What would happen if your atrium goes into tachycardia?

cardiac output decreases with atrial tachycardia because there is not enough time between beats to fill the ventricles. It isn't a very pleasant feeling to have this so the best thing would be to see a cardiologist. there are good drugs now that can control the atrial rate. If it is regular and about 150 it can be very tiring and cause shortness of breath. If it goes too fast you certainly can loose consciousness due to lack of blood flow to the brain etc. Have it check and treated.