Flight Physiology and Environmental Considerations
Core concepts
Altitude changes gas behavior. Boyle’s law explains expansion of trapped gas as barometric pressure falls; Dalton’s law explains partial pressures of oxygen and other gases; Henry’s law explains dissolved-gas bubbles and decompression sickness. At cabin altitude, alveolar oxygen partial pressure falls even if the fraction of inspired oxygen is unchanged, so hypoxia can develop. Candidates must anticipate expansion of pneumothorax, bowel gas, endotracheal tube cuffs, and air splints; understand oxygen delivery and altitude compensation; and recognize effects of noise, vibration, fatigue, thermal stress, and G-forces.
Traps
Questions often ask what happens to a pneumothorax or ileus during ascent: trapped gas expands and can worsen the patient. Cuff pressures may rise and should be checked. IV flow rates can change with altitude and pressure, and oxygen consumption calculations must account for altitude and system efficiency. Do not confuse cabin altitude with aircraft altitude. Spatial disorientation and fatigue are human-factor traps.
Rule of thumb
At altitude, suspect trapped gas expansion and hypoxia earlier. A useful rule is to maintain supplemental oxygen and monitor capnography and oxygen saturation closely during ascent, especially with thoracic or abdominal pathology.
Worked example
A patient with a small pneumothorax is transported by air. As cabin altitude rises, the trapped air expands and can worsen respiratory compromise. The crew should ensure a chest decompression plan, monitor closely, and consider a lower cabin altitude or ground transport if clinically indicated.