Electrocardiogram (ECG) - Practice Questions & MCQ

ECG

• The representation of the electric current produced by the excitation of the cardiac muscles in the form of a graph is called an electrocardiogram (ECG)
• The machine by which the electrocardiogram is recorded is called as electrocardiograph. 
• Electrocardiograph was invented by Einthoven and he is known as the father of electrocardiography
• An electrocardiogram shows three consecutive waves namely P wave, QRS wave and the T wave. 

1. P wave- 

• It is a small upward wave that indicates the depolarisation of the atria leading to contraction of both atria. It is caused by the activation of SA node

2. QRS wave

• It begins after a fraction of a second of the P wave 
• It begins as a small downward deflection (Q) and continues as large upright (R)  and triangular wave, ending as downward wave (S) at its base
•  QRS wave indicates depolarization of ventricles
• It is caused by the impulses of the contraction from AV node through the Bundle of His and Purkinje fibres and the contraction of ventricular muscles
• In this wave there is a spread of electrical impulses through ventricles
• The normal duration of the QRS complex is <0.10 seconds

3. T wave

• The T wave is dome-shaped and it indicates ventricular repolarization ( ventricular relaxation)
• The end of T waves marks systole 

P-R interval

• It is the time required for an impulse to travel through the atria and AV node to the remaining conductive tissue.
• The normal duration of P-R interval is <0.12 to 0.2 seconds
• The P-R interval is lengthened during conditions like arterosclerotic heart disease and rheumatic fever due to the inflammation of atria and AV node

S-T interval

• It is the time between the end of the spread of impulse through ventricles and its repolarization.
• It starts at the end of the S wave and terminates at the beginning of the T wave
• During acute myocardial infarction S-T segment is elevated while it is depressed during the insufficient supply of oxygen to the muscles

Q-T interval

• The QT interval represents the time of ventricular activity including both depolarization and repolarization. 
• It is measured from the beginning of the QRS complex to the end of the T wave. 
• The QT interval will vary with patient gender, age and heart rate.
• The normal duration of Q-T interval is <0.42 seconds