From the Guidelines
Different types of heart blocks affect the PR interval in distinct ways, with first-degree heart block showing a prolonged PR interval beyond 200 milliseconds, second-degree heart block type I (Mobitz I or Wenckebach) showing progressive PR interval prolongation, second-degree heart block type II (Mobitz II) showing a constant PR interval for conducted beats, and third-degree (complete) heart block showing no relationship between P waves and QRS complexes. The effects of heart blocks on the PR interval can be understood by examining the characteristics of each type of block.
- First-degree heart block is characterized by a prolonged PR interval, which remains constant at greater than 200 milliseconds, indicating delayed conduction through the AV node without dropped beats 1.
- Second-degree heart block type I (Mobitz I or Wenckebach) shows progressive PR interval prolongation until a P wave fails to conduct, creating a pattern of gradually lengthening PR intervals followed by a dropped QRS complex 1.
- Second-degree heart block type II (Mobitz II) is characterized by a constant PR interval for conducted beats, but some P waves suddenly fail to conduct without prior PR prolongation 1.
- Third-degree (complete) heart block shows no relationship between P waves and QRS complexes, with P waves occurring at regular intervals while ventricular beats occur independently at a slower rate; thus, the PR interval varies continuously and has no consistent measurement 1. These variations in PR interval reflect different levels of conduction system dysfunction, from simple delay to complete dissociation between atrial and ventricular activity, and help clinicians determine the location and severity of the conduction abnormality. Key points to consider when evaluating the PR interval in heart blocks include:
- The level of conduction block, which can be at the AV node, bundle of His, or below 1
- The presence of symptoms, such as syncope or fatigue, which can indicate more severe conduction system dysfunction 1
- The results of electrophysiological studies, which can help determine the site and severity of conduction block 1 By considering these factors and understanding the effects of different types of heart blocks on the PR interval, clinicians can provide more accurate diagnoses and effective treatments for patients with conduction system disorders.
From the Research
Effect of Heart Blocks on PR Interval
The PR interval is affected differently by various types of heart blocks. Here are the effects of different heart blocks on the PR interval:
- First-degree atrioventricular block: Characterized by a prolonged PR interval, typically greater than 0.20 seconds 2, 3, 4. This type of block is generally considered benign, but extreme forms (PR interval exceeding 0.30 seconds) can cause symptoms due to inadequate timing of atrial and ventricular contractions 2, 4.
- Second-degree atrioventricular block (Mobitz type II): Characterized by periodic atrioventricular block with constant PR intervals in the conducted beats 5. The PR interval in the conducted beats remains normal, but some atrial beats are blocked, resulting in a variable ventricular rate.
- Third-degree atrioventricular block: Not directly discussed in the provided studies, but it is known that this type of block is characterized by a complete block of electrical conduction between the atria and ventricles, resulting in an abnormal PR interval that is often variable and unrelated to the atrial rate.
Clinical Implications
The clinical implications of these changes in PR interval vary depending on the type and severity of the heart block:
- Prolonged PR interval: Associated with an increased risk of mortality, heart failure, and atrial fibrillation 3.
- First-degree atrioventricular block: Generally considered benign, but may require pacing in symptomatic patients with a very long PR interval 2, 4.
- Second-degree atrioventricular block: May progress to complete heart block and Stokes-Adams arrest, especially if the block is located below the AV node 5.