When is it dangerous to have a resting pulse of 45 while lying down?

Depends on person: A resting pulse of 45 is low, but a very athletic person who is well conditioned, such as the athlete lance armstrong in his prime, had a low resting heart rate in the 40's. If you have symptoms of lethargy, fatigue, lightheadedness, than it is a problem, and you need to get checked. Generally, with a low heart rate, you should get a check by your dr. To assure it is in fact "healthy" for you.