The CFRN outline gives transport physiology a single line, physiologic stressors on crew and patients. Behind it sit the gas laws, hypoxia, cabin altitude and the physical stresses of flight, each tested through what it does to a patient or a device.
Gas laws with clinical effects: Boyle (volume and pressure), Dalton (partial pressures), Charles and Gay-Lussac (temperature), Henry (gas dissolved in liquid), Graham and Fick (diffusion).
Hypoxia: its four types, the stages as altitude rises, and time of useful consciousness for the crew.
Cabin altitude: pressurized and unpressurized aircraft, and when to ask the pilot for a lower cabin.
Stresses of flight: falling pressure, temperature, low humidity, noise, vibration, fatigue, acceleration, spatial disorientation, flicker and fuel vapors.
Equipment: anything with a sealed air space, from cuffs and splints to drip chambers and ventilators.
How big the pressure change really is
Barometric pressure is about 760 mmHg at sea level, about 565 mmHg at 8,000 ft and, near 18,000 ft, about 380 mmHg (U.S. Standard Atmosphere, 1976). Two consequences follow.
Trapped gas expands as pressure falls: by about a third in an 8,000 ft cabin, and to roughly twice its volume near 18,000 ft.
Inspired oxygen pressure falls with total pressure. Transport texts restore it by scaling the oxygen fraction by the ratio of pressures, FiO2 × P1 ÷ P2. Room air at sea level, 0.21, becomes about 0.28 in an 8,000 ft cabin.
Hypoxia, for patient and crew
Four types: hypoxic (too little oxygen pressure, the altitude type), hypemic (too little carrying capacity), stagnant (too little flow) and histotoxic (cells that cannot use it). Questions mix them with causes on the ground, such as anemia or carbon monoxide.
Night vision goes first. The FAA's Aeronautical Information Manual (chapter 8) notes that night vision can deteriorate at cabin altitudes as low as 5,000 ft, well below the altitudes where other effects appear.
The outline names crew and patients separately. Expect questions about your own fatigue, hydration and hypoxia, not only the patient's.
16 questions on altitude and its effects
0 of 16 attempted · 0 correct · 0 missed
Transport physiology
A pressurized fixed-wing aircraft at 31,000 ft loses cabin pressure suddenly, and the cabin fogs. The flight nurse's FIRST action should be to
Answer and why, all 4 options
At this altitude crew members lose useful consciousness within seconds to a minute, so they must don their own oxygen first to be able to help the patient.
The patient needs 100% oxygen, but a nurse who loses consciousness while setting it up cannot deliver any care.
Descent is the pilot's immediate action and will happen anyway; the nurse's first task is to stay conscious to help.
Checking the patient first ignores the time of useful consciousness at 31,000 ft, which is under a minute after rapid decompression.
Item 564516
Transport physiology
The flight nurse is planning a fixed-wing transfer of a patient with decompensated heart failure. The patient's SpO2 is 90% on 4 L/min nasal cannula at sea level, with HR 118 beats/min and BP 96/60 mmHg. The planned cruise will produce a cabin altitude of 8,000 ft. Which action is MOST appropriate before departure?
Answer and why, all 4 options
Asking for a lower cabin altitude and raising FiO2 offsets the fall in inspired oxygen pressure for a patient with no reserve.
Diuresis treats the congestion, but leaving oxygen unchanged ignores the predictable drop in alveolar oxygen at 8,000 ft.
Absorption atelectasis is a minor concern next to hypoxemia in a patient already at 90% who is about to lose ambient pressure.
A cabin altitude of 8,000 ft is not sea level; inspired oxygen pressure falls by about a quarter, so 4 L/min will not hold the SpO2.
Item 566189
Transport physiology
At a cabin altitude of 8,000 ft, a ventilated patient on volume control with a set tidal volume of 450 mL now has a measured exhaled tidal volume of 560 mL. The ventilator has no altitude compensation. The patient's ETCO2 has fallen from 38 to 30 mmHg. The flight nurse should
Answer and why, all 4 options
A faster rate increases minute ventilation and drives ETCO2 even lower.
The ventilator without altitude compensation now delivers a larger true volume as the gas expands, so lowering the set volume restores the intended 450 mL and corrects the hypocapnia.
The set number no longer matches what the patient receives; the measured 560 mL and falling ETCO2 show overventilation.
PEEP does not limit tidal volume; adding it raises plateau pressure on top of the already larger breath.
Item 1695940
Transport physiology
A patient with bilateral pneumonia and pulmonary edema desaturates more at altitude than a healthy patient at the same cabin altitude and FiO2. Which gas law BEST explains why the thickened alveolar–capillary membrane worsens the altitude effect?
Answer and why, all 4 options
Charles' law links gas volume to temperature and says nothing about how oxygen crosses the alveolar–capillary membrane.
Henry's law describes how much gas dissolves at a given pressure; it is the basis of decompression sickness, not of diffusion block.
