Guide — Transport Respiratory Care
Altitude Physiology & Gas Laws in Transport
Two gas laws explain almost everything that goes wrong at altitude: Boyle expands the air trapped in your patient, and Dalton thins the oxygen they breathe. This guide turns those laws into pre-flight actions — decompress the air spaces, correct the FiO₂, and protect the cuff.
9 min read · Transport Respiratory Care
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
As altitude rises, barometric pressure falls. At sea level, atmospheric pressure is 760 mmHg. At 8,000 ft — a common pressurized cabin altitude — it drops to roughly 565 mmHg. That 25% reduction in pressure drives two physiological consequences that the transport RT must anticipate before wheels leave the ground: trapped gas expands (Boyle’s law) and inspired oxygen partial pressure falls (Dalton’s law). Neither problem announces itself; both must be managed pre-flight.
Safety note. An untreated pneumothorax and an air-filled ETT cuff are the two most time-critical altitude hazards the RT controls. Address both before departure — in-flight correction is far harder and the consequences of delay are life-threatening.
Key Concepts
Three gas laws frame altitude physiology. Boyle and Dalton are clinically dominant; Henry’s law appears in decompression contexts.
| Law | Statement | Transport Implication |
|---|---|---|
| Boyle's Law | At constant temperature, P × V = constant (P₁V₁ = P₂V₂) | Trapped gas expands as barometric pressure falls — ~30–35% at 8,000 ft; ~doubles at 18,000 ft |
| Dalton's Law | Total pressure = sum of partial pressures of all gases | PiO₂ falls with barometric pressure even though FiO₂ (0.21 room air) stays constant — producing hypobaric hypoxia |
| Henry's Law | Gas dissolved in liquid is proportional to its partial pressure | Lower partial pressures at altitude reduce dissolved gas — relevant to decompression physiology |
Boyle’s law in practice: a fixed volume of trapped gas will expand by approximately 30–35% at 8,000 ft and will roughly double at 18,000 ft. Dalton’s law in practice: even though room air is always 21% O₂ (FiO₂ = 0.21), the partial pressure of inspired oxygen (PiO₂ = FiO₂ × (P_baro − 47), after subtracting water-vapor pressure) falls with barometric pressure — from roughly 150 mmHg at sea level to about 109 mmHg at 8,000 ft. The result is hypobaric hypoxia even on unaltered supplemental oxygen.
Closed Gas Spaces at Risk
Any air-filled cavity in or on the patient will expand during ascent. The table below lists the spaces the transport RT must systematically screen before flight.
| Space | Altitude Risk |
|---|---|
| Pneumothorax | Can expand to tension pneumothorax in flight |
| Pneumocephalus | Expanding intracranial air worsens neurological status |
| Bowel gas / obstruction | Expansion causes pain, vomiting, and visceral ischemia risk |
| ETT cuff (air-filled) | Increased cuff pressure injures tracheal mucosa |
| Air splints / MAST trousers | Expanding air increases compartment-like pressure |
| Pneumoperitoneum | Post-operative abdominal air expands, increasing pain and diaphragm pressure |
| Middle ear and sinuses | Pressure differentials cause barotrauma if congested |
| IV drip chambers | Air in chamber expands — ensure air-free during flight |
Assessment & Findings
The pre-flight assessment targets altitude-specific hazards on top of the standard transport evaluation. Screen specifically for:
- Pneumothorax or other trapped air. Review the most recent chest radiograph. If a pneumothorax is present — even a small one — a functioning chest tube with water-seal or flutter valve drainage must be in place before departure. Heimlich valves are acceptable for transport.
- Recent abdominal or cranial surgery. Post-operative gas collections (pneumoperitoneum, pneumocephalus) may persist days after surgery and are not always visible clinically. Review operative reports and imaging.
- ETT cuff status. Note whether the cuff is air-filled. Plan to replace with saline or arrange in-flight monitoring and adjustment of cuff pressure.
- Oxygen requirement and cabin altitude. Determine the patient’s current FiO₂ and SpO₂. Calculate the FiO₂ that will be needed at the planned cabin altitude, and confirm that sufficient supplemental oxygen and delivery hardware are available for the entire mission.
- Middle ear and sinus patency. Congested or intubated patients cannot self-equalize. Assess for recent upper respiratory infection, nasal packing, or auditory pathology that would increase barotrauma risk.
RT Priorities & Interventions
- Decompress closed spaces pre-flight. Ensure any pneumothorax has a functioning chest tube and that it is draining freely. Vent the nasogastric tube to decompress the stomach. Deflate air splints and MAST trousers; plan to re-inflate and recheck at altitude.
- Replace ETT cuff air with saline. Deflate the cuff, fill with the equivalent volume of sterile normal saline, and verify a seal. Saline does not expand with decreasing pressure, so cuff pressure remains stable throughout ascent and descent. If saline is unavailable, use a cuff manometer to monitor and adjust pressure continuously during flight.
