CFRN Exam

This chapter turns Boyle's law into bedside actions. Expect several items asking which intervention protects a patient or a device from gas expansion, and at least one asking you to estimate how much a gas volume will grow.

Worked Example: An endotracheal tube cuff holds 8 mL of air at sea level. The helicopter will climb over a pass at 10,000 feet MSL. Expected cuff volume if the cuff could expand freely = 8 × 760 / 523 = about 11.6 mL. Because the trachea limits expansion, the real result is a rise in cuff pressure on the tracheal mucosa, which can cause ischemia, and the cuff can herniate or rupture.

Memory Aid: A rough rule is "a third larger at 8,000 feet, about half larger at 10,000 feet, double at 18,000 feet."

Trapped Gas in the Patient: Effects and Interventions

Ears deserve a careful look because the direction matters. On ascent, expanding middle ear gas pushes the eustachian tube open and vents easily. On descent, external pressure rises and the soft, valve-like eustachian tube tends to collapse, so the ear cannot equalize. Ear block (barotitis media) is therefore mostly a descent problem. Awake patients can swallow, yawn or perform a gentle Valsalva; infants can suck on a bottle or pacifier. An unconscious patient cannot clear the ears, which is one more reason to descend gradually when possible.

A pneumothorax that is small and stable on the ground can expand on climb and become a tension pneumothorax in the aircraft, where noise makes breath sounds useless. Many programs require a chest tube before flight for any pneumothorax in a patient who will be ventilated with positive pressure or flown to significant altitude; others accept close observation for very small pneumothoraces in spontaneously breathing patients with a low planned altitude. The exam answer is almost always: decompress before flight and be ready to decompress in flight.

Transport Scenario: You are flying a ventilated patient with a right chest tube to a trauma center. At 4,500 feet MSL crossing a ridge, peak inspiratory pressure rises from 24 to 38 cmH2O, SpO2 falls to 86% and blood pressure drops from 118/70 to 84/50. The chest tube drainage system shows no tidaling. Suspect a kinked or clotted tube with an expanding pneumothorax. Check the tube along its whole length, then needle-decompress or perform finger thoracostomy per protocol if the patient stays unstable, notify the pilot, and ask for a lower altitude if terrain allows.

Boyle's law acts on every closed air space in the cabin.

Equipment Affected by Altitude

On descent everything reverses: cuffs, splints and pneumatic devices soften and may leak, gravity drips slow, and pressure bags lose pressure. An ETT cuff filled with air at altitude and not rechecked can become underinflated on landing, causing an air leak and aspiration risk. A cuff filled with saline (a liquid, which does not expand) avoids both problems, though saline cuffs are harder to deflate fully and are used per program policy. The most universally correct exam answer is to measure cuff pressure with a manometer at cruise altitude and again on descent.

Exam Tip: If an option says "remove air from the ETT cuff on ascent and add air on descent," it is reasonable, but the best option usually names cuff pressure measurement or a saline-filled cuff. "Leave the cuff alone because the trachea will accommodate the change" is always wrong.

Common Trap: Gravity drip rates rise on ascent because air in the bag and drip chamber expands and pushes fluid out. Do not assume altitude has no effect on a gravity infusion. The fix is an infusion pump, not counting drops.

Some patients benefit from a cabin altitude restriction. In fixed-wing transport, request a sea-level or low cabin altitude for:

Recent intraocular gas injection or penetrating globe injury

Pneumocephalus or recent intracranial surgery with air

Bowel obstruction or severe ileus that cannot be fully decompressed

Suspected decompression illness or arterial gas embolism

Severe hypoxemia already on high FiO2 and PEEP

Undrained pneumothorax when a chest tube cannot be placed

Gas expands about one third at 8,000 feet and doubles at 18,000 feet; trapped gas in a rigid space raises pressure instead.

Decompress before you fly: chest tube for pneumothorax, gastric tube for the stomach and bowel, saline or measured cuffs.

Middle ear problems occur mainly on descent; awake patients swallow or Valsalva, infants suck on a bottle or pacifier.

Every air-containing device needs a check after climb and before landing: cuffs, splints, pressure bags, transducers.

Use pumps, not gravity, for critical infusions in the aircraft.