Simple ECG machines and the normal ECG waveform (A-level only) — AQA A-Level Physics

This topic covers the physics principles behind the electrocardiogram (ECG), a diagnostic tool that records the electrical activity of the heart. You will learn how the heart's depolarisation and repolarisation generate potential differences that can be detected by electrodes on the skin, and how these signals are processed and displayed as a characteristic waveform. The normal ECG waveform consists of the P wave, QRS complex, and T wave, each corresponding to specific electrical events in the cardiac cycle.

Understanding the ECG is important because it applies core physics concepts such as potential difference, electrical signals, and signal processing to a real-world medical context. It also links to biology topics on the cardiac cycle and provides a foundation for interpreting abnormal ECGs, which is crucial for diagnosing heart conditions. In AQA A-Level Physics, this topic is examined as part of the 'Medical Physics' option, where you may be asked to analyse ECG traces, calculate heart rate, or explain the origin of the waveform.

• →The heart generates electrical signals due to the depolarisation and repolarisation of cardiac muscle cells. These signals are detected as small potential differences (around 1 mV) using electrodes placed on the skin.
• →The normal ECG waveform consists of: P wave (atrial depolarisation), QRS complex (ventricular depolarisation, which masks atrial repolarisation), and T wave (ventricular repolarisation).
• →The time axis of an ECG is calibrated so that 1 large square = 0.2 s (or 1 small square = 0.04 s). The voltage axis is calibrated so that 1 large square = 0.5 mV (or 1 small square = 0.1 mV).
• →Heart rate can be calculated from the R-R interval: heart rate (bpm) = 60 / (number of large squares between R waves x 0.2 s).
• →The ECG is a voltage-time graph. The shape and size of the waves can indicate abnormalities such as arrhythmias, myocardial infarction, or hypertrophy.

• 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.

• 💡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.
• 💡Always use precise terminology: 'depolarisation' and 'repolarisation' rather than 'contraction' and 'relaxation'. Marks are awarded for correct scientific language.
• 💡When calculating heart rate, show your working clearly. State the R-R interval in large squares, convert to seconds using the calibration, then use heart rate = 60 / period. This ensures method marks even if the final answer is incorrect.
• 💡For 'explain' questions, link each wave to its electrical event and, where relevant, the mechanical consequence (e.g., P wave leads to atrial systole). Avoid vague statements like 'the heart beats'.

• Thinking the ECG measures the heart's mechanical contraction directly; correct this by stressing that it records electrical activity that precedes contraction.
• Labelling the T wave as atrial repolarisation; correct this 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; correct this by explaining that it is the combined depolarisation of both ventricles.
• Treating the trace as a direct measurement of blood pressure; correct this by noting that pressure is inferred separately and the ECG is an electrical record.
• Students often think the P wave represents atrial contraction, but it actually represents atrial depolarisation. Contraction (systole) follows depolarisation after a short delay.
• Students may believe that the QRS complex includes atrial repolarisation, but atrial repolarisation is masked by the larger ventricular depolarisation and is not visible on the ECG.
• Students sometimes confuse the T wave with ventricular contraction, but it represents ventricular repolarisation, which leads to relaxation (diastole).

1. 1Day 1-2: Learn the definitions of depolarisation and repolarisation, and how they relate to the ECG waveform. Use diagrams to label the P wave, QRS complex, and T wave.
2. 2Day 3-4: Practice interpreting ECG traces. Identify the waves and measure time intervals (e.g., R-R interval) using the grid calibration. Calculate heart rate from given traces.
3. 3Day 5-6: Study the calibration of ECG axes. Memorise that 1 large square = 0.2 s and 0.5 mV. Practice converting between small squares and large squares.
4. 4Day 7-8: Work through past paper questions on ECG. Focus on 'explain' and 'calculate' questions. Review mark schemes to understand what examiners expect.
5. 5Day 9-10: Revise common abnormalities (e.g., tachycardia, bradycardia) and how they appear on an ECG. Test yourself with active recall and flashcards.

• 📋Calculation of heart rate from an ECG trace: You will be given an ECG with a time calibration and asked to calculate the heart rate. Remember to measure the R-R interval in large squares, convert to seconds, and use heart rate = 60 / period.
• 📋Explanation of the origin of the ECG waveform: You may be asked to describe what each wave represents in terms of depolarisation and repolarisation. Use correct terminology and link to the cardiac cycle.
• 📋Analysis of abnormal ECG traces: You could be given an ECG with an abnormality (e.g., missing P wave, widened QRS) and asked to suggest a possible condition or explain the effect on the trace.
• 📋Practical question on ECG detection: You might be asked how electrodes detect the heart's electrical activity and why amplification is needed. Mention the small potential difference (around 1 mV) and the role of the amplifier.