Chart — Transport Respiratory Care
Altitude Effects on Gas Volume & Oxygenation
What altitude does to a patient in two columns: trapped gas expands (Boyle) and inspired oxygen falls (Dalton). This chart pairs altitude with barometric pressure, room-air PiO₂, and gas-volume expansion, then lists the air spaces to decompress before flight.
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.
Altitude, Pressure, Gas Volume, and Oxygenation
| Altitude | Barometric Pressure (mmHg) | Room-Air PiO₂ (mmHg) | Trapped-Gas Volume vs Sea Level | Clinical Implication |
|---|---|---|---|---|
| Sea level (0 ft) | 760 | ~150 | 1.0× (baseline) | Reference. |
| 5,000 ft | ~632 | ~123 | ~1.2× (about 20% larger) | Mild hypoxemia risk in marginal patients. |
| 8,000 ft (typical cabin altitude) | ~565 | ~109 | ~1.3× (about 30–35% larger) | Standard cabin altitude; expect lower SpO₂ and meaningful gas expansion. |
| 10,000 ft | ~523 | ~100 | ~1.45× (about 45% larger) | Significant hypoxemia without supplemental O₂. |
| 12,000 ft | ~483 | ~92 | ~1.6× (about 57% larger) | Supplemental oxygen essentially required. |
Closed Gas Spaces Affected by Altitude
Any non-communicating gas space expands as barometric pressure falls. Decompress or actively manage these before flight:
- Pneumothorax.Gas volume expansion risks conversion to tension pneumothorax — chest tube drainage before flight is strongly preferred.
- ETT cuff. Air-filled cuffs expand and can over-pressurize the trachea; use saline or a pressure-regulating device in-flight.
- Bowel gas & obstruction. Gaseous distension worsens with altitude; decompress nasogastrically when possible.
- Pneumocephalus.Intracranial air expands — consult neurosurgery before air transport.
- Middle ear & sinuses. Pressure equalization problems cause pain and potential injury, especially in sedated patients.
- Air splints & MAST trousers. Monitor and adjust pressure continuously during ascent and descent.
- Pneumoperitoneum. Post-laparotomy patients with residual abdominal air are at risk; defer non-urgent transport when possible.
- IV drip chambers. Air bubbles in drip chambers can expand and cause air embolism risk; use pressure bags and inline filters.
FiO₂ Correction & Notes
Because inspired oxygen tension falls with barometric pressure, patients who are adequately oxygenated at sea level may require supplemental O₂ at altitude. A practical field approximation for the FiO₂ needed to preserve the same inspired oxygen partial pressure:
- FiO₂ correction formula. FiO₂ needed ≈ FiO₂ current × (Pbaro current ÷ Pbaroat altitude). For a patient on room air (FiO₂ 0.21) flying to an 8,000 ft cabin: 0.21 × (760 ÷ 565) ≈ 0.28 — roughly the equivalent of low-flow O₂ at sea level.
- Trapped-gas doubles at ~18,000 ft.At that altitude barometric pressure is approximately half of sea level (~380 mmHg), so gas volume is twice its sea-level size by Boyle’s law. Cabin altitudes are far lower, but this landmark illustrates the physics quickly.
- Request a lower cabin altitude for critical patients. Fixed-wing aircraft can sometimes maintain a lower cabin pressure (closer to sea level) for critically hypoxemic patients; coordinate this with the pilot and medical team before wheels-up.
How to Use This Chart
Start with the patient’s current altitude and oxygen status, then work down the table to the transport altitude. The PiO₂ column tells you how much inspired oxygen the patient will have on room air; the volume column tells you how much any trapped gas space will expand. Use both columns together when clearing a patient for flight.
- Pre-flight assessment. Identify every closed gas space, confirm SpO₂ on the planned FiO₂, and calculate whether supplemental oxygen will be required at cabin altitude.
- Monitor continuously during ascent.Gas expansion is proportional to the change in pressure and happens progressively as the aircraft climbs — reassess at intervals, not just on level-off.
- Descent matters too. Pressure increases again on descent; ETT cuffs and pressure lines must be rechecked as altitude decreases.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Physics and physiology of respiration.
- Commission on Accreditation of Medical Transport Systems. Accreditation Standards of the Commission on Accreditation of Medical Transport Systems. 11th ed. CAMTS; 2018.