11.2 Secondary Brain Injury

The primary injury happened at impact. Secondary brain injury is everything you allow after that. Domain 3.B of the August 2026 Board of Certification for Emergency Nursing (BCEN) Certified Flight Registered Nurse (CFRN) outline tests those insults because they are the part a flight nurse can actually prevent. The 2026 outline also lists intracranial pressure (ICP) management under medical neurology (section 14.3). Keep drain and transport technique here. Leave medical space-occupying-lesion titration for that chapter so the two sections do not become the same paragraph twice.

What you can prevent in the cabin

The testable list

Every item below is a second hit. Name them on the ramp the way you name failed-airway gear.

• Hypoxia — Dalton's law drops partial pressure of inspired oxygen (PIO2) on climb; raise fraction of inspired oxygen (FiO2) or lower the cabin.
• Hypotension — a single systolic blood pressure under 90 mm Hg is associated with worse outcome; blood or a pressor, not a shrug.
• Hypercarbia or hypocarbia — high carbon dioxide dilates vessels and raises ICP; low carbon dioxide starves the penumbra. Target end-tidal carbon dioxide (ETCO2) 35–40 mm Hg.
• Hyperthermia, hyponatremia, seizures, hyperglycemia, and anemia — fever raises demand; free water swells brain; a convulsion is an ICP crisis; skip needless dextrose; oxygen content needs hemoglobin.
• Mechanical ICP spikes — coughing, fighting the tube, head-down loading, a tight collar, and high positive end-expiratory pressure (PEEP) all shove pressure into the vault.

A coughing, bucking patient on a tube is performing a Valsalva against a closed system. That is an ICP crisis you created. Sedate—and paralyze if protocol allows—before you congratulate yourself on a secure airway. Deep tracheal suction is the same spike. Pre-oxygenate, keep it short, and watch the pupils after.

Monro-Kellie in one cabin sentence

The Monro-Kellie doctrine says the adult skull is a rigid box. Brain, cerebrospinal fluid (CSF), and blood share a fixed volume. If one compartment grows—hematoma, edema, extra arterial blood from hypercarbia—another must shrink or ICP rises. Early compensation is CSF displacement into the spine and venous blood leaving the vault. When that reserve is gone, a few milliliters more produce a steep ICP rise, herniation, and a falling cerebral perfusion pressure (CPP).

CPP is mean arterial pressure (MAP) minus ICP. You often cannot measure ICP on a scene call. You still protect CPP by refusing hypotension and the mechanical spikes above. Hypercarbia is extra arterial blood in the box; a head-down slide is extra venous blood. Both are Monro-Kellie events you can see without a bolt.

External ventricular drain and ICP-drain transport

Level, clamp, and output

An external ventricular drain (EVD) is both a monitor and a treatment. It must stay leveled to the foramen of Monro—commonly the external auditory meatus or tragus—when you are using it as a transducer. If you raise the head of the bed and leave the transducer on the litter rail, the number is a lie. If you drop the collection burette below the ordered level, you overdrain. If you raise it too high, you underdrain and ICP climbs.

Do not clamp an EVD casually. Clamping for a short move is a program-specific, physician-ordered act, not a default because the cabin is tight. A clamped drain in a patient who was depending on CSF egress can produce a rapid ICP spike. If you must clamp for a seconds-long roll, watch pupils and blood pressure when you reopen, and document the output you see.

Note color and hourly volume. A sudden stop may be a clot in the tubing or a herniation that collapsed the ventricle. A sudden flood of bright blood is a new hemorrhage until the receiving neurosurgeon says otherwise. Keep the system sterile. Do not flush an EVD in the aircraft unless a written protocol and a physician order exist—that is a ventriculitis and overdrainage risk.

Intraparenchymal bolts measure but do not drain. They still need a leveled, zeroed cable so vibration does not invent a crisis.

Loading is where ICP care dies. A head-down slide, a collar cinched across the jugulars, and a fight with the tube undo an hour of osmotherapy. Reverse Trendelenburg if the aircraft allows; if the only path is downhill, shorten it, raise FiO2 first, and recheck pupils when the litter locks. Medical titration for tumors and hydrocephalus belongs in section 14.3; the hardware rules do not change.

• Prevent the list: oxygen, pressure, carbon dioxide, temperature, sodium, seizures, glucose, hemoglobin, and mechanical spikes.
• Monro-Kellie means a few extra milliliters can herniate a compensated brain.
• Level the EVD to the tragus; do not clamp by habit; write down the output.
• Do not load a tight-brain patient head-down if you have any other geometry.