Poor ventricular filling due to prolonged PR intervals may also result in mitral regurgitation, which exacerbates conditions such as heart failure.įirst-degree AV block is almost universally without associated symptoms. As the PR interval extends beyond 0.30 seconds, synchrony of atrial and ventricular systole worsens, potentially resulting in poor ventricular preload and symptoms of the “pacemaker syndrome,” further characterized below. Prolonged conduction is well-tolerated, especially when the PR interval remains shorter than 0.30 seconds. Patients with conduction abnormalities originating in the His or Purkinje systems are more likely to have prolonged QRS intervals as well as the prolonged PR interval of first-degree AV block. Delayed conduction in these areas is more often due to underlying heart disease and more frequently progresses to higher degree AV blockade. The conduction delay may also be due to dysfunction in the atria, at the bundle of His, or in the Purkinje system. Įven though conduction slows, every impulse originated from the atrium is passed to the ventricles. The presence of first-degree AV block on ECG represents prolonged conduction in the AV node, commonly due to increased vagal tone in younger patients and fibrosis of the conduction system in older patients. Morphology and size of the QRS complex reflect that the His Purkinje system is the site of conduction delay. However, the most commonly affected place is the AV node. Electrophysiological studies have shown that PR interval prolongation could be due to conduction delay located at the atrioventricular node, right atrium, or the His Purkinje system.
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