a normal trace instead of guessing a diagnosis.