Classic heart block, often referred to as atrioventricular (AV) block, represents a disruption in the normal electrical conduction pathway between the atria and ventricles. This condition implies a delay or complete cessation of impulse transmission at the atrioventricular node, a critical gateway for coordinated cardiac contraction. Understanding the nuances of this conduction abnormality is essential for clinicians managing patients with syncope, fatigue, or those found incidentally on rhythm monitoring.
Physiological Mechanisms and Anatomy
The heart's electrical system relies on a precise sequence, beginning with the sinoatrial node and progressing through the AV node. The AV node serves as a buffer, slowing the impulse to allow atrial systole to complete ventricular filling before ventricular depolarization. Classic heart block occurs when this delay becomes pathological, either slowing the impulse significantly or halting it entirely. The block is typically classified by the degree of conduction failure, ranging from a slight delay to a total disconnect between atrial and ventricular activity.
Classification and Grading
First-Degree Heart Block
First-degree heart block is characterized by a consistently prolonged PR interval on the electrocardiogram, exceeding 200 milliseconds. While the impulse eventually reaches the ventricles, the delay is noticeable. This grade is often asymptomatic and may be a normal variant, particularly in athletes, or a consequence of medications such as beta-blockers. Identification usually occurs during a routine physical exam or ECG for an unrelated issue.
Second-Degree Heart Block
Second-degree heart block is subdivided into two distinct patterns. Mobitz Type I, or Wenckebach, features a progressive lengthening of the PR interval until a beat is dropped, creating a cyclical pattern. This type is generally associated with a block within the node itself and is often transient. Mobitz Type II presents with a fixed PR interval followed by a sudden, non-conducted P wave. This variant suggests a block below the node in the His-Purkinje system and carries a higher risk of progressing to complete heart block.
Third-Degree (Complete) Heart Block
Third-degree heart block, or complete heart block, is the most severe form of classic heart block. Here, no atrial impulses reach the ventricles, resulting in complete dissociation between the P waves and QRS complexes. The ventricles generate an escape rhythm to maintain perfusion, but this rhythm is typically slow and unreliable. Symptoms can be profound, including dizziness, heart failure, and syncope, necessitating urgent intervention.
Etiology and Risk Factors
Acute myocardial infarction, particularly inferior wall infarction, is a leading cause of new-onset heart block due to ischemia affecting the conduction system. Chronic conditions, however, often involve fibrosis and calcification of the valve apparatus or the conduction system itself. Degenerative changes associated with aging, infiltrative diseases like sarcoidosis, or autoimmune disorders such as Lyme disease are common culprits. Additionally, athletes may exhibit transient first-degree block due to enhanced vagal tone.
Clinical Presentation and Diagnosis
The presentation of classic heart block is highly variable. Some individuals with first-degree block may remain entirely unaware of their condition. Those with higher-grade blocks might report exertional fatigue, presyncope, or near-syncopal episodes. A thorough physical exam, including carotid massage to assess the vagal tone, is the initial step. The definitive diagnosis is made via a 12-lead ECG, which visualizes the exact timing delay and the relationship between atrial and ventricular activity. For intermittent symptoms, Holter monitoring or event recorders provide dynamic data to correlate symptoms with rhythm abnormalities.
