Reserve is finite: pressure inside a closed box

The mechanism. The skull is a fixed container holding three things: brain tissue, blood, and cerebrospinal fluid. When one of the three grows, the other two are displaced to make room, and during that displacement the pressure barely moves. This is the flat part of the curve, and it is where your patient looks fine. Once displacement is exhausted, a tiny added volume produces a large pressure jump. Cerebral perfusion is what the mean arterial pressure has left over after intracranial pressure takes its share, so as the pressure climbs, perfusion falls even while the blood pressure on the monitor still reads normal. The perfusion pressure calculation is → BOOK 06 · P4.

Early, in order

• A change in level of consciousness, first, before anything else.
• Restlessness, irritability, or new confusion.
• Headache that is worse in the morning or with a Valsalva maneuver.
• Vomiting that arrives suddenly and is not preceded by nausea.
• A pupil that becomes sluggish, then fixed and dilated, on the same side as the lesion.

Late

• Cushing's triad: systolic pressure rising with a widening pulse pressure, bradycardia, and irregular respirations.
• Decorticate posturing progressing to decerebrate posturing.

Watch outCushing's triad is not an early warning. It means herniation is already underway, not beginning. If you are waiting for the triad to confirm your suspicion, you have already lost the interval that mattered.

What the nurse controls

• Head of bed at 30 degrees, head midline, hips unflexed, so venous drainage from the head is not obstructed.
• Suction passes kept brief and preceded by oxygenation.
• Care spaced out rather than clustered, because each stimulus adds pressure.
• Stool softeners and a quiet room to prevent Valsalva.
• Normothermia maintained, since fever raises cerebral metabolic demand.
• Hypotonic fluid avoided, because free water moves into brain tissue.

Memory hookUP, DOWN, IRREGULAR for the triad. Pressure up, pulse down, respirations irregular.

Margin notethe first sign is always how they answer you. Osmotic agents as a drug class are → BOOK 05 · P11.

Takeawaylevel of consciousness changes first, Cushing's triad arrives last, and everything the nurse does is aimed at the space between them.