MCQ – 24094

Correct Answer � D Explanation

D i.e. Ventricular depolarization

• QRS complex (wave) reflects ventricular depolarization.

• QT interval (or period of electrical systole of ventricles) is closely correlated with the mean action potential duration of ventricular myocytes. 
• Contraction of ventricles lasts from the beginning of Q wave (or R wave, if Q wave is absent) to the end of T wave: an interval k/a QT interval.

Normal Electrocardiogram (ECG):

P wave:

• P wave reflects atrial depolarization before atrial contraction begins. 

• QRS complex (wave) reflects ventricular depolarization before contraction. 
• In other words, QRS complex is caused by potentials generated when the depolarization wave spreads through the ventricles, but before ventricular contraction begins. 

• Therefore, both P waves and QRS complex are depolarization waves.

T wave:

• T wave represents ventricular repolarization.
• It is caused by potentials generated as the ventricles recover from the state of depolarization. 
• This process normally occurs 0.25- 0.35 seconds after depolarization in ventricular muscles. 
• T wave is also k/a repolarization wave.

Other important points:

• Current flows from one part of ventricle to another part (and therefore to the surface of body) to produce ECG only when the muscles is partly polarized and partly depolarized. 
• No ECG is recorded when the ventricular muscle is either completely polarized or completely depolarized.

• Depolarization must spread through the muscle before contraction of muscle can occur. 

• So the P wave occurs at the beginning of contraction of atria and the QRS complex occurs at the beginning of contraction of ventricles. 

• The ventricles remain contracted until after repolarization has occurred i.e. until after end of T wave.

• Atria repolarize about 0.15 to 0.20 seconds after termination of P wave i.e. approximately during QRS complex.

• Therefore atrial repolarization wave (atrial T wave) is usually masked by much larger QRS complex (of ventricular depolarization).

• Ventricular repolarization begins about 0.20 seconds after the beginning of depolarization (QRS complex) in some fibers and takes as long as 0.35 seconds in others. 

• Thus, ventricular repolarization (T wave) is prolonged process extending over 0.15 second, but the voltage of T wave is lesser than the voltage of QRS complex, partly b/o its prolonged length.

P-Q or PR interval: 

• • The time between the beginning of P wave and the beginning of QRS complexis called PR interval (when Q wave is absent) or more precisely P­Q interval. 
  • It is a measure of the time from the onset of atrial activation (ie beginning of electrical excitation of atria) to the onset of ventricular activation (or excitation).
  • The PR interval measures the time between the atrial and ventricular depolarization, including the physiological delay imposed by cells in AV junction area. 
  • PR interval is normally 0.16 second (0.12 – 0.20 second). 
  • It shortens as the heart rate increases. 

QT interval:

• Period of electrical systole of ventricles) is closely correlated with the mean action potential duration of ventricular myocytes.
• Contraction of ventricles lasts from the beginning of Q wave (or R wave, if Q wave is absent) to the end of T wave: an interval k/a QT interval.
• Normally QT interval is 0.4 second, but is inversely related with the heart rate because the duration of myocardial cell action potential varies inversely with the heart rate.
• A heart rate related (corrected) QT interval (QTc) can be calculated as QT,1^—Z – R and normally is 0.44 second.