Final answer:
A first-degree AV block is a type of cardiac conduction delay between the SA node and AV node marked by an elongated PR interval on an ECG, indicating that electrical impulses still reach the ventricles but with a delay.
Step-by-step explanation:
Definition of 1st Degree AV Block
A first-degree AV block is a type of heart block where there is a delay in the electrical conduction between the sinoatrial (SA) node and the atrioventricular (AV) node. This delayed conduction is identifiable on an electrocardiogram (ECG) by an abnormally long PR interval. It is considered a partial block because the electrical impulses eventually reach the ventricles, just later than normal, leading to a synchronization issue between atrial and ventricular contractions.
Unlike second- or third-degree AV blocks, where some or all of the impulses, respectively, do not reach the ventricles, in a first-degree AV block, all the impulses pass through but at a slower rate. Second-degree blocks show some P waves not followed by QRS complexes, while third-degree blocks demonstrate no correlation between P waves and QRS complexes on the ECG.
From a clinical perspective, AV nodal and infra-Hisian blocks are common types of heart blocks. The first-degree AV block comes under the category of AV nodal blocks, which involve the AV node itself but do not typically have the same impact on heart rate (pulse) as more severe blocks might.