CFRN Physiology and Gas Laws 3 — Questions and Answers
Question 1: A CFRN is transporting a patient with a closed bowel obstruction. At altitude, this patient is most at risk for which complication based on gas law principles?
- Bowel distension and perforation due to gas expansion (Correct answer)
- Bowel ischemia due to vasoconstriction
- Decreased gastric motility worsening the obstruction
- Hyperkalemia from ischemic bowel segments
Correct answer: Bowel distension and perforation due to gas expansion
Boyle's Law predicts trapped intestinal gas expands as cabin pressure decreases, increasing the risk of bowel distension and potential perforation.
Question 2: Charles' Law describes the relationship between gas temperature and volume. This law is most relevant during aeromedical transport when:
- Calculating changes in IV bag volumes during cold high-altitude flights
- Assessing expansion of air splints in cold weather (Correct answer)
- Determining oxygen flow rates from compressed cylinders
- Predicting arterial blood gas changes in hypothermic patients
Correct answer: Assessing expansion of air splints in cold weather
Charles' Law (volume varies directly with temperature at constant pressure) is relevant for air splints, which may contract in cold temperatures and require reassessment.
Question 3: During aeromedical transport, which factor does NOT directly contribute to hypoxic hypoxia in a patient?
- Carbon monoxide binding to hemoglobin (Correct answer)
- Decreased cabin partial pressure of oxygen
- Hypoventilation due to sedation
- Ventilation-perfusion mismatch from pneumonia
Correct answer: Carbon monoxide binding to hemoglobin
Carbon monoxide binding to hemoglobin causes hypemic (anemic) hypoxia, not hypoxic hypoxia, which specifically refers to inadequate oxygen in the inspired gas reaching alveoli.
Question 4: A patient's pulse oximeter reads 92% SpO2 during flight at cabin altitude equivalent to 8,000 feet. The flight nurse knows that the actual PaO2 is approximately:
- 60 mmHg (Correct answer)
- 80 mmHg
- 40 mmHg
- 100 mmHg
Correct answer: 60 mmHg
An SpO2 of 92% corresponds to approximately 60 mmHg PaO2 on the oxyhemoglobin dissociation curve, which is the critical inflection point.
Question 5: Graham's Law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. This explains why:
- Carbon dioxide diffuses 20 times more readily across the alveolar membrane than oxygen (Correct answer)
- Oxygen crosses the alveolar membrane faster than nitrogen at altitude
- Helium accumulates in body cavities faster than oxygen at altitude
- Nitrogen causes decompression sickness faster than carbon dioxide
Correct answer: Carbon dioxide diffuses 20 times more readily across the alveolar membrane than oxygen
CO2 has greater solubility (approximately 20× more soluble than O2) and diffuses across the alveolar-capillary membrane much more readily than oxygen.
Question 6: The alveolar gas equation (PAO2 = FiO2 × [PB - PH2O] - PaCO2/RQ) is used by the flight nurse primarily to:
- Estimate expected alveolar oxygen tension and calculate the A-a gradient (Correct answer)
- Determine total oxygen content in arterial blood
- Calculate minute ventilation requirements for a mechanically ventilated patient
- Predict hemoglobin saturation at any given altitude
Correct answer: Estimate expected alveolar oxygen tension and calculate the A-a gradient
The alveolar gas equation calculates PAO2, which when compared to measured PaO2 yields the A-a gradient, an indicator of pulmonary gas exchange efficiency.
Question 7: During rapid cabin decompression, the primary immediate threat to the flight crew and patients is:
- Time of useful consciousness rapidly decreasing due to acute hypoxia (Correct answer)
- Hypothermia from sudden temperature drop
- Dysbarism from trapped gas expansion
- Hypercapnia from loss of ventilation control
Correct answer: Time of useful consciousness rapidly decreasing due to acute hypoxia
Rapid decompression causes sudden severe hypoxia; at 35,000 feet, time of useful consciousness is only 15-20 seconds without supplemental oxygen.
CFRN Physiology and Gas Laws 3
A CFRN is transporting a patient with a closed bowel obstruction.
At altitude, this patient is most at risk for which complication based on gas law principles?