Electrocardiograms (ECGs) (OCR A Level Biology): Revision Note

Electrocardiograms (ECGs)

• 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

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

  • 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

  • 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.