What is box breathing? The deep breathing technique is one of the ways to calm down and lower heart rate fast, cardiologists say.

In TODAY.com's Expert Tip of the Day, a cardiologist explains why a lower resting heart rate can be a good sign of heart health and how to improve this vital sign.

A heart rate that averages above 100 beats per minute is called tachycardia. You can develop a high heart rate because of ...