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FP-C Flight Physiology Review: Gas Laws & Stressors of Flight
Flight physiology is the FP-C's signature content: the exam expects you to take 18th-century gas laws and apply them to a 21st-century patient — the pneumothorax at altitude, the ET cuff that tightens as you climb, the diver who flew too soon. The math is trivial; the test is whether you connect each law to its patient.
Frame it as: Boyle's law moves volumes (trapped gas expands as pressure falls), Dalton's law starves oxygen (partial pressure falls with altitude even though the percentage doesn't), and Henry's law moves dissolved gas (decompression). Layer the eight classic stressors of flight on top, and most questions answer themselves.
The gas laws, attached to patients
Boyle's law (volume inversely proportional to pressure): ascent expands every trapped gas — a simple pneumothorax can tension; pneumocephalus, bowel gas in obstruction, gastric air (vent the OG/NG), air in IV bags/pressure bags, ET tube cuffs (monitor/adjust per protocol or use saline-filled cuffs where directed), air splints, and balloon devices all grow with altitude. The applied answers: decompress/chest-tube the at-risk chest before flight, vent stomachs, monitor cuffs, and request lower cabin/cruise altitude when trapped gas can't be controlled. Descent reverses it — sinus/ear blocks squeeze on the way down.
Dalton's law (total pressure = sum of partial pressures): at altitude, oxygen remains ~21% of a smaller number — PaO2 falls, and the marginal patient desaturates first; the field consequence: increase FiO2 with altitude and anticipate hypoxemia in every compromised-gas-exchange patient. Henry's law (dissolved gas proportional to pressure above the liquid): the basis of decompression sickness — nitrogen leaves solution as pressure falls — which is why recent divers fly low (and why DCS/arterial gas embolism patients are flown at the lowest safe altitude, on high FiO2, toward hyperbaric care). Charles's/Gay-Lussac's handle temperature-volume/pressure (oxygen cylinder pressures read lower cold) — minor but quotable.
Hypoxia and the stressors of flight
Hypoxia stages (rough altitude bands for healthy subjects): indifferent (sea level–10,000 ft: night vision degrades first), compensatory (10–15k: heart rate/ventilation climb), disturbance (15–20k: impaired judgment, coordination), critical (above 20k: rapid incapacitation). The four hypoxia types map cleanly: hypoxic (altitude itself), hypemic (anemia, CO — your post-house-fire patient is hypemic even with normal SpO2-appearing oximetry), stagnant (shock, poor flow), histotoxic (cyanide). Time of useful consciousness shrinks with altitude and is a favorite recall item. HEMS rarely flies high enough for crew hypoxia — but your patient, already on the edge of the oxyhemoglobin curve, lives several 'physiologic thousand feet' higher than the aircraft does.
The eight stressors of flight: hypoxia, barometric pressure changes, thermal changes, decreased humidity (dry medical gas + altitude = thickened secretions; humidify long transports, lubricate eyes), noise (masks auscultation — rely on waveforms, monitors, and visual assessment; hearing protection for everyone), vibration (fatigue, pain, interferes with monitors and splints), fatigue (the crew stressor — AMRM territory), and G-forces (modest in HEMS but real in fixed-wing climbs/descents — positioning matters for ICP and cardiac patients). Exam stems love 'which stressor explains this finding': the intubated patient's plugging secretions (humidity), the unreadable BP (noise/vibration → use NIBP/art line trends), the crew's degraded night vision (hypoxia at cabin altitude + dark adaptation).
Practice questions with answers & rationales
Q1. Your patient has a small untreated apical pneumothorax and the flight will cruise at 8,000 ft cabin altitude. Which law applies and what's the plan?
Answer: Boyle's law: ambient pressure falls with altitude, so the trapped pleural gas expands — a small simple pneumothorax can enlarge or tension in flight. Plan: decompress/place a chest tube (or at minimum have decompression immediately ready per protocol) before departure, fly at the lowest safe altitude, monitor for tension signs (rising distress, falling BP, unilateral sounds, rising airway pressures if ventilated). 'Treat the trapped gas before you climb' is the credited pattern.
