Basic principles of the ECG

 Basic principles of the ECG

Voltage and timing intervals

• 1mV is represented by a deflection of 10 mm

• Each small square represents 40 milliseconds (ms)

• 5 small squares, thus representing 200 ms.

Heart rate estimation

• HR = 300/ Number of large boxes between two RR waves

• If the number of large boxes:

5 - the HR is 60 beats per minute.

3 - the HR is 100 per minute.

2 - the HR is 150 per minute.

• Normal heart rate = between 60 to 100 beats per minute

Electrical axis

• The overall direction of travel of the electrical depolarization wave through the heart is known as the electrical axis.

• Limb leads record ECG in the coronal plane, hence used to determine the electrical axis.

• The limb leads are I, II, III, AVR, AVL and AVF.

• The cardiac electrical axis is normally downward and to the left.

• The electrical axis is expressed in degrees and is normally in the range from -30 to + 90 degrees.

Normal ECG

P wave

• Depolarization of atria during sinus rhythm generates P wave 

PR interval 

• Short physiological delay at the level of atrioventricular (AV) node 

QRS complex

• Depolarisation of the ventricles produces QRS complex

ST-segment and the T wave

• Repolarisation of the ventricles

Normal intervals

• PR interval: 120 - 200 ms (3 – 5 small squares)

• QRS duration: 80 - 120 ms (up to 3 small squares)

• QT interval: up to 440 ms in males and 460 ms in females

Basic Electrophysiology of the Heart

• There is spontaneous depolarization of the sinus node, an area of specialized tissue situated in the high right atrium. 

• The wave of electrical depolarization then spreads through the RA and across the interatrial septum into the left atrium.

• The atria are separated from the ventricles by an electrically inert fibrous ring so that the only route of transmission of electrical impulse from atria to ventricles is through the AV node.

• The AV node delays the electrical signal for a short time, and then the wave of depolarisation spreads down the interventricular septum, via the bundle of His and the right and left bundle branches, into the right and left ventricles. 

• After complete depolarization of the heart, the myocardium must then repolarise, before it can be ready to depolarise again for the next cardiac cycle.