Clinical classification of AV block was done before the sites and mechanisms involved in impairing conduction between the atria and ventricles were understood. As a matter of fact, Wenkebach described his block prior to the development of ECGs by looking at the A and V waves in the jugular veins. Mobitz described his classification after the invention of the ECG but before His bundle recordings. This classification is too simple to categorize all the problems that may occur with AV conduction. However, this system is almost universally used in respect to their observations.
First-degree AV block is characterized by a delay in AV conduction manifest by a prolonged PR interval. Second-degree AV block is characterized by intermittent AV conduction—some atrial impulses reach the ventricles and others are blocked. Third-degree AV block is characterized by the complete interruption of AV conduction.
Precise localization of AV conduction blocks can be made with His bundle electrocardiography. Although this method is not available for use in the ED, correlations can be made between the clinical ECG, approximate location of the block, and risk of future progression.
AV blocks can also be divided into nodal and infranodal blocks. This is an important distinction because the clinical significance and prognosis vary with the site. AV nodal blocks (block at the AH area by the His bundle) are usually due to reversible depression of conduction, are often self-limited, and generally have a stable infranodal escape pacemaker pacing the ventricles. AV nodal blocks therefore do not usually have a serious prognosis. Infranodal blocks (block at the HV area by the His bundle) are usually due to organic disease of the His bundle or bundle branches; often the damage is irreversible. They generally have a slow and unstable ventricular escape rhythm pacing the ventricles, and they may have a serious prognosis depending on the clinical circumstance.
FIRST-DEGREE AV BLOCK In first-degree AV block, each atrial impulse is conducted into the ventricles, but more slowly than normal. This is recognized by a PR interval of greater than 0.20 s (Fig 2.4-27). The AV node is usually the site of conduction delay, although this may occur at any infranodal level.
Clinical Significance First-degree AV block is occasionally found in normal hearts. Other common causes include increased vagal tone (whatever the cause), digoxin toxicity, acute inferior MI, and myocarditis. Patients with first-degree AV block without evidence of organic heart disease appear to have no significant difference in mortality compared with matched controls.
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