Why the PR Interval Deserves Attention 🫀
In everyday ECG reading, attention gravitates toward drama. The PR interval, by contrast, sits patiently between the P wave and the QRS complex. It is easy to skip — and that is exactly why it matters. It tells you how efficiently the impulse travels from atria to ventricles and how close the system is to losing coordination.
A Short Clinical Story: The Calm ECG That Wasn’t
A young man arrives for preoperative assessment. No symptoms. Normal vitals. The ECG looks calm — until someone measures carefully: the PR interval is 280 milliseconds.
Weeks later, he presents with dizziness — then syncope. The atrioventricular node had been slowing down quietly long before it failed loudly. The PR interval warned. No one listened.
What the PR Interval Actually Represents
The PR interval represents the time taken for an electrical impulse to travel from the atria to the ventricles. Physiologically, it includes:
- Atrial depolarization
- Conduction through atrial myocardium
- Delay within the atrioventricular node
- Transit through the His bundle
- Arrival at the ventricular myocardium
The PR interval is a deliberate physiological pause — a timing buffer that lets atrial contraction finish before ventricular contraction begins.
Normal PR Interval: Numbers That Matter
A normal PR interval is 120 to 200 milliseconds. Shorter suggests the impulse is reaching the ventricles too quickly; longer suggests delay in conduction.
How to Measure the PR Interval Correctly
Measure from the beginning of the P wave to the beginning of the QRS complex. The best lead is often Lead II, where the P wave is typically clear and upright.
- Measure multiple beats
- Average if needed
- Beware baseline wander and poorly defined P onset
Physiology Behind the PR Interval Delay
The atrioventricular node contributes most of the PR duration. Its slower conduction is expected because of smaller cells, fewer gap junctions, and calcium-dependent depolarization.
This is not a defect — it is a safety feature that preserves mechanical coordination.
Short PR Interval: When the Heart Rushes ⚡
A PR interval shorter than 120 milliseconds suggests the impulse reaches the ventricles too quickly. This can reflect accessory pathways, junctional rhythms, or pre-excitation physiology.
Common causes
- Accessory pathways
- Junctional rhythms
- Pre-excitation syndromes
- High sympathetic tone
ECG clues
- PR interval less than 120 milliseconds
- May be associated with a delta wave
- May have widened QRS complex (in pre-excitation)
Prolonged PR Interval: When the Heart Hesitates 🐢
A PR interval greater than 200 milliseconds defines first-degree atrioventricular block. Every impulse conducts — just slowly.
Why it happens
- Increased vagal tone
- Degenerative conduction disease
- Myocardial ischemia
- Drugs affecting atrioventricular conduction
- Electrolyte disturbances
First-Degree AV Block: More Than a Benign Label
First-degree atrioventricular block was once dismissed as harmless. Modern data suggest it can be a marker of conduction system disease and is associated in selected populations with increased risk of atrial fibrillation and higher likelihood of pacemaker implantation.
PR Interval and Drugs: A Silent Interaction 💊
Many routinely used drugs prolong the PR interval by slowing atrioventricular nodal conduction:
- Beta-blockers
- Non-dihydropyridine calcium channel blockers
- Digoxin
- Antiarrhythmic agents
The critical issue is additive effect. A small prolongation in isolation can become dangerous when combined with other nodal-slowing drugs or disease.
PR Interval in Athletes and High Vagal Tone
In trained athletes, mild PR prolongation can be physiological due to increased parasympathetic tone. Context matters: asymptomatic status, reversibility with exercise, and normal QRS morphology support a benign interpretation.
PR Interval in Acute Settings: ICU and Anesthesia
In critical care and anesthesia, PR interval changes can function as an early warning sign for evolving conduction disease, drug toxicity, ischemic injury, or electrolyte imbalance. A lengthening PR interval may precede higher-grade atrioventricular block.
Treat a rising PR interval like smoke: it may be the earliest sign of a conduction fire.
When PR Interval Becomes Dangerous
Red flags include progressive PR prolongation, very long PR intervals (for example greater than 300 milliseconds), associated symptoms, or coexisting bundle branch block.
A Poetic Pause 🌿
The ventricle waits.
Between them —
a pause that keeps the heart honest.
Common Mistakes in PR Interval Interpretation 🚫
- Measuring from the wrong P wave onset
- Ignoring baseline wander
- Overlooking drug effects
- Dismissing prolonged PR as always benign
- Forgetting age and clinical context
Exam Perspective: How PR Interval Is Tested 🧠
- Short PR with delta wave suggests pre-excitation physiology
- Prolonged PR with all beats conducted suggests first-degree atrioventricular block
- Progressively lengthening PR raises suspicion for higher-grade atrioventricular conduction disease
How PR Interval Changes Your ECG Reading
Once you respect the PR interval, ECG interpretation becomes more physiological. You begin to anticipate conduction failure, evaluate medication effects, and recognize early patterns before crises declare themselves.
Quick Self-Check Before You Move On
- Can I measure PR interval accurately?
- Can I identify short versus prolonged PR?
- Can I correlate PR changes with drugs and disease?
- Do I know when PR prolongation is dangerous?
References
- Goldberger AL. Clinical Electrocardiography: A Simplified Approach.
- Braunwald E. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine.
- Marriott HJL. Practical Electrocardiography.
- Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice.
- Guyton AC, Hall JE. Textbook of Medical Physiology.
- American Heart Association: https://www.heart.org/
- UpToDate: https://www.uptodate.com/
