What is RPE and when do you use it?

Rating of Perceived Exertion

Explanation:

When used in a personal setting it can be a way to determine your training intensity, as opposed to using percentages. On a coaching level RPE can be a way to gain an understanding of an athlete's level of exhaustion, plus what they have left in the tank

the last statement is correct i.e.

when your heart is not pumping enough blood around your body because your heart is severely failing, you may need an artificial heart. an artificial heart is a mechanical device, about the size of an orange, that is connected to your heart or implanted in your chest to or replace a failing heart.

take a break move to stretching and slowly work back into physically exercising if the problem continues go to a doctor or physical therapist