What is the normal PR interval?

Appeared in: AIIMS Delhi NO - 2018

Explanation

12-0.20 seconds, which is the standard accepted range for a normal PR interval in adults.
The PR interval represents the time from the onset of atrial depolarization (the beginning of the P wave) to the onset of ventricular depolarization (the beginning of the QRS complex).
This duration includes the time it takes for the impulse to travel from the SA node, through the atria, and across the AV node, where a physiological delay occurs.
A normal PR interval of 0.12-0.20 seconds corresponds to 3-5 small squares on a standard ECG grid (where one small square is 0.04 seconds).

Option A: This range is too narrow. While a PR interval of 0.12-0.16 seconds is normal, it excludes the upper part of the normal range, which extends up to 0.20 seconds.
Option C: This range is incorrect because a PR interval consistently greater than 0.20 seconds is, by definition, a first-degree atrioventricular (AV) block, which is an abnormal finding.
Option D: This range is excessively long and physiologically incorrect for a PR interval. A duration of 0.6-0.8 seconds (600-800 ms) is more typical for the entire cardiac cycle (R-R interval) at a heart rate of 75-100 beats per minute.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Normal ECG Parameters: PR Interval as background academic context rather than a clinical decision trigger.
Nurses must be able to accurately measure the PR interval on an ECG strip to identify conduction abnormalities.
A prolonged PR interval (first-degree AV block) can be caused by medications (like beta-blockers, calcium channel blockers, digoxin), electrolyte imbalances, or ischemic heart disease. It requires monitoring for progression to higher-degree blocks.
A short PR interval can indicate a pre-excitation syndrome, which carries a risk for tachyarrhythmias. The nurse should be vigilant for signs of rapid heart rate, dizziness, or syncope.

How to Approach the Question

First, identify the core of the question. It is asking for a specific, factual piece of information: the normal value for the PR interval on an ECG.
This is a direct recall question. Access your knowledge of normal ECG parameters.
Recall that the PR interval reflects the time for the electrical impulse to travel from the atria to the ventricles.
Remember the standard range: 0.12 to 0.20 seconds, which corresponds to 3 to 5 small boxes on ECG paper.
Evaluate the given options against this standard value. Option '0.12-0.20' is an exact match.
Eliminate other options by identifying why they are incorrect (e.g., too narrow, too long, or representing a pathological state).

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 1 p. 1909-1911

Practise the full AIIMS Delhi NO - 2018

Attempt every question from this paper in a timed mock, then review the full solution for each one.