AQA A-Level Physics: ECG Waveform Principles and Characteristics — mark scheme explained

The question

Identify the P wave, QRS complex, T wave, PR interval, ST segment, and QT interval in the given ECG trace. Describe what each component represents. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Locate the small, rounded bump at the beginning of the cycle. This is the P wave, representing atrial depolarization.

• S2

  Identify the larger, more prominent wave with three components: Q (a small downward deflection), R (a large upward deflection), and S (a downward deflection following R). This is the QRS complex, representing ventricular depolarization.

• S3

  Find the flat portion of the ECG trace between the end of the QRS complex and the beginning of the T wave. This is the ST segment, representing the period when the ventricles are depolarized but not repolarizing.

• S4

  Locate the larger, rounded wave following the QRS complex. This is the T wave, representing ventricular repolarization.

• S5

  Measure the time from the onset of the P wave to the start of the QRS complex. This is the PR interval, which should be 0.12 to 0.20 seconds.

• S6

  Measure the time from the start of the QRS complex to the end of the T wave. This is the QT interval, which should be less than 0.44 seconds.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Locate the small, rounded bump at the beginning of the cycle. This is the P wave, representing atrial depolarization.

2. S2

   Identify the larger, more prominent wave with three components: Q (a small downward deflection), R (a large upward deflection), and S (a downward deflection following R). This is the QRS complex, representing ventricular depolarization.

3. S3

   Find the flat portion of the ECG trace between the end of the QRS complex and the beginning of the T wave. This is the ST segment, representing the period when the ventricles are depolarized but not repolarizing.

4. S4

   Locate the larger, rounded wave following the QRS complex. This is the T wave, representing ventricular repolarization.

5. S5

   Measure the time from the onset of the P wave to the start of the QRS complex. This is the PR interval, which should be 0.12 to 0.20 seconds.

6. S6

   Measure the time from the start of the QRS complex to the end of the T wave. This is the QT interval, which should be less than 0.44 seconds.

7. Final answer: P wave: Atrial depolarization; QRS complex: Ventricular depolarization; T wave: Ventricular repolarization; PR interval: 0.15 seconds; ST segment: Flat and at baseline; QT interval: 0.38 seconds.

Common mistakes

• Confusing the P wave with the T wave — Always identify the QRS complex first, as it is a prominent feature. The P wave will be before the QRS complex, and the T wave will follow it.
• Misinterpreting the PR interval — Start measuring the PR interval from the beginning of the P wave, where it first rises above the baseline, and continue to the start of the QRS complex.
• Failing to recognize a prolonged QT interval — Measure the QT interval from the start of the QRS complex to the end of the T wave. A normal QT interval is typically less than 0.44 seconds, but it can vary based on heart rate.
• Not identifying an elevated ST segment — Carefully examine the ST segment to ensure it is flat and at the baseline. Any elevation could indicate myocardial ischemia or infarction.
• Misinterpreting a widened QRS complex — Measure the duration of the QRS complex accurately. A widened QRS complex could indicate bundle branch block or ventricular hypertrophy.
• Failing to understand the role of electrodes and leads — Review the placement of electrodes on specific parts of the body and the function of leads in transmitting signals. Understand that each lead provides a different view of cardiac activity.

Where the marks go

• Full worked solution (all marking points)6 marks

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