What Is ECG? Its Uses, Leads, Waves, Intervals And Segments Of ECG



Electrocardiography is the technique by which electrical activities of the heart are studied. The spread of excitation through myocardium produces local electrical potential. This low-intensity current flows through the body, which acts as a volume conductor. This Current can be picked up from surface of the body by using suitable electrodes and recorded in the form of electrocardiogram. This technique was discovered by Dutch physiologist, Einthoven Willem, who is considered the father of electrocardiogram (ECG).


Electrocardiograph is the instrument (machine) by which electrical activities of the heart are recorded.


Electrocardiogram (ECG or EKG from electrocardiogram in Dutch) is the record or graphical registration of electrical activities of the heart, which occur prior to the onset of mechanical activities. It is the summed electrical activity of all cardiac muscle fibers recorded from surface of the body.

Uses Of ECG

Electrocardiogram is useful in determining and diagnosing the following:

  • Heart Rate
  • Heart Rhythm
  • Abnormal Electrical Conduction
  • Poor Blood Flow To Heart Muscle (Ischemia)
  • Heart Attack
  • Coronary Artery Disease
  • Hypertrophy Of Heart Chambers

Electrocardiographic Grid

The paper that is used for recording ECG is called ECG paper. ECG machine amplifies the electrical signals produced from the heart and records these Signals on a moving ECG paper. Electrocardiographic grid refers to the markings (lines) on ECG paper. ECG paper has horizontal and vertical lines at regular intervals of 1 mm. Every 5th line (5 mm) is thickened.


Time duration of different ECG waves is plotted horizontally on X-axis.

On X-axis

1 mm  = 0 04 second
5 mm = 0 20 second


Amplitude of ECG waves is plotted vertically on Y-axis.

On Y-axis

1 mm = 0.1 mV
5 mm = 0.5 mV

Speed Of The Paper

Movement of paper through the machine can be adjusted by two speeds, 25 mm/second and 50 mm/ second. Usually, speed of the paper during recording is fixed at 25 mm/second. If heart rate is very high, speed of the paper is changed to 50 mm/second.

ECG Leads

ECG is recorded by placing series of electrodes on the surface of the body. These electrodes are called ECG leads and are connected to the ECG machine. Electrodes are fixed on the limbs. Usually, right arm, left arm and left leg are chosen. Heart is said to be in the center of an imaginary equilateral triangle drawn by connecting the roots of these three limbs. This triangle is called Einthoven triangle.

Einthoven Triangle And Einthoven Law

Einthoven triangle is defined as an equilateral triangle that is used as a model of standard limb leads used to record electrocardiogram. Heart is presumed to lie in the center of Einthoven triangle. Electrical potential generated from the heart appears simultaneously on the roots of the three limbs, namely the left arm, right arm and the left leg.

ECG is recorded in 12 leads, which are generally classified into two categories.

  • Bipolar Leads
  • Unipolar Leads

1- Bipolar Limb Leads

Bipolar limb leads are otherwise known as standard limb leads. Two limbs are connected to obtain these leads and both the electrodes are active recording electrodes, i.e. one electrode is positive and the other one is negative.

Standard limb leads are of three types:

  • Limb lead I
  • Limb lead II
  • Limb lead Ill.

Limb Lead I

Limb Lead I is obtained by connecting right am and left arm. Right arm is connected to the negative terminal of the instrument and the left arm is connected to the positive terminal.

Limb Lead II

Limb Lead II is obtained by connecting right arm and left leg. Right arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal

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