2.1 Flight Physiology and Stressors for Crew and Patients
Key Takeaways
- Boyle's law expands trapped gas as cabin altitude rises—air-filled endotracheal cuffs, residual pneumothorax, bowel gas, glass intravenous bottles, and intra-aortic balloon pump helium all enlarge.
- Dalton's law explains altitude hypoxia: fraction of inspired oxygen stays about 0.21 while the partial pressure of oxygen falls, so raise FiO2 and/or lower cabin altitude.
- Most civilian helicopters are unpressurized; pressurized fixed-wing cabins commonly sit near 6,000–8,000 feet unless the crew can honor a lower-cabin request.
- Self-imposed crew stressors—smoking, alcohol, hypoglycemia, and self-medication—stack on top of hypoxia, noise, vibration, fatigue, G forces, and third-spacing; use IM SAFE before accepting a flight.
- The 2026 CFRN outline still tests gas-law physiology, including Henry's law, but no longer treats evolved gas disorders as a standalone medical-content topic.
Flight physiology is a bedside tool, not trivia. The gas laws explain why an endotracheal tube (ETT) cuff goes rock-hard on climb, why a "small" pneumothorax becomes a crisis, and why a patient who was 96 percent on a nonrebreather at the scene desaturates as an unpressurized helicopter climbs. Domain 1 of the August 2026 Board of Certification for Emergency Nursing (BCEN) outline tests transport physiology as stressors on crew and patients. Treat every law as a lever: change fraction of inspired oxygen (FiO2), change cabin altitude, or evacuate trapped gas.
The gas laws as flight-nurse tools
Boyle's law: trapped gas
Boyle's law states that at constant temperature, pressure and volume are inversely related (P1V1 = P2V2). As barometric pressure falls, every closed or poorly vented gas space expands. Cabin altitude—the physiologic altitude inside the aircraft—is the number that matters, not the cockpit flight level.
| Approximate altitude | Relative gas volume versus sea level | Flight-nurse implication |
|---|---|---|
| Sea level | 1.0 | Baseline cuff, chest, and bowel volumes |
| 5,000 feet | About 1.2 (roughly 20 percent larger) | Early ear, sinus, and cuff-pressure change |
| 8,000 feet (common pressurized-cabin equivalent) | About 1.35 (roughly 35 percent larger) | Typical fixed-wing cabin; ETT cuff and pneumothorax expand |
| 18,000 feet | About 2.0 (volume doubles) | Unpressurized high mountain crossings; high barotrauma risk |
Watch air-filled ETT and laryngeal-mask cuffs, glass intravenous bottles, chest tubes and residual pneumothorax, bowel gas, pneumocephalus, and intraocular gas after retinal surgery. A pressurized King Air at 25,000 feet may still hold a cabin near 6,000 to 8,000 feet. An unpressurized helicopter at 7,000 feet mean sea level is at 7,000 feet of cabin altitude.
Dalton's law: hypoxia is a partial-pressure problem
Dalton's law states that total pressure equals the sum of partial pressures. Oxygen remains about 21 percent of dry air at altitude, but the partial pressure of inspired oxygen (PIO2) falls because barometric pressure falls. Alveolar oxygen tension (PAO2) therefore declines if FiO2 is unchanged. That is hypoxic hypoxia—the dominant altitude threat in anemia, shock, lung disease, or high intracranial pressure. Raise FiO2 first; fly lower or request a lower cabin altitude next. Treat falling saturation, harder breathing, and new agitation as altitude until proven otherwise.
Charles, Gay-Lussac, Henry, and Graham
Charles's law (volume varies with absolute temperature) and Gay-Lussac's law (pressure varies with absolute temperature) explain night-flight reality: oxygen-cylinder gauges fall in the cold, then volumes expand as the cabin warms. Recheck tank pressure after temperature swings.
Henry's law states that the amount of gas dissolved in liquid is proportional to its partial pressure. As ambient pressure falls, dissolved nitrogen can leave solution—the physiology of evolved gas and decompression. Know the mechanism so a recent scuba diver or a rapid unpressurized climb makes sense. The August 2026 CFRN outline removed evolved gas disorders as a tested medical topic; do not memorize decompression-sickness algorithms as if they were still a Domain 4 disease list.
Graham's law states that lighter gases diffuse faster than heavier ones. Helium in many intra-aortic balloon pump (IABP) balloons therefore changes volume aggressively under Boyle's law. Watch IABP consoles and ventilator circuits for altitude compensation rather than assuming sea-level calibration.
Stressors of flight: crew versus patient
The classic stressors are hypoxia, barometric change, thermal stress, low humidity, noise, vibration, fatigue, gravitational (G) forces, and third-spacing.
| Stressor | What it does | Typical countermeasure |
|---|---|---|
| Hypoxia | Falls in PAO2 and oxygen delivery | Increase FiO2; fly lower or lower the cabin |
| Barometric change | Expands trapped gas | Evacuate gas; unclamp chest tubes; adjust cuffs |
| Thermal | Cold soak at altitude; heat on the ramp | Pre-warm the cabin; cover the patient; watch burns and neonates |
| Low humidity | Thick secretions and mucous plugs | Humidify the ventilator; protect eyes and mucosa |
| Noise | Often 90–110+ decibels in rotor-wing; auscultation dies | Use end-tidal carbon dioxide (ETCO2), saturation, and visual chest rise |
| Vibration | Dislodges tubes; worsens pain and bleeding | Pad, splint, and secure every line |
| Fatigue | Crew error and missed trends | Crew resource management; sterile cockpit |
| G forces | Transient shifts in venous return and intracranial pressure | Smooth profile; thoughtful loading orientation |
| Third-spacing | Capillary leak, edema, burn-fluid shifts | Watch limbs, abdomen, and urine output |
Patients meet these stressors with less reserve. Crew add self-imposed stressors: smoking (carboxyhemoglobin already acts like extra altitude), alcohol, hypoglycemia, and self-medication. Use IM SAFE—Illness, Medication, Stress, Alcohol, Fatigue, Emotion/Eating—before you accept a flight.
Most civilian helicopters are unpressurized. Dedicated fixed-wing ambulances are usually pressurized and can often honor a lower-cabin request, at a cost in fuel and true altitude. Say the reason out loud: expanding pneumothorax, bowel obstruction, recent intraocular gas, IABP, or a patient already hypoxic on the ramp.
Actions that change outcomes
- Increase FiO2 early; do not wait for a saturation you know will fall on climb.
- Ask for a lower cabin altitude or a lower en-route altitude.
- Evacuate trapped gas: a working nasogastric/orogastric (NG/OG) tube for obstruction, an unclamped chest drain, and a saline-filled or manometer-managed ETT cuff.
- Prefer plastic fluid bags; vent glass bottles if you have no alternative.
- Pad and splint; ear and sinus pain on descent needs a slower profile and a patent eustachian tube.
- Confirm IABP altitude compensation and recheck ventilator volume, peak pressure, and ETCO2 after climb.
A flight nurse is packaging an intubated scene patient for an unpressurized rotor-wing flight. Which action best reduces tracheal injury from Boyle's law as cabin altitude rises?
A patient on a nonrebreather desaturates as an unpressurized helicopter climbs through 7,000 feet mean sea level. Which statement best explains the change?
A patient with a small residual pneumothorax and a functioning chest tube must fly on a pressurized fixed-wing aircraft. What is the most appropriate first-line flight plan?