Flight and Transport Physiology: Gas Laws, Altitude and Stressors of Flight

Air medical transport takes the patient through falling pressure. Gas in closed spaces expands (Boyle's law). The partial pressure of inspired oxygen falls (Dalton's law). Add noise, vibration, fatigue and motion, and both crew and patient feel it. Each change can alter oxygenation, device readings and gas-filled spaces. That is why the plan gets made before the aircraft leaves the ground.

Gas Laws That Matter in Flight

Flow Path

1. Climb, or let cabin pressure fall, and barometric pressure (PB) drops. Near 18,000 ft the atmosphere weighs about half of what it does at sea level.
2. Inspired PO₂ (PIO₂) falls in step with PB. The oxygen fraction of air stays put. Only the pressure around it falls.
3. Alveolar PO₂ (PAO₂) falls further. Arterial PO₂ follows it down unless the patient ventilates more or receives a higher fraction of inspired oxygen (FiO₂).
4. Gas in a sealed space, such as the ETT cuff or a pneumothorax, expands. It pushes harder against its walls.

Forms of Hypoxia

What Altitude Does to Healthy Oxygenation

• 8,000 ft (2,438 m): SpO₂ stays above 90% in most healthy people. This is the maximum cabin altitude regulators allow under normal operating conditions.
• 10,000 ft (3,048 m): hypoxemia becomes more prominent, and SaO₂ falls to about 89% in healthy people.
• Above 10,000 ft: symptoms worsen and time of useful consciousness (TUC) shrinks rapidly; see the table below.

Time of Useful Consciousness Without Oxygen

Values from FAA-H-8083-25C, Figure 17-1. Individuals vary. Do not count on symptoms as your warning.

Symptoms

Early hypoxia can feel like euphoria, or simply carefree. Then the signs stack up. Headache, impaired judgment, drowsiness and visual change follow. Numbness and cyanosis come as oxygen starvation builds. Symptoms vary by person, so a patient or crew member who feels fine can still be hypoxic.

Stressors of Flight

• Hypoxia: Covered above. It impairs judgment before the person notices it.
• Acceleration: Heavy G-forces can cause stagnant hypoxia.
• Hyperventilation: Breathing too hard lowers CO₂. Dizziness follows. In severe cases, the patient can lose consciousness.
• Ear and sinus gas: Middle ear and sinus problems are a known in-flight hazard.
• Low humidity: Dry cabin air is a respiratory hazard for passengers with lung disease.
• Cold, fatigue, dehydration and motion: cut circulation and performance. Motion sickness and spatial disorientation are listed flight hazards.
• Noise and vibration: The sources here do not put numbers on noise or vibration. Use the operator's guidance for hearing protection and alarm audibility.

The equipment risk is gas in closed spaces. As ambient pressure falls, each one swells. Each one needs a look before takeoff and again in the air.

Oxygen Delivery Equipment

• Ventilators: BTS 2022 wants you to ask whether the ventilator settings need changing at altitude.
• Portable oxygen concentrators: The airline or operator must sign off before flight. Pulse-dose and continuous modes do not deliver the same flow.
• Cylinders: confirm supply duration and spare capacity for the planned flight time, as your protocol requires.

Who Needs a Pre-Flight Assessment

• Severe or poorly controlled obstructive airway disease (forced expiratory volume in 1 second, FEV₁, below 50% predicted; symptoms, oxygen needs or frequent exacerbations).
• Recent (under 6 weeks) hospital treatment for a respiratory condition, or patients who need domiciliary oxygen.
• Recent (under 6 weeks) pneumothorax, or higher risk of pneumothorax such as cystic lung disease or recurrent pneumothorax.
• Recent (under 6 weeks) pulmonary embolism or deep vein thrombosis, or increased venous thromboembolism (VTE) risk.

Contraindications to Air Travel

• Untreated respiratory failure.
• Untreated pneumothorax.
• Active infection that risks others, such as TB, SARS, MERS or COVID-19.
• Bronchogenic cysts.

Hypoxic Challenge Test Thresholds (Adults)

• PaO₂ at or above 50 mmHg (6.6 kPa), or SpO₂ at or above 85%: in-flight oxygen not required.
• PaO₂ below 50 mmHg, or SpO₂ below 85%: in-flight oxygen recommended. Titrate to keep PaO₂ at or above 50 mmHg or SpO₂ at or above 85%.

Those thresholds come from passenger guidance for commercial aircraft. They are not a transport target. For a patient in an air ambulance, the medical director sets the oxygen plan.

Key Evidence

Normal Values and Targets

Before Takeoff

1. Confirm whether the aircraft is pressurized and the planned maximum altitude.
2. Record SpO₂ on the planned oxygen source, and on room air where the patient's order allows.
3. Measure and record ETT cuff pressure after the tube is secured.
4. Calculate the FiO₂ needed at the planned altitude (see Calculations & Derived Values).
5. Confirm oxygen supply duration, spare cylinders and the pneumothorax plan.
6. Confirm the patient is not carrying a contraindication from the list above.

Two formulas explain most of the altitude effect on oxygen. Both run on barometric pressure (PB), and aircraft altitude sets that number.

