Sinus bradycardia is defined as...

• A. HR greater than 100 with abnormal ECG
• B. Normal HR with abnormal rhythm
• C. HR less than 60, normal ECG otherwise
• D. HR between 60 and 100 with prolonged QT

Answer: (C)

Sinus bradycardia means the heart is beating slower than normal, but the rhythm is still a true sinus rhythm. That requires a rate below 60 beats per minute with P waves before every QRS and a regular, consistent rhythm, and no other abnormal ECG findings. In other words, you have a normal sinus rhythm that just runs a bit slow. This fits the description of "HR less than 60, normal ECG otherwise."

Think about why the other patterns aren’t sinus bradycardia: a heart rate over 100 is tachycardia, not bradycardia; an abnormal rhythm with a normal rate isn’t the clean, sinus pattern; and a normal rate with a prolonged QT signals a different conduction or repolarization issue rather than bradycardia. Sinus bradycardia can be normal in athletes or during sleep and is only concerning if symptoms or other signs of hemodynamic compromise appear.