ECG Basic – PR Interval

Clinical calculation support · Verify results before clinical use

Appcardio ECG Academy · ECG Basics

PR Interval

Understand how to measure the PR interval, recognize normal and abnormal values, and use a short or prolonged PR interval to guide ECG interpretation.

Measurement & interpretationCore level~7 min read

Learning objectives

✓Define the PR interval and distinguish it from the PR segment.

✓Measure the interval using ECG paper calibration.

✓Identify normal, prolonged, and shortened PR intervals.

✓Recognize common ECG patterns associated with PR abnormalities.

✓Relate PR findings to AV conduction and rhythm diagnosis.

✓Avoid common measurement and interpretation pitfalls.

1. What is the PR interval?

The PR interval is measured from the first deflection of the P wave to the first deflection of the QRS complex. It represents the time required for an electrical impulse to travel from the onset of atrial depolarization until the onset of ventricular depolarization. This includes conduction through the atria, AV node, and His–Purkinje system.

Original schematic: the interval includes the P wave and PR segment; its endpoint is the beginning of the QRS complex.

Important distinction: The PR interval and PR segment are not interchangeable. The PR segment begins at the end of the P wave and ends at the start of the QRS complex. The PR interval includes both the P-wave duration and the PR segment.

2. Normal PR interval

In adults, the normal PR interval is generally 120–200 milliseconds (0.12–0.20 seconds), equivalent to approximately three to five small ECG squares at a paper speed of 25 mm/s.

120 msLower reference limit

200 msUpper reference limit

40 msOne small square at 25 mm/s

How to measure

Identify a lead in which the onset of the P wave and QRS complex are clearly visible. Lead II is often useful.
Mark the earliest beginning of the P wave, then identify the earliest beginning of the QRS complex.
Count the small squares between these two points and multiply by 40 ms at 25 mm/s.
Measure several representative beats when the rhythm or conduction varies; report the pattern as well as the value.

3. ECG paper conversion

At the standard paper speed of 25 mm/s:

Horizontal distance	Time
1 small square (1 mm)	40 ms
2 small squares	80 ms
3 small squares	120 ms
5 small squares	200 ms
1 large square (5 mm)	200 ms

Quick calculation:
PR interval = number of small squares × 40 ms.

If the ECG is recorded at a different paper speed, recalculate the time per square before measuring.

4. Prolonged PR interval (>200 ms)

A PR interval longer than 200 ms indicates delayed atrioventricular conduction. When every atrial impulse is conducted to the ventricles and the PR interval remains prolonged, the ECG pattern is classified as first-degree AV block.

Schematic only: a prolonged but constant PR interval with 1:1 AV conduction.

First-degree AV block

PR interval is greater than 200 ms.
Each P wave is followed by a QRS complex (1:1 AV conduction).
The PR interval is prolonged but remains essentially constant from beat to beat.

Clinical interpretation: First-degree AV block describes delayed conduction rather than a complete failure of conduction. Assess the patient, medications, rhythm, QRS width, and any associated conduction abnormalities rather than interpreting the PR value in isolation.

PR prolongation with second-degree AV block

A prolonged PR interval can also be seen in second-degree AV block. In Mobitz I (Wenckebach), the PR interval progressively lengthens over consecutive conducted beats until a P wave is not followed by a QRS complex; after the pause, the sequence starts again with a shorter PR interval.

Do not label every prolonged PR interval as first-degree AV block. Check whether all P waves conduct. A non-conducted P wave changes the diagnosis and requires assessment of the AV conduction pattern.

5. Short PR interval (<120 ms)

A PR interval shorter than 120 ms may occur when ventricular activation begins earlier than expected through an accessory pathway, or when the atrial impulse originates close to the AV node or His region, as can occur with a junctional rhythm.

Pre-excitation pattern

In Wolff–Parkinson–White (WPW) pattern, an accessory atrioventricular pathway activates part of the ventricle early. The classic resting ECG combination is a short PR interval, a delta wave (slurred initial QRS upstroke), and a widened QRS complex.

