ECG morphology

Each beat of the heart can be observed as a series of deflections away from the baseline on the ECG. These deflections reflect the time evolution of electrical activity in the heart which initiates muscle contraction. A single sinus (normal) cycle of the ECG, corresponding to one heart beat, is traditionally labelled with the letters P,Q,R,S and T on each of its turning points (Fig. 1). The ECG may be divided into the following sections:

• P-wave: a small low-voltage deflection away from the baseline caused by the depolarisation of the atria prior to atrial contraction as the activation (depolarisation) wave-front propagates from the SA node through the atria.
  
• PQ-interval: the time between the beginning of atrial depolarisation and the beginning of ventricular depolarisation.
  
• QRS-complex: the largest-amplitude portion of the ECG, caused by currents generated when the ventricles depolarise prior to their contraction. Although atrial repolarisation occurs before ventricular depolarisation, the latter waveform (i.e. the QRS-complex) is of much greater amplitude and atrial repolarisation is therefore not seen on the ECG.
  
• QT-interval: the time between the onset of ventricular depolarisation and the end of ventricular repolarisation. Clinical studies have demonstrated that the QT-interval increases linearly as the RR-interval increases [4]. Prolonged QT-interval may be associated with delayed ventricular repolarisation which may cause ventricular tachyarrhythmias leading to sudden cardiac death [9].
• ST-interval: the time between the end of S-wave and the beginning of T-wave. Significantly elevated or depressed amplitudes away from the baseline are often associated with cardiac illness.
• T-wave: ventricular repolarisation, whereby the cardiac muscle is prepared for the next cycle of the ECG.