It is a condition characterised by sudden sharp chest pains that last only a minute or two and are not related to activity. When the pain is present it hurts to cough or breathe deeply. They may occur several times a day but more usually once or twice a week.
Those with cyanotic heart disease ("blue" due to reduced oxygen in the blood) can still travel but aircraft at high altitudes have less oxygen in the air than at ground level and thus, the blueness may be more apparent. This does not usually cause symptoms but if necessary, airlines can arrange for additional oxygen to be available on the aircraft.