Going to a higher altitude increases the viscosity of the blood coupled with increased coagulability increases the risk of stroke and venous and pulmonary clots. There is no evidence the converse is true, but more likely coagulability decreases and less risk for pulmonary thrombosis (clot) pulmonary emboli (clots). However, you should be on anti-platelet medication if you have atrial flutter, but that would be with your doctor's advisement.
Thanks for the question, take care,
Ken