Which dysrhythmia is characterized by "regular R-R intervals" and "narrow QRS"?

The characteristic of "regular R-R intervals" and "narrow QRS" strongly indicates that the rhythm is most likely organized and originates within the atria or the ventricles, leading to a specific interpretation of the heart's electrical activity. In sinus tachycardia, the heart rate increases due to physiological stress or other factors, yet it retains the normal sinus rhythm pattern. This means that the R-R intervals remain consistent, indicating a regular rhythm, while the QRS complexes are narrow, as they indicate normal conduction through the ventricles.

In contrast, atrial fibrillation typically presents with irregular R-R intervals, which does not align with the description provided. Ventricular tachycardia often results in wide QRS complexes due to abnormal conduction pathways originating in the ventricles. Normal sinus rhythm, while characterized by regular R-R intervals and narrow QRS complexes, usually indicates a heart rate within the normal range (60-100 bpm) and doesn't typically refer to the faster heart rates seen in sinus tachycardia.

Thus, the combination of regular R-R intervals and narrow QRS complexes at a higher heart rate precisely defines sinus tachycardia.