Understanding ECG Waveforms Revision Notes | A Level AQA Physics

1. Why the Heart Produces an Electrical Signal

Every heartbeat begins with an electrical event. Cardiac muscle cells depolarise — that is, they undergo a rapid change in electrical charge across their membranes — and this wave of electrical activity spreads through the heart in a precise, coordinated sequence. Because the body's tissues conduct electricity, these potential differences can be detected right at the skin surface.

Think of the heart as a battery that charges and discharges roughly once per second. The changing voltage it produces spreads outward through the conducting tissues of the body, much like ripples spreading across water. Place electrodes on the skin and you can pick up those ripples as a varying voltage signal over time — that signal is the electrocardiogram, or ECG.

The ECG does not measure the mechanical contraction of the heart directly. It measures the electrical events that trigger contraction. This distinction matters enormously in exam answers — the ECG is an electrical recording, not a pressure or force recording.

2. Principles of Operation: How an ECG is Obtained

To obtain an ECG, electrodes are placed at specific positions on the body surface — typically on the limbs and across the chest. Each electrode detects the electrical potential at that point on the skin. The ECG machine measures the potential difference between pairs of electrodes over time and plots this as a continuous waveform.

The key principle is that as the wave of depolarisation moves through the heart, it creates a dipole — a separation of positive and negative charge. When this dipole moves towards a particular electrode, the recorded potential rises; when it moves away, the potential falls. The shape of the resulting trace therefore encodes both the direction and the magnitude of the electrical activity at each moment.

• Electrodes are placed on the skin at standardised positions (limbs and chest).
• The ECG machine amplifies the tiny potential differences detected — typically in the millivolt range.
• Potential difference between electrode pairs is plotted on the vertical axis (voltage, in mV).
• Time is plotted on the horizontal axis, allowing the sequence of electrical events to be read left to right.
• A standard ECG paper speed is 25 mm per second, so each small square (1 mm) represents 0.04 s.

The amplification stage is essential because the signals reaching the skin are very small — on the order of 1 mV for the largest deflections. Without amplification, the signal would be too weak to display or analyse reliably.

3. The Normal ECG Waveform: A Feature-by-Feature Guide

Now that you know how the signal is captured, the next step is to read what it tells you. A normal ECG trace has a characteristic repeating pattern for each heartbeat. Three named features dominate that pattern: the P wave, the QRS complex, and the T wave. Each one maps directly onto a specific electrical event in the cardiac cycle.

Reading left to right across one cycle: the trace starts at the isoelectric baseline (zero potential difference), rises into the P wave, returns to baseline, then shows the sharp spike of the QRS complex, returns briefly to baseline, and finally produces the broader T wave before settling back to baseline ready for the next cycle.

The P wave is relatively small because the atria have a modest muscle mass compared to the ventricles. Its duration is typically around 0.08–0.10 s. After the P wave, there is a short flat segment — the PR interval — during which the impulse is delayed at the atrioventricular (AV) node. This delay allows the atria to finish contracting and push blood into the ventricles before ventricular contraction begins.

Next comes the most visually striking feature of the trace. The QRS complex is a rapid, large deflection that dominates the waveform. It consists of a small downward Q wave, a tall upward R wave, and a small downward S wave — though in many standard recordings the Q and S deflections are subtle and the R spike is the dominant feature you will be asked to identify.

After the QRS complex, the trace returns briefly to the baseline during the ST segment — a period when the ventricles are fully depolarised and contracting. Displacement of this segment above or below the baseline is clinically significant, but for the purposes of the AQA specification the key point is that it represents the plateau phase between depolarisation and repolarisation.

Notice that there is no separate wave for atrial repolarisation. The electrical signal from atrial repolarisation is present, but it occurs at the same time as the QRS complex and is completely masked by the much larger ventricular signal. This is a detail examiners sometimes probe — do not invent a separate atrial repolarisation wave on your diagram.

