ECGs & Diagnosing CVD (Edexcel A (SNAB) A Level Biology)

• 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