Given below are different intervals/ durations of electrocardiogram of a human subject (column A) and events in heart during process (column B).

COLUMN A		COLUMN B
a.	PR interval	i.	Ventricular action potential
b.	QRS duration	ii.	Atrioventricular conduction
c.	QT interval	iii.	Ventricular depolarization
d.	ST interval	iv.	Plateau portion of ventricular action potential

Which one of following options is a correct match of entries in columns A and B?

The correct answer is Option 2 i.e.a - ii: b - iii; c - i; d - iv

Explanation-

• The QRS complex represents rapid ventricular depolarization, as the action potential spreads through ventricular contractile fibers.
• The S–T segment, which begins at the end of the S wave and ends at the beginning of the T wave, represents the time when the ventricular contractile fibers are depolarized during the plateau phase of the action potential. PR includes both atria and ventricular conduction.
•  The P wave represents atrial depolarization, which spreads from the SA node through contractile fibers in both atria.
• The second wave, called the QRS complex, begins as a downward deflection, continues as a large, upright, triangular wave, and ends as a downward wave. The QRS complex represents rapid ventricular depolarization.
• The Q–T interval extends from the start of the QRS complex to the end of the T wave. It is the time from the beginning of ventricular depolarization to the end of ventricular repolarization.

Key PointsThe electrocardiogram (ECG or EKG) is a graphical representation of the electrical activity of the heart over time. Different intervals and durations are measured on the ECG to assess the cardiac cycle. Here are some of the key intervals and durations observed in a typical human electrocardiogram:

P Wave:Represents atrial depolarization, or the contraction of the atria.

• Duration: Usually around 80 milliseconds.

PR Interval: Represents the time it takes for the electrical impulse to travel from the atria to the ventricles.

• Duration: Normal range is 120 to 200 milliseconds.

QRS Complex: Represents ventricular depolarization, or the contraction of the ventricles.

• Duration: Typically less than 120 milliseconds.

ST Segment: Represents the early part of ventricular repolarization.

• Duration: Normally isoelectric (flat).

T Wave: Represents ventricular repolarization.

• Duration: Typically around 160 milliseconds.

QT Interval:Represents the total time for ventricular depolarization and repolarization.

• Duration: Varies with heart rate, corrected QT (QTc) is used to account for heart rate variations.

These durations can vary among individuals and may be influenced by factors such as age, sex, and overall health. It's important to note that abnormalities in these intervals can indicate various cardiac conditions and are used in the diagnosis and monitoring of heart diseases.