Fick's law: diffusion is inversely proportional to membrane thickness, so edema and inflammation cut transfer further when the alveolar oxygen pressure falls at altitude.
Gay-Lussac's law links the pressure of a fixed volume of gas to its temperature; it says nothing about oxygen crossing the alveolar–capillary membrane.
Item 1882816
Transport physiology
At a cabin altitude of 8,000 ft, barometric pressure is 565 mmHg. For a patient breathing room air (FiO2 0.21) with a PaCO2 of 40 mmHg, what is the approximate alveolar PO2? (Water vapor pressure 47 mmHg; respiratory quotient 0.8.)
Answer and why, all 4 options
69 mmHg comes from subtracting PaCO2 without dividing by the respiratory quotient.
100 mmHg is roughly the sea-level alveolar value; it ignores the drop in barometric pressure.
109 mmHg is the inspired oxygen pressure (0.21 × 518) before subtracting CO2 (40/0.8 = 50).
A patient with pneumonia has a PaO2 of 95 mmHg and an SpO2 of 97% at sea level. After climbing to a cabin altitude of 6,000 ft, the SpO2 is 95%. The flight nurse recognizes that the PaO2 has MOST likely
Answer and why, all 4 options
Hyperventilation can raise PaO2 slightly, but it cannot offset the lower inspired oxygen pressure; the falling SpO2 already shows PaO2 has dropped.
Starting at a PaO2 of 95 mmHg on the plateau, a 2-point SpO2 drop corresponds to a PaO2 fall of roughly 20–30 mmHg.
Barometric pressure at 6,000 ft is lower, so alveolar and arterial oxygen pressure fall even when SpO2 looks steady.
SpO2 and PaO2 do not move together on the flat part of the curve; a 2-point SpO2 drop there hides a much larger fall in PaO2.
Item 1882931
Transport physiology
A patient with a femur fracture in a traction splint reports worsening pain during a helicopter flight. The flight nurse attributes it to rotor vibration. Which action BEST reduces the effect of vibration on the patient?
Answer and why, all 4 options
Loosening straps lets the fracture ends move more with vibration and loses the traction that controls pain and bleeding.
Padding and securing the splint and limb to the stretcher dampens transmitted vibration and stops the fracture ends from moving.
Removing traction in flight lets the femur shorten and the fragments move, increasing pain, bleeding and nerve risk.
Sedation dulls the patient's response but leaves the limb moving with each vibration and does not treat the pain.
Item 1882934
Transport physiology
Before a commercial air medical escort flight, which patient is the MOST appropriate candidate for a hypoxic challenge (altitude simulation) test?
Answer and why, all 4 options
A patient on high-flow oxygen at FiO2 0.8 already needs oxygen at sea level; the question is how much more to plan, not whether to test.
A ventilated patient needs an air medical crew and in-flight ventilation; a hypoxic challenge test does not apply.
A healthy patient with an SpO2 of 99% has ample reserve and needs no testing before a commercial flight.
A stable patient with chronic lung disease and borderline room-air saturation is the group in which a hypoxic challenge decides whether in-flight oxygen is needed.
Item 1882937
Transport physiology
The flight nurse reviews preflight data for four patients to be flown with a cabin altitude of 8,000 ft. Which finding places the patient at the HIGHEST risk of serious hypoxemia in flight?
Answer and why, all 4 options
A PaO2 of 72 mmHg is low but still near the shoulder of the curve; it will fall at altitude, though less dangerously than 55 mmHg.
An SpO2 of 95% on room air means a PaO2 near 80 mmHg with some reserve for the climb.
A room-air PaO2 of 55 mmHg is already on the steep part of the curve, so the drop in inspired oxygen at 8,000 ft will cause serious hypoxemia.
An ETCO2 of 38 mmHg reflects normal ventilation and says nothing about oxygenation risk at altitude.
Item 1882940
Transport physiology
During cruise at a cabin altitude of 8,000 ft, the flight nurse adjusted an air-filled ETT cuff to 25 cmH2O. On final descent, an audible leak develops and the cuff pressure now reads 14 cmH2O. Which action is MOST appropriate?
Answer and why, all 4 options
A tube exchange during descent is high risk and unnecessary; the cuff simply needs more air.
A fully deflated cuff gives no seal or aspiration protection; low pressure is the problem here, not high.
On descent the higher cabin pressure compresses the cuff air, so the cuff pressure falls; adding air back to 20–30 cmH2O with a manometer restores the seal.
The pressure will keep falling to the end of descent, not recover, and the leak exposes the airway to aspiration.
Item 1882958
Transport physiology
During a helicopter transport, a patient with a rate-responsive permanent pacemaker has a paced heart rate that rises from 70 to 120 beats/min. The patient is comfortable, SpO2 is 97%, and the rhythm is 100% paced with appropriate capture. The MOST likely cause is
Answer and why, all 4 options
Failure to sense causes pacing despite native beats, not a smooth rise in the paced rate with consistent capture.
