What is the PR (Prolonged Rhythm) interval in a patient with first-degree atrioventricular (AV) block?

PR Interval in First-Degree AV Block

In first-degree atrioventricular block, the PR interval is prolonged to greater than 200 milliseconds (0.20 seconds), with every atrial impulse conducted to the ventricles. 1

Defining Characteristics

• The diagnostic threshold is a PR interval exceeding 200 ms, as established by the European Society of Cardiology and American College of Cardiology guidelines 1
• Every P wave is followed by a QRS complex—there is delay but no actual block of conduction 1, 2
• The conduction delay occurs most commonly at the AV node level, though it can also occur in the His-Purkinje system 1, 3

Clinical Significance Based on PR Duration

Standard First-Degree Block (200-300 ms)

• PR intervals between 200-280 ms are usually well tolerated if left ventricular ejection fraction and heart rate are normal 1
• These patients typically remain asymptomatic and do not require pacing 1

Marked First-Degree Block (≥300 ms)

• PR intervals ≥300 ms can cause hemodynamic compromise even without higher degrees of block 1, 4
• When atrial systole occurs close to or simultaneous with the previous ventricular systole, it produces symptoms similar to pacemaker syndrome 1
• This timing abnormality reduces left ventricular filling, increases pulmonary capillary wedge pressure, and decreases cardiac output 1
• Permanent pacing is reasonable (Class IIa indication) for marked first-degree AV block causing pacemaker syndrome symptoms 1, 4

Anatomical Localization by QRS Width

• If the QRS complex is narrow, the conduction delay is usually in the AV node and rarely within the His bundle 1
• If the QRS is wide, the delay may be either in the AV node or His-Purkinje system, and only His bundle electrography can localize it precisely 1

Prognostic Implications

• Each 20-millisecond increment in PR interval is associated with increased risk: 11% increased risk of atrial fibrillation, 22% increased risk of pacemaker requirement, and 8% increased risk of all-cause mortality 5
• Individuals with first-degree AV block have a 2-fold risk of atrial fibrillation, 3-fold risk of pacemaker implantation, and 1.4-fold risk of all-cause mortality compared to those with normal PR intervals 5

Special Considerations

E-A Wave Fusion

• When PR intervals exceed 280 ms in patients with impaired relaxation or faster heart rates, fusion of mitral E and A velocities occurs on echocardiography 1
• This fusion happens when atrial contraction occurs before early diastolic flow velocity decreases to ≤20 cm/sec 1
• The resulting pattern can be misinterpreted as impaired relaxation filling pattern 1

Cardiac Resynchronization Therapy

• Approximately 50% of CRT patients have PR intervals ≥200 ms 6
• Patients with first-degree AV block undergoing CRT are more prone to electrical desynchronization and have poorer outcomes than those with normal PR intervals 6, 7