Short PR + delta wave + wide QRS

Junctional rhythm

A junctional rhythm originates in or near the AV junction. P waves may be absent, occur before the QRS with a short PR interval, or appear after the QRS when atrial activation is retrograde.

Absent or retrograde P waves

Pre-excitation: recognizing the WPW pattern

Conceptual waveform: the delta wave is the slurred initial upstroke of the QRS in ventricular pre-excitation.

Accessory pathway conduction can provide the substrate for atrioventricular re-entrant tachycardia (AVRT). Patients may present with episodes of paroxysmal supraventricular tachycardia.

Interpret the whole ECG: A short PR interval alone does not establish WPW. Look for the delta wave and QRS widening, and interpret the tracing in the context of the clinical presentation.

Lown–Ganong–Levine (LGL) terminology

The supplied reference describes the historical LGL pattern as a very short PR interval with normal P-wave morphology, a normal-width QRS, and no delta wave. This label is included to reflect the source material; a short PR with a narrow QRS should not, by itself, be taken as proof of a discrete accessory pathway or a definitive contemporary syndrome diagnosis.

6. Junctional rhythm and the PR interval

In a junctional rhythm, the impulse arises from the AV junction rather than the sinus node. Atrial activation may occur retrogradely, producing inverted P waves in leads where sinus P waves are normally upright. Depending on the timing of atrial activation relative to ventricular activation, the P wave may be close to, hidden within, or follow the QRS complex.

Simplified example of retrograde atrial activation; actual P-wave timing and polarity vary with the junctional focus and conduction.

Before the QRS: a retrograde P wave may produce a short PR interval.
Within the QRS: atrial activity may be obscured and the P wave may not be identifiable.
After the QRS: an inverted retrograde P wave may follow the ventricular complex.

7. Practical interpretation framework

PR finding	Common ECG association	What to assess next
120–200 ms	Within the usual adult reference range	Rhythm, P-wave morphology, and AV conduction pattern
>200 ms, constant	First-degree AV block when every P conducts	Symptoms, medications, QRS width, and associated conduction disease
Progressively lengthening	Mobitz I pattern when followed by a non-conducted P wave	Sequence of PR intervals and dropped ventricular response
<120 ms with delta wave	Ventricular pre-excitation (e.g., WPW pattern)	QRS width, delta wave, symptoms, and tachyarrhythmia history
<120 ms with altered/absent P	May occur in junctional rhythm	P-wave timing and polarity in multiple leads

Common pitfalls

Measuring from the peak of the P wave instead of its onset.
Ending the measurement at the R-wave peak rather than the first onset of the QRS.
Confusing the PR segment with the full PR interval.
Interpreting a single PR measurement without checking for dropped beats or changing conduction.
Diagnosing pre-excitation from a short PR interval without assessing QRS morphology.

8. Key takeaways

The PR interval extends from the onset of the P wave to the onset of the QRS complex.
The usual adult reference range is 120–200 ms (three to five small squares at 25 mm/s).
A constant PR interval >200 ms with 1:1 AV conduction is the pattern of first-degree AV block.
Progressive PR lengthening followed by a non-conducted P wave suggests Mobitz I AV block.
A PR interval <120 ms should prompt assessment for pre-excitation or a junctional rhythm; interpret the P wave and QRS morphology together.

References

Cadogan M. PR Interval. Life in the Fast Lane (LITFL), ECG Library Basics. Published August 1, 2020. https://litfl.com/pr-interval-ecg-library/

Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. 13th ed. Wolters Kluwer; 2021.

Hampton JR, Adlam D. The ECG Made Practical. 7th ed. Elsevier; 2019.

Surawicz B, Knilans TK. Chou's Electrocardiography in Clinical Practice: Adult and Pediatric. 6th ed. Saunders; 2008.

Related ECG Academy lessons

Continue with the ECG components that help complete rhythm and conduction assessment.

Medical education disclaimer: This page is intended for educational purposes and does not replace clinical assessment, specialist consultation, current guidelines, or local institutional protocols. ECG interpretation should always be integrated with the patient's symptoms, clinical context, and the complete 12-lead tracing.