When PaO2 decreases below 50 mmHg (6.7 kPa) cerebral hypoxia may ensue. An increase in cerebral blood flow may actually occur due to the vasodilatory effects of hypoxia, despite the tachypnea and vasoconstriction related to the decrease in PaCO2. Initially, patients may present with excitation, loquaciousness, euphoria, hyperactivity, and restlessness, followed by decreased attention, amnesia, deterioration of visual fields or depth perception, depression, impaired judgment, and confusion. In patients with recent ocular injury, ocular surgery, or progressive retinal disease, there is a high oxygen requirement because of the sensitivity of the retina.
The effective performance time of an experienced crew member exposed to an altitude of 18 000 ft (5400 m) without oxygen or pressurization is 20 to 30 min; at 25 000 ft (7500 m) it is 3 to 5 min and at 40 000 ft (12 000 m) it is 15 to 30 s. Arterial line monitoring and pulmonary artery catheter monitoring for volume status, blood pressure, oxygenation, and ventilation may be of value in prolonged flights with an unstable patient.
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