In Humans the maximal heart rate is about 200 BPM. Sinus tachycardiac is a heart rate of more than 100 BPM.
Fast heart rate decreases coronary blood flow fill leading to an increased demand of oxygen on the heart.
This means that you should not keep your heart beating at its maximal rate for too long.
Fibbrilation is a high heart rate which occurs in a disease of the heart that causes the electrical signals in the heart to become very rapid and irregular the heart rate for fibbrilation is about 300 -600 BPM.
Dont worry though your heart cannot go into fibbrilation unless there is a heart disease which causes it to happen and a heart rate of 200 BPM is NOT fast enough to causes fibbrilation in the heart.