ELECTROCARDIOGRAM (ECG)

Electrocardiography is the procedure of recording the electrical activity of the heart. The recording itself is known as an electrocardiogram. The 1st ECG machine was designed by Einthoven. Abnormal ECG is composed of a:

  • P wave
QRS complex
  • T wave

The QRS complex is often 3 separate waves

  • Q wave
  • R wave
  • S wave

The normal ECG tracking shows deflections or waves and isoelectric segments. The waves (deflections) may be positive or negative (above or below the zero lines, respectively). When depolarization moves towards a positive electrode, an upward deflection is produced. When it moves towards the negative electrode, downward deflection recorded. The record shows depolarization followed by repolarization of different regions of the heart. The ECG can be divided into 3 types of recordings which consist of complexes or waves, intervals, and segments.


P wave

This is caused by electrical potentials generated as the atria depolarize prior to contraction, which means the P wave shows atrial depolarization.

Duration: 0.1 sec or less

Amplitude: 2 mm or less

Voltage: 0.1 - 3mV


QRS complex

This is caused by the potentials generated when the ventricles depolarize prior to contraction. Normally, the Q is initially a negative deflection followed by a positive deflection (R wave). A negative S wave follows the R wave.

Duration: 0.11 sec

Amplitude: 0.5 mm

Voltage: 1 mv


T wave

This is caused by the potentials generated as the ventricles recover from depolarization. This wave represents ventricular repolarization.

Voltage: 0.2 - 0.3 mv

U wave

This is a slight positive deflation seen after the T wave and preceding the next P wave. Its origin is probably due to the slow repolarization of the inter-ventricular Purkinje conducting system of papillary muscles - characteristically seen in hypokalemia.

Voltage: 1 mv


Normal Intervals

1. PR - interval

This is measured from the onset of the P wave to the beginning of the QRS complex. During this interval, the cardiac impulses travel from the SA node to the ventricles. Its duration is 0.12 - 0.20 sec. This interval tends to be longer in a slow heart. It is prolonged in acute rheumatic fever showing delayed AV conduction. The PR interval occurs during atrial depolarization and conduction through the AV node.


2. QRS - interval

This occurs during ventricular depolarization and atrial repolarization. It is measured from the onset of the Q (or R) wave to the end of the S wave. Its duration is 0.08 - 0.10 sec.


3. QT - interval

This is measured from the beginning of the Q wave to the end of the T wave. This occurs during ventricular depolarization and ventricular repolarization. Its duration is 0.40 - 0.43 sec.


4. ST - interval

This begins from the end of the S wave and lasts up to the T wave. It occurs during ventricular repolarization and its duration is 0.32 sec.


Clinical significance of the ECG

By studying the ECG, we can note the following points related to the cardiovascular system:

  • Heart rate
  • Rhythm
  • Position of the heart (axis deviation)
  • Ventricular hypertrophy
  • Ischemic heart diseases
  • P wave
  • PR interval
  • Q wave QRS complex 
  • ST segments
  • T wave