What characterizes first degree heart block?

First-degree heart block is characterized by a fixed PR interval that is prolonged, specifically greater than 0.20 seconds. This means that there is a consistent delay in the conduction from the atria to the ventricles as the electrical impulses traverse through the atrioventricular (AV) node. This consistent delay results in the characteristic prolonged PR interval that remains the same throughout the cardiac cycle.

Unlike other types of heart blocks, such as second-degree or third-degree heart block, first-degree heart block does not involve dropped beats or completely independent P waves and QRS complexes. In those conditions, the relationship between atrial and ventricular electrical activity becomes more disorganized. In first-degree heart block, the relationship is maintained (all P waves are followed by QRS complexes), but the timing of that relationship (the PR interval) is excessively prolonged.

This prolonged PR interval indicates a degree of conduction delay but typically does not lead to any significant symptoms or hemodynamic instability, making first-degree heart block often an incidental finding on an electrocardiogram.