What is typically observed with PR intervals in sinus tachycardia?

In sinus tachycardia, the PR interval is typically observed to be constant and falls within the normal range of 0.12 to 0.20 seconds. This is because, in this rhythm, the electrical impulses originate from the sinoatrial node, and as the heart rate increases, the time it takes for the impulse to conduct through the atria to the ventricles remains relatively stable. The PR interval represents the time taken for electrical conduction from the atria to the ventricles, and while the heart is functioning properly and the conduction pathways are intact, the PR interval will show this consistency even at elevated heart rates.

Different circumstances, such as varying conduction through the atrioventricular node (AV node) or other arrhythmias, can lead to varying PR intervals. However, in the context of sinus tachycardia, one would not expect to see PR intervals that are not measurable, exceed 0.20 seconds, or vary significantly. Thus, the observation of constant PR intervals within the specified range is a hallmark feature of sinus tachycardia, reflecting a well-functioning conduction system despite the increased heart rate.