CFRN Physiology and Gas Laws 5 — Questions and Answers
Question 1: A patient with COPD relying on hypoxic drive is being transported at altitude. The flight nurse should be MOST concerned about administering high-flow oxygen because:
- Eliminating the hypoxic drive may cause apnea or severe hypoventilation (Correct answer)
- High FiO2 worsens V/Q mismatch through pulmonary vasoconstriction
- Oxygen toxicity will occur within the transport window
- High-flow oxygen increases CO2 production via the Haldane effect
Correct answer: Eliminating the hypoxic drive may cause apnea or severe hypoventilation
In COPD patients dependent on hypoxic drive, high-flow oxygen can remove the primary stimulus to breathe, causing dangerous hypoventilation or apnea.
Question 2: The flight nurse calculates oxygen cylinder duration using the formula: Duration = (Cylinder pressure × Cylinder factor) / Flow rate. For a D cylinder (factor 0.16) at 1,800 psi with a flow rate of 4 L/min, the duration is:
- 72 minutes (Correct answer)
- 45 minutes
- 120 minutes
- 30 minutes
Correct answer: 72 minutes
Duration = (1,800 × 0.16) / 4 = 288 / 4 = 72 minutes; always plan for at least 30 minutes extra beyond transport time.
Question 3: Third-spacing of fluids at altitude most commonly occurs because of:
- Reduced oncotic pressure relative to increased hydrostatic pressure from hypoxia-induced pulmonary vasoconstriction (Correct answer)
- Boyle's Law expanding extracellular fluid compartments
- Henry's Law causing plasma gas release into interstitial space
- Charles' Law decreasing plasma volume in cold aircraft cabins
Correct answer: Reduced oncotic pressure relative to increased hydrostatic pressure from hypoxia-induced pulmonary vasoconstriction
Altitude-induced hypoxia causes pulmonary vasoconstriction and increased hydrostatic pressure, promoting fluid movement from capillaries to interstitial spaces.
Question 4: Which of the following BEST explains why a patient breathing spontaneously at altitude will have a lower PaCO2 than at sea level?
- Hypoxia stimulates hyperventilation causing CO2 to be blown off (Correct answer)
- CO2 dissolves more readily in blood at lower atmospheric pressure per Henry's Law
- Reduced atmospheric pressure directly lowers CO2 partial pressure
- Decreased metabolic rate at altitude reduces CO2 production
Correct answer: Hypoxia stimulates hyperventilation causing CO2 to be blown off
Hypoxia at altitude stimulates the carotid body chemoreceptors, increasing respiratory rate and depth, which blows off CO2 and causes hypocapnia.
Question 5: A patient with a pneumothorax that was 15% on chest X-ray at sea level will be approximately what size at a cabin altitude of 8,000 feet (PB ~565 mmHg)?
- Approximately 20% (Correct answer)
- Approximately 30%
- Approximately 15% — no change
- Approximately 50%
Correct answer: Approximately 20%
Using Boyle's Law: V2 = V1 × (P1/P2) = 15% × (760/565) ≈ 20%; small pneumothoraces can expand significantly and should be drained before flight.
Question 6: During rapid ascent to transport altitude, a patient with an air-filled ETT cuff experiences tracheal mucosal ischemia. This is BEST explained by:
- Air in the cuff expands at altitude increasing cuff pressure against the tracheal wall (Correct answer)
- Tracheal vessels vasoconstrict due to hypoxia at altitude
- Charles' Law warming the cuff air and increasing pressure
- Increased respiratory effort forcing the cuff to press harder
Correct answer: Air in the cuff expands at altitude increasing cuff pressure against the tracheal wall
Per Boyle's Law, air in the ETT cuff expands as ambient pressure decreases at altitude, increasing lateral tracheal wall pressure and risking ischemic injury.
Question 7: The oxyhemoglobin dissociation curve shifts LEFT during aeromedical transport primarily as a result of:
- Hyperventilation-induced respiratory alkalosis and hypocapnia (Correct answer)
- Metabolic acidosis from tissue hypoperfusion
- Increased body temperature from transport stress
- Elevated 2,3-DPG from chronic altitude adaptation
Correct answer: Hyperventilation-induced respiratory alkalosis and hypocapnia
Altitude-induced hyperventilation causes respiratory alkalosis (increased pH, decreased PaCO2), shifting the curve left and increasing hemoglobin's affinity for oxygen.
CFRN Physiology and Gas Laws 5
A patient with COPD relying on hypoxic drive is being transported at altitude.
The flight nurse should be MOST concerned about administering high-flow oxygen because: