Magic, obviously.
But seriously, I doubt there is a very practical way, besides them having carved out many vent holes along the perimeter. The Medial Screw, for example, would probably be sufficient ventilation for the higher pressure generated by all the breathing, but you need somewhere for it to equalize from.
Then again, I recall having read that if Westeros has a higher air density, it is actually plausible that dragons could fly there. So, perhaps the same is true of Eärwa...