- Increase FiO₂ to compensate for reduced PiO₂. Apply the field correction: FiO₂ needed ≈ FiO₂ current × (P_baro current ÷ P_baro at altitude). Titrate upward from that starting point using continuous SpO₂ monitoring. For critically hypoxemic patients where FiO₂ 1.0 is insufficient, request a lower cabin altitude from the flight crew.
- Request a sea-level cabin when indicated. Fixed-wing aircraft can sometimes be pressurized to sea level, eliminating Boyle and Dalton effects. This option increases fuel consumption and structural stress on the airframe; confirm availability and lead time with the transport coordinator before departure.
- Monitor and reassess continuously in flight. Cabin altitude is not always constant, and patient status can change during ascent. SpO₂, end-tidal CO₂ (ETCO₂), cuff pressure, and chest tube drainage should be monitored throughout the flight.
Common Pitfalls
- Flying with an untreated pneumothorax. The single most dangerous pre-flight omission. A pneumothorax that is stable at sea level can convert to a tension pneumothorax within minutes of ascent. It is never safe to transport a patient with an undecompressed pneumothorax by air.
- Leaving the ETT cuff air-filled. This is common because it requires an active, deliberate intervention before departure. Building it into a pre-flight checklist reduces omission.
- Assuming sea-level FiO₂ settings will hold. A patient stable on FiO₂ 0.35 at sea level will desaturate at cabin altitude on the same setting. Pre-calculate and pre-set the corrected FiO₂ before ascent begins.
- Overlooking bowel gas in post-operative patients. Expanding bowel gas causes pain, vomiting, and diaphragmatic splinting. Nasogastric decompression before departure is a simple, effective intervention that is frequently omitted.
- Confusing a pressurized cabin with a sea-level environment. Pressurized transport aircraft still operate at 6,000–8,000 ft cabin altitude. Boyle and Dalton effects are reduced but not eliminated — pre-flight preparation is still required.
Board Exam Pearls
- Boyle’s law and pneumothorax: trapped gas expands as pressure drops — a pneumothorax MUST be decompressed with a functioning chest tube before any air transport.
- ETT cuff rule: use saline (not air) in the ETT cuff for air transport. Saline does not expand with altitude; air does.
- Dalton’s law and hypoxia: PiO₂ = FiO₂ × (P_baro − 47). At altitude, P_baro falls, so PiO₂ falls even though FiO₂ is unchanged. Correct oxygenation by increasing FiO₂ or lowering cabin altitude.
- FiO₂ correction formula: FiO₂ needed ≈ FiO₂ current × (P_baro sea level ÷ P_baro at altitude). Know how to apply it to a clinical scenario.
- Closed gas spaces to know: pneumothorax, pneumocephalus, pneumoperitoneum, bowel gas, air splints, air-filled ETT cuff, middle ear, and sinuses.
FAQ
Why is even a small pneumothorax dangerous at altitude?
Boyle's law dictates that trapped gas expands as barometric pressure falls. At a typical cabin altitude of 8,000 ft, a pneumothorax can expand by 30–35%. A small, clinically insignificant pneumothorax at sea level can become a tension pneumothorax in flight, compressing the mediastinum and collapsing the opposite lung. Any pneumothorax must be decompressed with a functioning chest tube before air transport.
Why fill the ETT cuff with saline instead of air for air transport?
Air in the ETT cuff expands as cabin altitude rises, increasing cuff pressure against the tracheal mucosa. This can cause ischemic injury or necrosis of the tracheal wall. Saline is a liquid and, unlike a gas, does not compress or expand with changes in barometric pressure — cuff volume and pressure remain stable throughout the flight.
How do I estimate the FiO₂ needed to maintain oxygenation at cabin altitude?
Use the field approximation: FiO₂ needed ≈ FiO₂ current × (P_baro current ÷ P_baro at altitude). For example, a patient requiring FiO₂ 0.40 at sea level (760 mmHg) flying to a cabin altitude of 8,000 ft (565 mmHg) would need approximately 0.40 × (760 ÷ 565) ≈ 0.54. This gives you a starting point; titrate based on SpO₂ monitoring throughout the flight.
Does a pressurized aircraft cabin eliminate altitude physiology concerns?
No. Commercial and fixed-wing medical aircraft cabins are pressurized, but typically maintained at an equivalent altitude of 6,000–8,000 ft — not sea level. That still represents a meaningful drop in barometric pressure. Trapped gas still expands and PiO₂ still falls. Only a request for a true sea-level cabin pressure eliminates these effects, and that option comes with significant operational constraints.
Put it to work
Altitude raises the flow you need and shortens how long a cylinder lasts. Plan the supply with the Oxygen Tank Duration calculator.
Open the Oxygen Tank Duration calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Physics and physiology of respiration chapters.
- Commission on Accreditation of Medical Transport Systems. Accreditation Standards of the Commission on Accreditation of Medical Transport Systems. 11th ed. CAMTS; 2018.