I've had an accelerated heartrate for 12 hours now. up to 120 laying down. had ekg xray and blood work done. all normal. healthy female 18?

Tachycardia: What was your diagnosis? Did the doctor say you have supra ventricular tachycardia? This would be the most common reason for this condition in someone your age? Did you have your thyroid hormones checked? Your resting HR is 120 and have abdominal obesity. That is not a "healthy female 18". If valsalva maneuver does not work, you may need ablation and/or pacing. See a cardiologist.

Hear t rate: If you are 18 and have a healthy heart, look to see what you may be doing that could accelerate your hear rate. Evaluate you caffeine intake - energy drinks, coffee, starbucks. Check to see if you are taking any supplements to lose weight, medications for ADD or ADHD that are amphetamine based or weight loss medications, Also have your physician check your thyroid.