Electrocardiogram

Electrocardiogram

Define electrocardiogram. Draw a neat diagram and briefly describe different waves. What is the significance of P-R interval? (or) Draw and label a normal electrocardiogram. What is P-R interval?
Answer:

Electrocardiogram (ECG):

Electrocardiogram Definition:

• It is the graphical registration of the electrical fluctuations of the heart occurring during each cardiac cycle.

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Waves: of Normal Electrocardiogram (ECG):

• The waves of normal ECG are represented by letters P, Q, R, S,T and U.

1. ‘P’ wave:
Synonym:

1st wave:

It is positive wave.

Direction and shape:

Directed upwards.

It is rounded or pointed.

Causes:

It is produced due to depolarization of atrial musculature.

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Duration: 0.1 sec.

Significance:

Represents the spread of cardiac impulse from SA node to atrial muscle.

Peak – represents invasion of AV node by excitation process.

Height – represents functional activity of atrial muscles.

What is an electrocardiogram

2. QRS complex:

• Synonym: Initial ventricular complex.
• Q-is small negative deflection.
• Ris prominent, pointed positive wave.
• S-Negative deflection following ‘R’ wave.
  Causes:
• QRS complex – Due to depolarization of mid-portion of the interventricular septum.
• ‘R’ – due to depolarization of apical portion of the inteventricular septum and that of ventricular muscle.
• S’-due to depolarization of more basal portion of ventricular musculature.
• Duration: 0.08 to 0.12 sec.
  Significance:
• If its duration exceeds 0.12 sec, it leads to heart block.

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3. T wave:

• It is positive wave.
• It is rounded.
• It is final ventricular complex.
  Causes:
• Repolarization of ventricular musculature.
  Duration: 0.27 Sec.
  Significance:
• End of T wave coincides with the closure of semilunar valves.

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4. ‘U’ wave:

• It is rarely seen.
• It is positive small round wave.
  Cause:
• May be due to slow repolarization of papillary muscles.
• Duration: 0.08 sec.
  Significance:
• It is insignificant wave of ECG.

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Significance of PR interval:

• PR interval is the interval from the beginning of `P wave to the beginning of Q and R wave.
• It denotes the time interval between completion of atrial depolarization and beginning of ventricular depolarization.
• It signifies the conduction of impulses through AV node.
• It also indicates the duration of conduction of the impulse from the SA node to ventricles.
• Its normal duration is 0.12-0.16 sec.
• Duration decreases with increases in heart rate.
• If duration is more than 0.2 sec. it indicates delayed conduction, and if it is less than 0.12 sec – it indicates that impulse must have arise in AV node.