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First-degree atrioventricular (AV) block is a condition characterized by a delay in the conduction of electrical impulses from the atria to the ventricles through the AV node. While conduction is slowed, every atrial impulse still reaches the ventricles.
Analogy: “A slight delay in the communication between the upper and lower sections of the orchestra.”
The ECG in first-degree AV block shows normal P waves and QRS complexes. The defining feature is a prolonged PR interval, which measures greater than 0.20 seconds (or more than 5 small squares on the ECG paper). Importantly, the PR interval remains consistent from beat to beat, indicating a fixed delay in AV nodal conduction. Each P wave is consistently followed by a QRS complex, signifying that all atrial impulses are eventually conducted to the ventricles. The delay in conduction can occur at the level of the AV node, the right atrium, or the His-Purkinje system, with the AV node being the most common site.
First-degree AV block is usually asymptomatic, and most individuals are unaware they have this condition. It is often discovered incidentally during a routine ECG. However, if the PR interval becomes very prolonged (greater than 0.30 seconds), some individuals may experience symptoms such as fatigue or exercise intolerance due to the loss of optimal synchronization between atrial and ventricular contractions. First-degree AV block can be a physiological finding, particularly in younger individuals with high vagal tone and in well-trained athletes. It can also be caused by certain medications that slow AV nodal conduction or by underlying cardiac conditions.
“Like a message taking a little longer to travel between musicians, the electrical signal is delayed at the AV node.”