Flight Physiology
Altitude physiology
At altitude, barometric pressure falls while the fraction of oxygen in air remains 0.21. Dalton's law explains that the partial pressure of oxygen (PiO2) falls, so PaO2 and saturation fall even though the percentage is unchanged. Henry's law explains dissolved gas coming out of solution, and Boyle's law explains expansion of gas-filled spaces as pressure drops. Pressurized cabins reduce but do not eliminate this; cabin altitude may be 6,000–8,000 ft. Traps include equating FiO2 with PaO2, trusting a normal SpO2 when perfusion, carboxyhemoglobin, or anemia is abnormal, and forgetting that endotracheal cuffs, chest tubes, pneumothoraces, bowel gas, sinuses, and IV bags can expand. Vent cuffs and chest tubes per protocol; avoid nitrous oxide with trapped gas. Hypoxia may present as agitation, confusion, tachycardia, or somnolence. Noise, vibration, fatigue, G-forces, and spatial disorientation compound performance. Worked example: at about 8,000 ft, barometric pressure is roughly 565 mmHg; PiO2 is about 0.21 × (565 − 47) ≈ 109 mmHg versus ≈149 mmHg at sea level. Supplemental oxygen raises PiO2 but does not change the cabin altitude. Rule of thumb: new agitation, hypoxemia, or a worsening gas-filled space at altitude should prompt evaluation for gas expansion and hypoxia.