Which condition is defined by a heart rate of less than 60 bpm?

A heart rate of less than 60 beats per minute is classified as bradycardia, and specifically, the term "sinus bradycardia" refers to this condition when it originates from the sinus node, which is the heart's natural pacemaker. In sinus bradycardia, the rhythm is regular, but the heart beats slower than the normal range for adults. This can occur due to various factors such as increased vagal tone, certain medications, or underlying medical conditions that affect heart function. It is important to differentiate this from other heart rhythms, where the heart rate may be elevated, irregular, or caused by ectopic pacemakers.

Sinus tachycardia, by contrast, refers to an elevated heart rate, atrial flutter results in a rapid and irregular rhythm, and PAC (premature atrial contraction) is a specific type of arrhythmia involving early beats originating from the atria. These other conditions do not match the definition of a heart rate less than 60 bpm.