4. Linking the Waveform to the Cardiac Cycle

The ECG waveform only makes full sense when you map it onto the sequence of mechanical events in the heart. Each electrical event precedes the corresponding mechanical event by a short interval — the electrical signal triggers the contraction, so it always comes first on the trace.

The AV node delay — visible as the PR interval on the trace — is physiologically important. Without it, the ventricles would begin contracting before the atria have finished emptying, reducing the volume of blood available for ejection. The delay ensures the atrial contribution to ventricular filling is complete before systole begins.

5. Why the Waveform Has Its Characteristic Shape

Students often accept the ECG shape as a given without asking why it looks the way it does. Understanding the reasons behind the shape is precisely what separates a recall-level answer from an analysis-level one — and it is the kind of depth AQA rewards in longer-mark questions.

The amplitude of each deflection reflects the mass of muscle undergoing depolarisation. The ventricles contain far more muscle than the atria, which is why the QRS complex is so much taller than the P wave. The ventricles have substantially greater muscle mass than the atria, and this difference is directly reflected in the relative heights of the two features.

The duration of each feature reflects how long the depolarisation process takes. The QRS complex is narrow (under 0.12 s) because the Purkinje fibre network distributes the impulse almost simultaneously across the entire ventricular wall — it is a fast, synchronised event. The T wave is broader because repolarisation is a slower, less synchronised process that spreads across the ventricular wall over a longer period.

The direction of deflection — upward or downward — depends on the orientation of the electrode relative to the direction of the depolarisation wavefront. For a standard limb lead, depolarisation moving towards the positive electrode produces an upward deflection; movement away produces a downward deflection. This is why the precise shape of the QRS complex varies slightly between different lead positions, even though the underlying electrical event is the same.

6. Reading and Interpreting an ECG Trace

In an exam, you may be given an ECG trace and asked to identify features, measure intervals, or calculate heart rate. The approach below gives you a reliable method for each task.

Worked example: Calculating Heart Rate from an ECG Trace

An ECG trace is recorded at the standard paper speed of 25 mm per second. The distance between two consecutive R peaks (the R-R interval) is measured as 20 mm. Calculate the heart rate in beats per minute.

1. Paper speed = 25 mm per second, so 1 mm = $\frac{1}{25}$ s = 0.04 s R-R interval = 20 mm $\times$ 0.04 s mm$^{-1}$ = 0.80 s — Always convert the measured distance to time using the paper speed. Standard paper speed is 25 mm/s, giving 0.04 s per mm. [M1]
2. One cardiac cycle = R-R interval = 0.80 s — The R-R interval is the time between two successive ventricular depolarisations, which equals the period of one complete heartbeat. [B1]
3. Heart rate = $\frac{60 \text{ s}}{0.80 \text{ s per beat}}$ = 75 beats per minute — Divide 60 seconds by the period of one cycle to find how many cycles occur per minute. 60 $\div$ 0.80 = 75. [M1] [A1]
4. Heart rate = 75 beats per minute (bpm) — Always include units. Heart rate is expressed in beats per minute. A resting heart rate of 60-100 bpm is within the normal range, so 75 bpm is a plausible answer — a useful sanity check.

Answer: Heart rate = 75 beats per minute

The same logic applies if you are given the R-R interval already in seconds — you skip step 1 and go straight to the division. The key relationship to remember is: heart rate (bpm) = 60 $\div$ R-R interval (s).

7. Exam Technique and Common Mistakes

ECG questions appear in the medical physics section of AQA A Level Physics and reward precise use of terminology. The marks are often lost not through misunderstanding the physics, but through imprecise language or confusion between closely related features. The guidance below targets the most frequently penalised errors.

Bringing this together: the ECG waveform is a direct window onto the electrical sequence of the cardiac cycle. Each named feature — P wave, QRS complex, T wave — maps onto a specific depolarisation or repolarisation event, and the shape of each feature (amplitude, duration, direction) is determined by the mass of muscle involved, the speed of conduction, and the geometry of the electrode placement. Mastering these links between electrical event, waveform feature, and physiological consequence is the foundation of every ECG question at this level.