In type 1 heart block, how does the PR interval change?

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In type 1 heart block, also known as first-degree AV block, the defining characteristic is that the PR interval progressively lengthens with each successive beat until a QRS complex is dropped. This phenomenon occurs due to a gradual delay in the conduction through the atrioventricular (AV) node. It reflects a situation where the electrical signals from the atria struggle to pass through the AV node, leading to an incremental increase in the time it takes for these signals to trigger the ventricular contraction, represented by the QRS complex.

The increase in the PR interval is a key diagnostic feature, as it indicates an underlying issue with the heart's conduction system, specifically within the AV node. Eventually, after a series of beats where the PR interval lengthens, a beat will be dropped, meaning that a QRS complex will not be produced despite an atrial contraction being present. This characteristic makes it distinct from other types of heart blocks where the PR intervals behave differently or do not exhibit a consistent pattern such as merely remaining constant or varying randomly.