What is the key factor in differentiating atrial arrhythmias from junctional arrhythmias?

The duration of the PR interval is indeed a key factor in differentiating atrial arrhythmias from junctional arrhythmias. Atrial arrhythmias, such as atrial fibrillation or atrial flutter, typically have a normal PR interval whenever a QRS complex is present, often because the impulses are originating from the atria, where there is effective conduction through the atrioventricular (AV) node.

In contrast, junctional arrhythmias originate from the AV junction and can often manifest as either absent or significantly shortened PR intervals. This occurs because the electrical impulses start in the junctional tissue rather than the sinus node in the atria. Therefore, the length of the PR interval serves as a crucial diagnostic marker for distinguishing between these two types of arrhythmias.

Other factors, such as the amplitude of the QRS complex, frequency of the T waves, and overall heart rate, may provide additional information but are not as definitive as the PR interval in distinguishing atrial from junctional arrhythmias.