Final answer:
The average P-R interval is 120 to 200 ms. An AV node block can lead to a prolonged P-R interval in a first-degree block, dropped QRS complexes in a second-degree block, and a complete dissociation between P waves and QRS complexes in a third-degree block. In pre-excitation syndromes, the P-R interval may be shorter.
Step-by-step explanation:
The average length of the P-R interval, which measures the duration from the beginning of atrial depolarization to the initiation of ventricular depolarization, is approximately 120 to 200 milliseconds (ms). When an atrioventricular (AV) node block occurs, this interval can be affected in several ways. In a first-degree AV node block, there is a delay in conduction between the SA and AV nodes, leading to an abnormally long P-R interval on an electrocardiogram (ECG). A second-degree block may result in some P waves not being followed by a QRS complex, implying that some impulses do not pass through the AV node. In a third-degree block, there is no correlation between the P waves and QRS complexes; P waves may occur without subsequent QRS complexes because impulses are not transmitted through the AV node at all. Pre-excitation syndromes like Wolff-Parkinson-White (WPW) can also affect the P-R interval by causing it to be shorter than normal, as the electrical impulse bypasses the AV node through an accessory pathway, leading to earlier ventricular depolarization.