Question
QRS complex as shown in the image indicates :
A. Atrial repolarization
B. Atrial depolarization
C. Ventricular repolarization
D. Ventricular depolarization
Show Answer
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Correct Answer » D Explanation |
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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.
| Intervals /Wave | Normal Duration (sec)
Average Range |
Events in the heart | |
| P wave | Atrial depolarizationQ | ||
| T wave | Ventricular RepolarizationQ | ||
| PR (PQ) interval | 0.18 | 0.12-0.20 | Atrio ventricular conduction (from onset of atrial activation to onset of ventricular excitation) |
| QRS complex (wave) | 0.08 Q | to 0.10 | Ventricular depolarization (& atrial repolarization is masked) |
| QT (RT) interval | 0.40 | to 0.43 | Ventricular action potential or
contractionQ |
| (Ventricular depolarization + | |||
| Ventricular repolarization) | |||
| ST interval (QT minus
QRS) |
0.32 | Platue phase of ventricle action
potential |
|
| (Ventricular repolarization) | |||