Hypoxia would show as falling SpO2 or symptoms; the patient is comfortable at 97%.
Pain and anxiety raise the native sinus rate; a rhythm that stays 100% paced points to the device driving the rate.
Rate-responsive pacemakers with motion sensors read rotor vibration as activity and raise the paced rate in a calm, well-oxygenated patient.
Item 565936
Transport physiology
During ascent to a cabin altitude of 8,000 ft, a ventilated trauma patient develops spreading subcutaneous emphysema over the right chest and neck. Peak airway pressure rises from 28 to 42 cmH2O and ETCO2 from 38 to 52 mmHg. Preflight imaging showed no pneumothorax. What is the MOST appropriate immediate action?
Answer and why, all 4 options
Climbing further lets the trapped air expand more, and extra PEEP feeds the leak and pushes toward tension physiology.
A lower cabin altitude lets the trapped air shrink, and with signs of an evolving pneumothorax the crew prepares to decompress the chest.
Sedation and paralysis address dyssynchrony, but the rising pressure here comes from air expanding in the chest as the aircraft climbs.
Holding altitude leaves an expanding pneumothorax untreated while pressures and CO2 are already rising.
Item 1941214
Transport physiology
The ground temperature at the base is 20 °C (68 °F). The helicopter climbs to 5,000 ft above the base, and the cabin heater fails. Using the standard lapse rate, the flight nurse should expect the outside air temperature to be about
Answer and why, all 4 options
15 °C uses 1 °C per 1,000 ft, half the standard rate.
0 °C would require a lapse of 4 °C per 1,000 ft, double the standard rate.
The standard lapse rate is about 2 °C per 1,000 ft, so 5,000 ft above a 20 °C base gives about 10 °C.
Temperature does not stay the same with altitude; it falls in the standard atmosphere.
Item 566218
Transport physiology
A flight nurse went recreational scuba diving this morning without decompression stops and is scheduled for a rotor-wing shift that starts 4 hours after the dive. Which action is MOST appropriate?
Answer and why, all 4 options
Nitrogen absorbed during the dive comes out of solution as ambient pressure falls (Henry's law); waiting at least 12 hours after a no-stop dive lets it clear before flying.
Being symptom-free on the ground does not prevent bubbles from forming once the aircraft climbs.
The waiting period applies to flights below 8,000 ft as well; even modest altitude can trigger decompression sickness after a dive.
Oxygen reduces but does not remove the risk, and a crew member who develops symptoms in flight cannot care for the patient.
Item 9730022
Transport physiology
A patient with a nasogastric tube and nothing by mouth faces a 3-hour fixed-wing flight in a pressurized cabin. Which effect of the cabin air should the nurse plan for?
Answer and why, all 4 options
Pressurization limits gas expansion but not the dryness of cabin air.
Recirculated cabin air is dry, not humid, and adds no fluid.
Lower pressure expands trapped gas; it does not make the body retain water.
Cabin air at altitude is very dry, often under 20% humidity, so breathing and skin lose more water; fluids, eye care and mouth care are planned for a long flight.
Item 9730031
Transport physiology
A patient with type 1 diabetes is wearing an insulin pump during an unpressurized helicopter flight that climbs 6,000 ft. Which effect should the flight nurse anticipate during the climb?
Answer and why, all 4 options
As cabin pressure falls, air bubbles in the pump cartridge expand and push out extra insulin, so glucose is checked during and after the climb.
Reduced delivery happens on descent, when bubbles shrink, not during the climb.
The cartridge holds dissolved and trapped air that responds to cabin pressure changes.
Doubling the basal rate would add to the extra insulin already being pushed out on ascent.
Item 9730046
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Where candidates slip on physiology
Cabin altitude is not flight altitude. A pressurized airplane cruising high keeps its cabin far lower; the federal certification rule for transport-category airplanes caps cabin altitude at 8,000 ft at maximum operating altitude (14 CFR 25.841). Helicopters are unpressurized, so the patient is at the aircraft's true altitude, mountain terrain included.
Pressurization reduces expansion without removing it: at the usual 8,000 ft cabin ceiling, trapped gas still grows by about a third.
Definitions rarely score. Stems describe a device or a symptom and expect the direction of change: on ascent, gas spaces grow and partial pressures fall; on descent, both reverse.
Transport texts list falling barometric pressure (with falling oxygen pressure), temperature change, low humidity, noise, vibration, fatigue, acceleration forces, spatial disorientation, flicker vertigo and fuel vapors. The CFRN outline calls them physiologic stressors on crew and patients.
Does the CFRN ask for gas-law formulas?
The outline names no formulas. The questions on this site apply the laws to devices and patients, with simple arithmetic such as scaling oxygen for cabin pressure.
Is flight physiology on the CTRN?
As one line, physiologic stressors of transport on crew and patients, inside a general domain of 22 scored questions.