Q2. Why does a patient desaturate at altitude when the oxygen percentage of air hasn't changed?
Answer: Dalton's law: 21% of a lower total pressure is a lower partial pressure of oxygen — and partial pressure, not percentage, drives diffusion into blood. At 8,000–10,000 ft the alveolar PO2 of a healthy person drops them to the steep shoulder of the oxyhemoglobin curve; a patient with pneumonia, shock or anemia starts further down the curve and falls off it. Consequence: raise FiO2 proactively with ascent for every gas-exchange-compromised patient.
Q3. A scuba diver from this morning needs interfacility transport tonight. What's the concern and the flight plan?
Answer: Henry's law: residual nitrogen remains in solution post-dive; reducing ambient pressure (altitude) pulls it out as bubbles — provoking or worsening decompression sickness. Plan: screen for dive history within 24 hours, fly at the lowest safe altitude (or go by ground), high-flow oxygen (accelerates nitrogen washout), and for actual DCS/AGE patients, destination is hyperbaric-capable care with the flight conducted as low as safely possible. Flying a fresh diver high is the planted error.
Q4. Match the four hypoxia types to: a hemorrhaging trauma patient, a house-fire victim, a cyanide exposure, and a healthy crew member at high cabin altitude.
Answer: Hemorrhage → stagnant-plus-hypemic territory, classically taught as hypemic (lost hemoglobin) with stagnant features from shock — fewer carriers and poor flow; house fire (CO) → hypemic: carriers are occupied, and standard oximetry reads falsely normal; cyanide → histotoxic: oxygen arrives but cells can't use it; crew at altitude → hypoxic hypoxia: inadequate PO2 at the lung. The treatment logic differs accordingly (volume/blood, 100% O2 regardless of oximetry, antidote kits per protocol, descent/supplemental O2) — which is why the classification matters.
Q5. Why do flight clinicians distrust pulse oximetry and auscultation more than ground providers do?
Answer: Environmental stressors degrade both: vibration and cold fingers corrupt the pleth signal; noise makes stethoscopes nearly useless in rotor-wing flight. The compensations: waveform capnography as the continuous airway/ventilation truth, invasive or oscillometric pressures trended rather than single values, visual assessment (chest rise, color, monitor waveforms), and securing assessment data before engines turn. Exam stems that present 'can't hear breath sounds at altitude' are testing whether you reach for the waveform rather than the stethoscope.
Q6. An intubated patient's secretions thicken and begin plugging during a long fixed-wing transfer. Which stressor, and what's the prevention?
Answer: Decreased humidity — cabin air at altitude is extremely dry, and medical oxygen is anhydrous; together they desiccate the airway, thickening secretions toward plugging (and drying eyes/mucosa). Prevention: humidification (HME on the circuit), scheduled suctioning, adequate patient hydration per protocol, lubricating eye care for sedated patients. It's the least glamorous stressor and therefore a reliable exam discriminator.
Q7. Time of useful consciousness: what is it, and what happens to it during rapid decompression?
Answer: The interval from interrupted oxygen supply at altitude until performance becomes useless (not unconsciousness — uselessness): minutes in the low-20s of thousands of feet, shrinking to seconds in the 30s–40s. Rapid decompression roughly halves it — the pressure gradient pulls alveolar oxygen out of the blood momentarily. Crew answer: oxygen on first, then troubleshoot; for patients, anticipate immediate hypoxia and descend. Mostly a fixed-wing/military number, but a standard FP-C recall item.
Common mistakes to avoid
- Quoting gas laws without attaching the patient consequence — the exam tests applications (cuffs, pneumos, dressings, NG tubes), not definitions.
- Forgetting that descent reverses Boyle effects (ear/sinus blocks, shrinking splint pressure) — both directions are tested.
- Trusting standard pulse oximetry in CO exposure, or skin-color assessment under cabin lighting.
- Missing the dive-history screen before flight, or flying suspected DCS patients at normal altitudes.
- Treating noise/vibration as comfort issues rather than assessment-system failures with named workarounds.
- Ignoring humidity: plugged tubes and corneal injuries on long transports are preventable, and tested.