- Final air volume and final ICP refers to those estimated for cabin altitude 8000 feet in ascending order for amount intracranial air. Shaded areas show where intracranial pressure (ICP) would be expected to increase over 20 mm Hg at cruising altitude (cabin pressure = 8000 ft).Abbreviations: Intracranial (IC), total (tot), volume (vol), calculated (Calc), starting intracranial pressure at sea level 5 mm Hg (ICP5), starting intracranial pressure at sea level 10 mm Hg (ICP10), starting intracranial pressure at sea level 15 mm Hg (ICP15)