At this risk of repeating myself, I'll just paste the link in from when you asked this recently:
http://www.medhelp.org/posts/Heart-Rhythm/Does-Metroprolol-Help/show/2347387#post_11360341
As I mentioned there, Metoprolol Tartrate has a serum half life of approximately 5 hours. let me add this though: Let's say that you take your 25mg dose at 6am. BY 11am it has halved in strength, by 4pm it has halved again, by 9pm halved again. By 1am, you have the equivalent of 1.5 milligrams remaining in your blood, a miniscule amount.
Your physician had a reason to prescribe the twice daily dose. I wouldn't self medicate as you are now doing. Take the prescribed dose. If you need to change it, it's not a problem as the effects of it are not long lasting.
Ok first of all, the problem.
One, you dont know what is causing the racing.
Two, there are some symptoms that are "mysterious". eg it occurs when asleep and is associated with being on your left side.
Three. You are using a few medications, changing doses at the moment and are unclear on what the right strategy is.
1. You have a structurally normal heart. But I see no indication that you were checked for heart disease. This might be because you are young or do not have the personal profile associated heart disease. Instead of that, the racing could be caused by SVT, a heart arrhythmia that is usually curable. They check you for that using a portable heart monitor that you take home with you. Arryhtmias occur sporadically and have way of not happening when the ecg machine is hooked up. A heart monitor can be worn all the time. You could ask for a heart monitor.
2. Mysterious symptoms. OK I've had the left side thing happen to me myself and I am stumped. However, I generally feel better when I am off of caffeine for a full month. While I am getting off of caffeine I have a nightmare of symptoms. Headaches, heart palpitation, breathlessness, sweating, light headed or dizzy, you name it. Weird stuff. I take no drugs of any type, but still get the symptoms. Some of these symptoms also improve if I do some very light exercise. Ask you doctor if that would be ok. I definitely feel better after several weeks of no caffeine though.
3. I would expect some symptoms to be jerking you around until you guys get your dosing figured out. Just keep working with your doctor on this and be patient.
Best of luck.