PaCO₂ is arterial CO₂ tension. The 47 mmHg term is water-vapour pressure in the airway at 37°C. RQ, the respiratory quotient, is usually taken as 0.8. Hyperventilation pulls PaCO₂ down. Lower PaCO₂ raises PAO₂ in the second formula.

Reading Trends, Not Single Numbers

• SpO₂: drops with altitude even when the patient looks well. Chart the trend across the climb and the descent.
• ETT cuff pressure: climbs as the cuff gas expands on the way up. It falls on the way down, as the gas contracts (Boyle's law).
• Ventilation: in CO₂ retainers, a rising PaCO₂ on higher oxygen is the pattern to watch.

Abnormal Patterns

Crew Factors

• Crew hypoxia is hard to catch in yourself. The rule sits in 14 CFR 91.211(a), which covers U.S.-registered civil aircraft. Above 12,500 ft and up to 14,000 ft cabin pressure altitude, the minimum flight crew uses supplemental oxygen for any stretch longer than 30 minutes. Above 14,000 ft, they use it the entire time.
• Part 91 goes further above 15,000 ft. Each occupant must be provided with supplemental oxygen there. Section 91.211(b) adds reserve and mask rules for pressurized cabins at higher flight levels. Air ambulance operators answer to Part 135 and their own certificate rules, so confirm them.

Crew hydration, rest and oxygen use go on the same checklist as the patient's. Nobody gets a pass on it.

In flight, most of the drugs that matter are oxygen and passenger-level preventives. Doses vary by protocol; follow local protocols and medical direction.

Before You Climb

1. Confirm the aircraft type and whether it is pressurized.
2. Check the oxygen plan for the planned altitude.
3. Measure cuff pressure and confirm the protocol target.
4. Confirm equipment can run at altitude, including ventilator settings.

Helicopter and Fixed-Wing Differences

• Unpressurized helicopter: the patient sits at the altitude the aircraft flies. For one HEMS service, the median maximum was 3,000 ft.
• Pressurized aircraft: normal operation holds the cabin at or below 8,000 ft. Check the aircraft you actually get, because the operator may change aircraft.

Handoff

• Cuff pressure readings and the target used.
• SpO₂ trend and FiO₂ used at altitude.
• Pneumothorax status and the plan for it.
• Anticoagulation or VTE prophylaxis given.

Pediatric

Pediatric cuff management at altitude varies widely by region and organization. Children were historically moved almost always with uncuffed endotracheal tubes. They were also left out of the adult cuff studies, so pediatric evidence is thin. With a cuffed tube in place, the same Boyle's-law expansion applies. Measure cuff pressure and follow your pediatric protocol. Practice varies, including liquid cuff inflation. For tube selection, see the pediatric airway page.

BTS 2022 flags children with anemia, right-to-left heart shunts, neuromuscular disorders or lung disease as the most at risk for hypoxia.

Pregnancy

Maternal hypoxemia cuts oxygen delivery to the fetus. Set the SpO₂ target and oxygen plan with the obstetric team. Late in pregnancy, left lateral positioning relieves aortocaval compression.

Conditions That Lower Altitude Tolerance

• Anemia: the blood carries less oxygen (hypemic hypoxia, FAA-H-8083-25C).
• Carbon monoxide poisoning: CO latches onto hemoglobin and blocks oxygen transport (hypemic hypoxia, FAA-H-8083-25C).
• These sources do not cover other conditions, such as heart disease or sickle cell disease. Decide those with medical direction.

Chronic Lung and Heart Disease

• COPD: hypoxic challenge testing should be considered if resting SpO₂ is 95% or lower, Medical Research Council (MRC) dyspnea grade is 3 or higher, or SpO₂ falls below 84% on a walk test. When CO₂ retention is a concern, titration testing is advised.
• Pulmonary hypertension, New York Heart Association (NYHA) class III or IV: the usual advice is in-flight oxygen.
• Interstitial lung disease: consider testing if SpO₂ falls below 95% on exercise and resting PaO₂ is 9.42 kPa or lower, or transfer factor (TLCO) is 50% or lower.

Pneumothorax and Bullous Lung Disease

A history of pneumothorax or bullous lung disease needs a specialist assessment before flight. After full resolution on chest X-ray, the BTS 2022 statement advises waiting 7 days. Cystic lung disease raises pneumothorax risk. A pneumothorax in a patient with chronic lung disease is a significant challenge.

Systematic Walk-Through

1. Altitude and aircraft type.
2. Barometric pressure and PIO₂.
3. FiO₂ and SpO₂ target.
4. Pneumothorax and bullae.
5. ETT cuff pressure and the plan for altitude.
6. VTE risk and any prophylaxis ordered.
7. Handoff and equipment checks.

Pearls

• Boyle's law runs backward on descent. Cuff pressure falls as the gas contracts.
• Judgment slips before the person notices the hypoxia.
• At altitude, give oxygen during CPR. Compression depth falls in hypoxia.
• The BTS 50 mmHg PaO₂ figure is a passenger test cut-off. It is not a transport target.