Edexcel A (SNAB) A Level Biology:复习笔记7.3.3 ECGs & Diagnosing CVD

ECGs & Diagnosing CVD

  • Electrocardiography can be used to monitor and investigate the electrical activity of the heart
  • Electrodes that are capable of detecting electric signals are placed on the skin
  • These electrodes produce an electrocardiogram (ECG)
  • An ECG shows a number of distinctive electrical waves produced by the activity of the heart
  • A healthy heart produces a distinctive shape in an ECG

The ECG of a healthy heart

  • The P wave
    • Caused by the depolarisation of the atria, which results in atrial contraction (systole)
  • The QRS complex
    • Caused by the depolarisation of the ventricles, which results in ventricular contraction (systole)
    • This is the largest wave because the ventricles have the largest muscle mass
  • The T wave
    • Caused by the repolarisation of the ventricles, which results in ventricular relaxation (diastole)
  • The U wave
    • Scientists are still uncertain of the cause of the U wave, some think it is caused by the repolarisation of the Purkyne fibres
  • The bigger the wave, the greater the electrical activity passing through the heart, which results in a stronger contraction

Using ECGs to diagnose heart problems

  • If someone has a suspected heart problem a doctor will often use an ECG as a diagnostic tool
  • Some heart problems produce certain shapes or waves in an ECG which allow for a diagnosis
  • Tachycardia
    • When the heart beats too fast it is tachycardic
    • An individual with a resting heart rate of over 100 bpm is said to have tachycardia
  • Bradycardia
    • When the heart beats too slow it is bradycardic
    • An individual with a resting heart rate below 60 bpm is said to have bradycardia
    • A lot of fit individuals or athletes tend to have lower heart rates and it is usually not dangerous
  • Ectopic heartbeat
    • This condition is caused by an early heartbeat followed by a pause
    • This could be due to an earlier contraction of either the atria or ventricles
    • It is common in the population and usually requires no treatment unless very severe
  • Fibrillation
    • An irregular heartbeat will disrupt the rhythm of the heart
    • The atria or ventricles stop contracting properly
    • Severe cases of fibrillation can be very dangerous, even fatal


Each of these ECGs shows different faulty heartbeats. The speed or rhythm/regularity of the heartbeat is very important