The normal PR interval is:
A.
0.04-0.20 seconds
B.
0.12-0.20 seconds
C.
Less than half the R-to-R ratio
D.
0.08-0.10 seconds
The Correct answer and Explanation is:
The correct answer is B. 0.12-0.20 seconds.
Explanation:
The PR interval is a key component of the electrocardiogram (ECG) that measures the time between the onset of atrial depolarization and the onset of ventricular depolarization. It reflects the duration of electrical conduction through the atria, the atrioventricular (AV) node, and the bundle of His. The normal PR interval is an important diagnostic parameter in assessing the heart’s electrical conduction system.
A normal PR interval ranges from 0.12 to 0.20 seconds (120 to 200 milliseconds). This range represents the time it takes for the electrical impulse to travel from the sinoatrial (SA) node through the atria, the AV node, and into the ventricles. If the PR interval is longer than 0.20 seconds, it indicates a delay in conduction through the AV node, which can be a sign of first-degree heart block. Conversely, a PR interval shorter than 0.12 seconds may suggest an accessory pathway or pre-excitation syndrome, such as in Wolff-Parkinson-White syndrome.
Here’s a breakdown of the options:
- A. 0.04-0.20 seconds: This range is incorrect. A PR interval of 0.04 seconds is too short and does not accurately represent the normal conduction time through the AV node.
- B. 0.12-0.20 seconds: This is the correct range for the normal PR interval. It reflects the expected duration for normal conduction through the heart’s electrical system.
- C. Less than half the R-to-R ratio: This option is not a standard measure for the PR interval. The PR interval is not typically compared to the R-to-R interval (which is the time between successive R-wave peaks in the ECG) in this manner.
- D. 0.08-0.10 seconds: This range is too short for a normal PR interval. A PR interval of 0.08 seconds does not accommodate the full duration of atrial and AV node conduction.
In summary, a PR interval within the range of 0.12-0.20 seconds is considered normal and ensures proper electrical conduction between the atria and ventricles.