### Date : 2024-11-01 13:25 ### Topic : AV block #cardiology #ECG ---- **Atrioventricular (AV) Block** refers to a delay or complete interruption in the transmission of electrical impulses from the atria to the ventricles through the AV node. This disruption can lead to bradycardia (slow heart rate) and, in severe cases, compromise cardiac output. AV blocks are classified into three degrees based on the severity of the conduction delay. ### 1. **First-Degree AV Block** - **Definition**: In first-degree AV block, every atrial impulse reaches the ventricles, but there is a prolonged delay in the AV node. - **ECG Characteristics**: - **Prolonged PR Interval**: The PR interval is >0.20 seconds (5 small boxes on an ECG). - **Consistent PR Interval**: The PR interval is prolonged but consistent in every beat. - **Clinical Significance**: Usually asymptomatic and often benign. It does not typically require treatment but may indicate an underlying issue if associated with other symptoms or more advanced blocks. ### 2. **Second-Degree AV Block** Second-degree AV block is divided into two types: **Mobitz Type I (Wenckebach)** and **Mobitz Type II**. - **Mobitz Type I (Wenckebach)**: - **Mechanism**: There is a progressive delay in AV conduction until an impulse is blocked entirely and does not reach the ventricles. - **ECG Characteristics**: - **Progressively Lengthening PR Interval**: The PR interval gradually increases with each beat until a QRS complex is dropped. - **Grouped Beating**: This pattern of progressive prolongation and a subsequent dropped beat repeats in a cycle. - **Clinical Significance**: Usually benign and often asymptomatic. It may occur in healthy individuals, especially during sleep, or be caused by medications. No treatment is needed unless symptomatic. - **Mobitz Type II**: - **Mechanism**: Some impulses are blocked at a fixed ratio (e.g., 2:1, 3:1) without progressive prolongation of the PR interval. - **ECG Characteristics**: - **Consistent PR Interval**: PR intervals remain constant for conducted beats. - **Dropped Beats**: Intermittent dropped QRS complexes without a preceding increase in the PR interval. - **Clinical Significance**: More concerning than Mobitz I. It often indicates underlying structural disease in the conduction system and can progress to complete heart block (third-degree). Treatment with a **pacemaker** is generally recommended, especially if symptomatic. ### 3. **Third-Degree AV Block (Complete Heart Block)** - **Definition**: In third-degree (or complete) AV block, there is a complete disconnection between atrial and ventricular activity, with no impulses transmitted from the atria to the ventricles. - **ECG Characteristics**: - **P Waves and QRS Complexes are Independent**: There is no relationship between the P waves and QRS complexes, as the atria and ventricles beat independently. - **Atrial Rate > Ventricular Rate**: The atrial rate (P wave rate) is usually faster than the ventricular rate. - **Escape Rhythm**: A slower escape rhythm originating from a lower part of the conduction system (junctional or ventricular) takes over to maintain some level of cardiac output. - **Clinical Significance**: Third-degree AV block is a medical emergency, as it can result in severe bradycardia, syncope, heart failure, or sudden cardiac arrest. Immediate treatment with a **temporary or permanent pacemaker** is necessary. ### Causes of AV Block AV blocks can arise from various causes, including: - **Medications**: Drugs like beta blockers, calcium channel blockers, digoxin, and antiarrhythmics can depress AV node function. - **Ischemic Heart Disease**: Myocardial infarction, especially in the inferior wall, can affect the AV node and lead to AV block. - **Degenerative Changes**: Fibrosis and calcification of the conduction system, common with aging, can cause AV block. - **Infections and Inflammatory Conditions**: Conditions like myocarditis, Lyme disease, and rheumatic fever may affect the AV node. - **Congenital AV Block**: Some people are born with AV block due to genetic factors or conditions like maternal lupus. ### Management of AV Block - **First-Degree AV Block**: Generally does not require treatment unless caused by medications, in which case medication review may be helpful. - **Second-Degree AV Block**: - **Mobitz Type I**: Often no treatment is needed unless symptomatic. In symptomatic cases, underlying causes are treated, and medications are adjusted if contributing to the block. - **Mobitz Type II**: Pacemaker implantation is usually recommended because of the risk of progression to third-degree AV block. - **Third-Degree AV Block**: Requires immediate intervention with a pacemaker. Temporary pacing may be used in emergencies, followed by permanent pacemaker placement. ### Summary - **First-Degree**: Prolonged PR interval, benign, no treatment typically required. - **Second-Degree**: - **Mobitz I**: Progressive PR prolongation with dropped beats, generally benign. - **Mobitz II**: Dropped beats without PR prolongation, more serious, often requires a pacemaker. - **Third-Degree**: Complete dissociation between atria and ventricles, emergency treatment with pacemaker required. ### Reference: - ### Connected Documents: -