The PR interval extends from the onset of the P wave to the onset of the QRS complex and represents the electrical conduction time from the atria to the ventricles. Within it there is a specific portion called the PR segment, which corresponds only to the isoelectric stretch after the end of the P wave.

Although they share the same landmark, interval and segment are not synonyms. The former includes all atrioventricular conduction measurable on the ECG; the latter represents only the isoelectric phase related to nodal delay.

In this schematic summary, we will review the definition of each, the normal values, and the main associated abnormalities, focusing on practical application during systematic ECG reading.

Representation of the PR interval and PR segment on the electrocardiogram
PR interval and PR segment.

PR Interval

Definition

Interval extending from the onset of the P wave to the onset of the QRS complex.

Includes:

  • Atrial depolarization
  • Intra-atrial conduction
  • Nodal delay in the AV node
  • Conduction through the proximal His bundle

What It Represents

Reflects the total atrioventricular conduction time.

The main determinant of its value is the physiological delay in the AV node, which is modulated by the autonomic nervous system. For this reason, the PR interval varies inversely with heart rate.

Normal Values

  • Normal (adults): 120-200 ms (3 to 5 small boxes)
  • < 120 ms: short PR
  • > 200 ms: prolonged PR

Age-related variation:

  • Children: the PR interval tends to be shorter, ranging approximately from 90 to 170 ms, depending on age and heart rate.
  • Older adults: there may be slight physiological PR prolongation, related to greater fibrosis of the conduction system and a higher prevalence of degenerative nodal disease.

In addition to age, the PR interval is influenced by heart rate and autonomic tone.

Main Abnormalities

Prolonged PR (>200 ms)

Electrocardiogram showing first-degree atrioventricular block with a prolonged PR interval
First-degree atrioventricular block.

Main mechanisms/causes:

  • Increased vagal tone
  • Drugs that depress nodal conduction (beta-blockers, verapamil, diltiazem, digoxin)
  • Degenerative disease of the conduction system
  • Ischemia or inflammation involving the AV node

Progressively lengthening PR with a blocked QRS

Electrocardiogram showing second-degree atrioventricular block type 1 with progressive PR prolongation
Second-degree atrioventricular block type 1.

Fixed PR with sudden blocked QRS complexes

Electrocardiogram showing second-degree atrioventricular block type 2 with sudden conduction failure
Second-degree atrioventricular block type 2.

Short PR (<120 ms)

Electrocardiogram with ventricular preexcitation showing a delta wave
Ventricular preexcitation.

PR Segment

Definition

Isoelectric stretch between the end of the P wave and the onset of the QRS complex.

Corresponds to the period when there is no atrial electrical activity detectable at the body surface, predominantly reflecting nodal and early infranodal conduction.

What It Represents

Represents the AV nodal delay itself.

Serves as the baseline reference for ST-segment analysis.

During sinus tachycardia, it may show a slight downward slope, with no pathological significance on its own.

Normal Values

Should be isoelectric relative to the TP segment.

Small variations depend on how the baseline is defined and on heart rate.

Main Abnormalities

Diffuse PR-segment depression

Focal PR deviation

  • May occur in atrial injury
  • May accompany localized inflammatory processes

PR-segment deviation should not be interpreted in isolation; its analysis should always consider the clinical context and concomitant ST-segment changes.

References

  1. Scheffer MK, De Marchi MFN, de Alencar Neto JN, Felicioni SP. Eletrocardiograma de A a Z. São Paulo: Manole; 2024.
  2. Surawicz B, Knilans TK. Chou's electrocardiography in clinical practice. 6th ed. Philadelphia: Saunders Elsevier; 2008.