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•  Mapping of the cardiac cycle to a normal ECG trace

The cardiac cycle describes the series of events that take place in the heart over the duration of a single heart beat

  • It is comprised of a period of contraction (systole) and relaxation (diastole)

The cardiac cycle can be mapped by recording the electrical activity of the heart with each contraction

  • Activity is measured using a machine called an electrocardiograph to generate data called an electrocardiogram
  • Hint:  In a similar fashion, you use a telegraph (machine) to send a telegram (data)

Each normal heart beat should follow the same sequence of electrical events:

  • The P wave represents depolarisation of the atria in response to signalling from the sinoatrial node (i.e. atrial contraction)
  • The QRS complex represents depolarisation of the ventricles (i.e. ventricular contraction), triggered by signals from the AV node
  • The T wave represents repolarisation of the ventricles (i.e. ventricular relaxation) and the completion of a standard heart beat
  • Between these periods of electrical activity are intervals allowing for blood flow (PR interval and ST segment)

Electrical Activity of the Heart


Examples of Heart Conditions

Data generated via electrocardiography can be used to identify a variety of heart conditions, including:

  • Tachycardia (elevated resting heart rate = >120 bpm) and bradycardia (depressed resting heart rate = < 40 bpm)
  • Arrhythmias (irregular heart beats that are so common in young people that it is not technically considered a disease)
  • Fibrillations (unsynchronised contractions of either atria or ventricles leading to dangerously spasmodic heart activity)

Cardiac Rhythm Diagnoses

ECG data