What characterizes sinus bradycardia?

Sinus bradycardia is characterized by a heart rate that is less than 60 beats per minute. This reflects a slower than normal pace of electrical impulses generated by the sinoatrial node, which is responsible for initiating each heartbeat in a healthy heart.

Within the context of bradycardia, it's important to note that a decreasing trend in heart rate can suggest a potential concern, particularly if it is coupled with symptoms like dizziness or fatigue. However, the defining characteristic remains the heart rate itself falling below the threshold of 60.

The other descriptions do not align with the definition of sinus bradycardia. A heart rate over 100 indicates tachycardia, indicating an accelerated heart rate. Normal heart rate with a regular rhythm signifies a typical sinus rhythm, not bradycardia. Only atrial activity present would imply a complete block or other abnormal rhythm, which also deviates from the parameters of sinus bradycardia. Hence, identifying a heart rate of less than 60 is pivotal to diagnosing this particular arrhythmia.