Which statement correctly distinguishes sinus bradycardia from sinus tachycardia on ECG?

• A. Sinus bradycardia has rate <60 bpm; sinus tachycardia has rate >100 bpm; both have regular rhythm and upright P waves with normal PR and narrow QRS.
• B. Sinus bradycardia and sinus tachycardia always present irregular rhythms.
• C. Sinus tachycardia shows inverted P waves.
• D. Bradycardia shows a wide QRS complex.

Answer: (D)

The main idea is that the difference between sinus bradycardia and sinus tachycardia on ECG is the heart rate, while the rhythm and conduction stay sinusoidal. In true sinus rhythm, you see upright P waves before every QRS, a normal PR interval, and a narrow QRS complex. Sinus bradycardia is simply a slower rate, under 60 beats per minute, with these same normal sinus features. Sinus tachycardia is a faster rate, over 100 beats per minute, again with the same normal sinus features. The width of the QRS is not what separates the two; a wide QRS would point to a conduction block or a ventricular origin, not to sinus rhythm. Inverted P waves would suggest an abnormal atrial rhythm rather than normal sinus rhythm. So the key distinguishing factor is the rate: slower than 60 for sinus bradycardia, faster than 100 for sinus tachycardia, with all other sinus characteristics preserved.