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7.3.3Topic 7 · Run for your Life

ECGs and Diagnosing CVD — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 17 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.3.3 ECGs and Diagnosing CVD — Edexcel A Level Biology A (SNAB) 9BI0 notes: Interpret electrocardiogram traces and use them to identify abnormal heart rhythms and cardiovascular disease.An electrocardiogram (ECG) is a recording of the electrical activity of the heart made by ele…

  • QRS complex
  • atherosclerosis
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Tachycardia

Section 1 of 8What an ECG records

An electrocardiogram (ECG) is a recording of the electrical activity of the heart made by electrodes placed on the skin of the chest, wrists and ankles.

What an ECG records
The electrodes detect small voltage changes caused by waves of depolarisation and repolarisation spreading through cardiac muscle.
An ECG does not record the mechanical contraction itself, but the electrical activity that precedes it.
It is non-invasive, quick and useful to diagnose heart problems.

Section 2 of 8The normal trace

A normal ECG shows a repeating pattern of waves:

P wave
P wave depolarisation of the atria, which leads to atrial systole.
QRS complex
QRS complex a large, sharp spike caused by depolarisation of the ventricles, which leads to ventricular systole. It is large because the ventricle muscle mass is much greater than the atria.
T wave
T wave repolarisation of the ventricles, which leads to ventricular diastole.

Atrial repolarisation happens during the QRS complex and is hidden. The flat section between the T wave and the next P wave is the resting period. The gap between the P wave and the start of QRS reflects the delay at the AVN.

To calculate heart rate from a trace, measure the time for one cardiac cycle (for example the time between two R peaks) and use heart rate (beats per minute) = 60 ÷ time for one cycle in seconds. If the paper speed is 25 mm per second, a cycle of 20 mm lasts 0.8 s, giving 75 beats per minute.

Section 3 of 8Abnormal rhythms

ConditionECG appearanceExplanation
TachycardiaNormal waves, close together; rate above 100 beats per minuteHeart beating too fast; may be due to fever, stress or heart disease
BradycardiaWaves far apart; rate below 60 beats per minuteSlow; may be normal in athletes or due to a problem with the SAN
Atrial fibrillationNo clear P waves; irregular QRS spacingAtria contract rapidly and irregularly so less efficient filling; raises stroke risk
Ectopic beatAn early or extra beat followed by a pauseAn extra beat starts outside the SAN; often harmless
Heart blockMore P waves than QRS complexesThe wave from the atria is not conducted to the ventricles
Myocardial infarctionRaised or depressed segment between QRS and TPart of the muscle is damaged by lack of blood supply

Section 4 of 8Cardiovascular disease and diagnosis

Cardiovascular disease (CVD) includes coronary heart disease, stroke and high blood pressure.

Atherosclerosis
A common cause is atherosclerosis: damage to the artery endothelium (often by high blood pressure or smoking) leads to inflammation, build-up of white blood cells and cholesterol-rich plaques (atheromas) that narrow the lumen and make the artery wall less elastic.
If a plaque ruptures a blood clot (thrombus) can block the artery.
A blocked coronary artery deprives the heart muscle of oxygen, causing a heart attack (myocardial infarction); in the brain it can cause a stroke.

Risk factors include genetic predisposition, age, sex, high blood pressure, a diet high in saturated fat and salt, smoking, inactivity and obesity.

Cardiovascular disease and diagnosis
An ECG helps to diagnose conditions but is often combined with other tests such as blood tests for cholesterol or troponin, echocardiograms and angiograms.
A resting ECG can miss problems that appear only under load, so an exercise ECG may be used; a portable monitor worn for 24 hours detects rhythm problems that come and go.

Section 5 of 8Evaluating ECG evidence

Exam questions often show several traces and ask you to compare them.

Evaluating ECG evidence
Use a structured approach: first check the rate (distance between peaks), then the regularity (are intervals equal?), then check that each QRS is preceded by a P wave and that the shape of the waves is normal.
State your conclusion and link it to the heart's function, for example "the interval between the P wave and QRS is lengthened, which suggests slower conduction through the AVN".

You should also be able to discuss the limitations.

Evaluating ECG evidence
A single ECG records only a few seconds, so intermittent problems can be missed; muscle movement, poor skin contact and breathing alter the signal; and an ECG shows electrical but not structural problems, so it cannot show the size of a clot or the condition of the valves.
A cardiologist therefore uses an ECG together with blood pressure, history and other imaging methods before deciding on treatment such as drugs (beta-blockers, statins, anticoagulants), angioplasty with a stent, or bypass surgery.

Section 6 of 8Exam-style questions

  1. An ECG trace shows 4 complete cycles in 3.2 seconds. Calculate the heart rate in beats per minute. [2]2 marks

  2. Explain what the QRS complex shows and why it is larger than the P wave. [3]3 marks

  3. A patient's ECG has no clear P waves and irregular spacing between QRS complexes. Suggest the condition and explain the effect on the heart. [3]3 marks

Section 7 of 8Key terms

TermMeaning
ElectrocardiogramRecording of electrical activity of the heart from the skin surface
P waveECG wave showing atrial depolarisation
QRS complexECG spike showing ventricular depolarisation
T waveECG wave showing ventricular repolarisation
TachycardiaResting heart rate above 100 beats per minute
BradycardiaResting heart rate below 60 beats per minute
Atrial fibrillationRapid, irregular atrial contraction
AtherosclerosisBuild-up of plaque in artery walls narrowing the lumen

Section 8 of 8Exam technique

1
Saying the P wave is ventricular contraction: it is atrial depolarisation.
2
Saying an ECG measures the force of contraction: it measures electrical activity only.
3
Forgetting to convert the time per cycle to seconds before calculating beats per minute.
4
Saying the T wave is atrial repolarisation: it is ventricular repolarisation.
5
Saying atherosclerosis is caused by cholesterol alone: damage to the endothelium and inflammation start it.

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