PR prolongation, also known as first-degree atrioventricular (AV) block, is an electrocardiogram (ECG) finding that indicates a delay in the heart’s electrical signals as they travel from the upper to lower chambers. This common observation often presents without noticeable symptoms and is discovered incidentally during routine check-ups.
The Heart’s Electrical System and the PR Interval
The heart’s electrical system coordinates its contractions. This system begins with the sinoatrial (SA) node, the heart’s natural pacemaker, which generates electrical impulses in the right atrium. These impulses then spread across both upper chambers, causing them to contract and pump blood into the lower chambers.
The electrical signal then reaches the atrioventricular (AV) node, located between the atria and ventricles. The AV node briefly delays the impulse, ensuring the upper chambers fully empty before the ventricles begin to contract. From the AV node, the impulse travels through the bundle of His, bundle branches, and Purkinje fibers, rapidly spreading throughout the ventricles to initiate contraction.
On an ECG, the PR interval measures the time for this electrical impulse to travel from the beginning of atrial depolarization (P wave) to the start of ventricular depolarization (QRS complex). A normal PR interval is 0.12 to 0.20 seconds (120 to 200 milliseconds). An interval exceeding this range is considered prolonged.
Causes of a Prolonged PR Interval
A prolonged PR interval can stem from various factors, ranging from physiological adaptations to underlying medical conditions or medications.
Some individuals, particularly highly trained athletes, may exhibit a prolonged PR interval due to increased vagal tone, a physiological response that naturally slows heart conduction. This is often observed in athletes as part of “athletic heart syndrome” and is generally considered a benign finding.
Common culprits include beta-blockers, calcium channel blockers, and digoxin, which are frequently prescribed for various heart conditions or high blood pressure. Other drug classes, such as certain antiarrhythmics like Class Ia, Ic, and III agents, can also have this effect.
A prolonged PR interval can also be a sign of various medical conditions affecting the heart or other body systems. These include ischemic heart disease (including a history of heart attack), inflammation of the heart muscle (myocarditis), rheumatic fever, Lyme disease, and other infections like endocarditis, Chagas disease, and diphtheria. Infiltrative diseases such as sarcoidosis, amyloidosis, or hemochromatosis can also affect the heart’s conduction system.
Electrolyte imbalances, specifically elevated potassium levels (hyperkalemia), can disrupt normal heart function and prolong the PR interval. Less commonly, low levels of potassium or magnesium have also been implicated. Additionally, congenital heart conditions, thyroid disorders like hypothyroidism, and systemic lupus erythematosus have been associated with this finding. Sometimes, a prolonged PR interval can appear following certain cardiac procedures, such as valvular surgery or catheter ablation.
Associated Symptoms and Diagnosis
Most isolated first-degree AV block cases do not cause symptoms and are discovered incidentally during a routine electrocardiogram (ECG). Individuals often remain unaware because their heart continues to beat effectively, despite a slight delay. When symptoms do occur, they are uncommon and may indicate a more significant underlying issue or a progression of the conduction delay. Potential symptoms include dizziness, fatigue, or shortness of breath. In marked cases where the PR interval exceeds 0.30 seconds, patients might experience lightheadedness, chest pain, or fainting, sometimes due to poor coordination between atrial and ventricular contractions. Diagnosis is made solely through an ECG, which visually displays the prolonged interval between the P wave and the QRS complex.
Understanding Different Degrees of Heart Block
PR prolongation is classified as First-Degree Atrioventricular (AV) Block, the mildest form of heart block. In this condition, every electrical impulse from the atria successfully reaches the ventricles, but conduction is slowed, resulting in a consistently prolonged PR interval on the ECG. There are no missed beats, and the heart maintains a regular rhythm.
Second-Degree AV Block
More advanced forms of AV block indicate greater disruption in the heart’s electrical pathway. Second-degree AV block involves some atrial impulses failing to reach the ventricles, leading to skipped beats. This category is divided into two types.
Mobitz Type I, also known as Wenckebach, shows progressive lengthening of the PR interval with each beat until one beat is dropped. This type is generally less severe. Mobitz Type II is characterized by intermittent dropped beats without prior PR interval lengthening. This form carries a higher risk of progressing to a complete block.
Third-Degree AV Block (Complete Heart Block)
This is the most severe form, with total electrical dissociation between the atria and ventricles, meaning no impulses from the atria reach the ventricles. The ventricles then rely on their own backup pacemaker, resulting in a very slow and independent heart rate, typically between 20 and 40 beats per minute.
Management and Treatment Approaches
For most individuals with asymptomatic, isolated first-degree AV block, specific treatment is not required. The standard approach involves observation and periodic ECG monitoring to track any changes or new symptoms.
If a reversible cause for the prolonged PR interval is found, treatment focuses on addressing that underlying issue. This may involve adjusting or discontinuing medications known to slow heart conduction, such as beta-blockers or calcium channel blockers. Correcting an electrolyte imbalance, like high potassium levels, would also be a primary intervention.
More invasive treatments, such as pacemaker implantation, are reserved for patients who experience symptoms from higher-degree AV blocks (second or third degree). While not indicated for isolated first-degree AV block, a pacemaker may be considered in marked cases where the PR interval exceeds 0.30 seconds and the patient experiences symptoms like significant fatigue or dizziness. However, a pacemaker is not recommended for asymptomatic first-degree AV block.