![]() Among the causes of first-degree AV block with a prolonged AH interval are increased vagal tone, calcium channel blockers (which block the inward calcium current responsible for depolarization), digoxin (acting via its vagotonic action), and beta-blockers. Over 90 percent of patients with a PR interval greater than 300 ms have slowed AV nodal conduction. Conduction through the AV node is approximated quite well by the atrial-His (AH) time determined by His bundle ECG. The AV node is the most common site of conduction delay in first-degree block. The levels of conduction delay associated with first-degree AV block, and potential related etiologies, includes the following: However, the presence of additional evidence of conduction abnormalities (eg, bundle branch block) may be an indicator of more widespread conduction disease. Clinically, because first-degree AV block is most often asymptomatic and diagnosed by electrocardiogram (ECG), there is usually little clinical significance to the level of conduction delay. LEVEL OF CONDUCTION DELAY - First-degree AV block, or prolonged AV conduction, can occur at a variety of levels within the heart. (See "Etiology of atrioventricular block", section on 'Idiopathic'.) Infiltrative and dilated cardiomyopathies (eg, sarcoidosis).In comparison, an inferior wall myocardial infarction is more likely to be associated with delay in the AV node since the AV node and inferior wall are usually supplied by the right coronary artery. Myocardial infarction – When seen in the setting of an anterior wall myocardial infarction, this finding usually indicates bundle and fascicular block.Drugs that impair or slow nodal conduction including digoxin, beta blockers, and non-dihydropyridine calcium channel blocking agents.An increase in vagal tone that causes a reduction in the rate of impulse conduction.Underlying structural abnormalities of the node.A partial list of pathologic causes of first-degree AV block include the following: However, prolongation of the PR interval often represents underlying cardiac pathology. The conduction delay is most frequently in the AV node but may also be in the His-Purkinje system.ĮTIOLOGY - Patients who have a slow resting heart rate, such as highly conditioned endurance athletes, may have evidence of first-degree AV block simply due to increased vagal tone and a lower resting heart rate. ![]() Because of this, it is more appropriate to use the term "prolonged AV conduction" rather than AV block. (See "ECG tutorial: Basic principles of ECG analysis", section on 'PR interval'.)įirst-degree AV block, defined as a prolonged PR interval (>200 ms at resting heart rates), is not a true block but is rather delayed or slowed AV conduction. However, some apparently healthy persons have longer PR intervals, with PR intervals as long as 280 ms having been reported in 1.6 percent of healthy aviators. The normal PR interval is considered to be between 120 and 200 ms (0.12 to 0.20 s) and tends to shorten with increases in heart rate due in part to rate-related shortening of action potentials. (See "Etiology of atrioventricular block" and "Second-degree atrioventricular block: Mobitz type I (Wenckebach block)" and "Second-degree atrioventricular block: Mobitz type II" and "Third-degree (complete) atrioventricular block" and "Congenital third-degree (complete) atrioventricular block".)ĭEFINITION - The PR interval, which measures conduction between the atria and the ventricles, includes atrial depolarization (the P wave) and subsequent conduction through the AV node, His bundle, bundle branches and fascicles, and terminal Purkinje fibers ( figure 1). The etiology of AV block in general, and the management of other specific types of AV block, are discussed separately. The clinical presentation, evaluation, and management of first-degree AV block will be reviewed here. High-grade AV block – Intermittent atrial conduction to the ventricle with two or more consecutive blocked P waves but without complete AV block.Third-degree (complete AV) block – No atrial impulses conduct to the ventricle.Second-degree AV block – Intermittent atrial conduction to the ventricle, often in a regular pattern (eg, 2:1, 3:2), or higher degrees of block, which are further classified into Mobitz type I (Wenckebach) and Mobitz type II second-degree AV block.First-degree AV block – Delayed conduction from the atrium to the ventricle (defined as a prolonged PR interval of >200 ms) without interruption in atrial to ventricular conduction.The conduction disturbance can be transient or permanent, with conduction that is delayed, intermittent, or absent. INTRODUCTION - Atrioventricular (AV) block is defined as a delay or interruption in the transmission of an impulse from the atria to the ventricles due to an anatomic or functional impairment in the conduction system.
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