Edexcel A (SNAB) A Level Biology

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7.3.3 ECGs & Diagnosing CVD

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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

Healthy 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

Faulty ECGs (1)_2Faulty ECGs (2)_2

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

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Marlene

Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.