CFRN Physiology and Gas Laws 4 — Questions and Answers
Question 1: A flight nurse is managing a ventilated patient at altitude. Tidal volume has NOT been changed, yet peak airway pressures have increased. The most likely gas law explanation is:
- Gas in the circuit expanded due to lower cabin pressure, increasing effective delivered volume (Correct answer)
- Charles' Law causing gas to contract in cold aircraft temperatures
- Henry's Law causing CO2 to come out of solution in the ventilator circuit
- Dalton's Law altering the partial pressure driving ventilation
Correct answer: Gas in the circuit expanded due to lower cabin pressure, increasing effective delivered volume
Boyle's Law explains that gas in humidified circuits or bellows can expand at altitude, effectively increasing delivered tidal volume and airway pressures.
Question 2: The concept of 'equivalent oxygen percentage' at altitude means that to maintain a PaO2 equivalent to sea-level room air at 8,000 feet cabin altitude, a patient needs supplemental FiO2 of approximately:
- 30% (Correct answer)
- 21%
- 50%
- 100%
Correct answer: 30%
At 8,000 feet (PB ~565 mmHg), an FiO2 of approximately 30% is needed to achieve the same PiO2 as breathing 21% oxygen at sea level (760 mmHg).
Question 3: Which patient condition poses the GREATEST risk of expanding gas complications during aeromedical transport?
- Recent pneumocephalus following neurosurgical procedure (Correct answer)
- Stable chronic obstructive pulmonary disease
- Controlled diabetes mellitus
- Treated urinary tract infection
Correct answer: Recent pneumocephalus following neurosurgical procedure
Pneumocephalus contains intracranial air that expands at altitude per Boyle's Law, potentially causing increased intracranial pressure and brain herniation.
Question 4: Hypemic hypoxia during aeromedical transport would MOST likely occur in a patient with:
- Severe anemia with hemoglobin of 5 g/dL (Correct answer)
- Complete airway obstruction
- Cyanide poisoning blocking cellular respiration
- Flash pulmonary edema
Correct answer: Severe anemia with hemoglobin of 5 g/dL
Hypemic hypoxia results from reduced oxygen-carrying capacity of blood, most commonly from severe anemia, methemoglobinemia, or carboxyhemoglobin binding.
Question 5: A patient on 100% non-rebreather mask at sea level has a PaO2 of 500 mmHg. During transport at 8,000 feet cabin altitude (PB 565 mmHg), the expected PaO2 on the same mask would be approximately:
- 372 mmHg (Correct answer)
- 500 mmHg
- 250 mmHg
- 100 mmHg
Correct answer: 372 mmHg
PaO2 at altitude scales with barometric pressure; 500 × (565/760) ≈ 372 mmHg, reflecting the proportional decrease in available oxygen partial pressure.
Question 6: Stagnant hypoxia in the aeromedical setting is BEST described as hypoxia resulting from:
- Inadequate circulation delivering oxygenated blood to tissues (Correct answer)
- Insufficient oxygen in the inspired atmosphere
- Inability of cells to utilize delivered oxygen
- Reduced hemoglobin oxygen-carrying capacity
Correct answer: Inadequate circulation delivering oxygenated blood to tissues
Stagnant (circulatory) hypoxia occurs when blood flow to tissues is inadequate despite normal arterial oxygen content, as seen in shock or heart failure.
Question 7: When transporting a SCUBA diver who surfaced too quickly, FAA altitude restrictions typically require flying NO HIGHER than what pressure altitude to prevent worsening decompression sickness?
- 1,000 feet (Correct answer)
- 8,000 feet
- 5,000 feet
- Sea level only — any altitude is contraindicated
Correct answer: 1,000 feet
Divers with decompression sickness should ideally be transported at the lowest possible altitude (≤1,000 feet) to minimize further nitrogen gas bubble expansion.
CFRN Physiology and Gas Laws 4
A flight nurse is managing a ventilated patient at altitude.
Tidal volume has NOT been changed, yet peak airway pressures have increased.
The most likely gas law explanation is: