10.2 Traumatic Brain Injury & ICP Management

The Monro-Kellie Doctrine

The skull is a rigid box of fixed total volume occupied by three components: brain tissue (~80%), blood (~10%), and cerebrospinal fluid (CSF, ~10%). The Monro-Kellie doctrine states that because the vault cannot expand, an increase in any one component — or the addition of a mass such as a hematoma or cytotoxic edema — must be offset by a decrease in another, or intracranial pressure (ICP) rises. Early compensation shunts CSF toward the spinal thecal sac and pushes venous blood out. Once these buffers are exhausted, small additional volume produces steep ICP spikes, because the pressure-volume curve bends sharply upward. This is why a patient can look stable and then decompensate abruptly.

ICP and CPP Targets

Normal ICP is 5-15 mmHg; sustained ICP above 20-22 mmHg is treated. The number that determines whether the brain is actually being perfused is cerebral perfusion pressure (CPP), calculated as:

CPP = MAP - ICP (MAP = mean arterial pressure)

The CCRN expects you to compute this instantly and to target a CPP of 60-70 mmHg.

A CPP below 60 risks ischemia; chasing a CPP above 70 with aggressive fluids and pressors risks ARDS and is avoided. Because CPP depends on both MAP and ICP, you can raise it by lowering ICP or supporting MAP — but the priority is always to first lower a treatable elevated ICP.

TBI Severity and Preventing Secondary Injury

TBI severity is graded by the Glasgow Coma Scale (GCS): severe 3-8, moderate 9-12, mild 13-15. A GCS of 8 or less is the trigger to secure a definitive airway (less than eight, intubate). Two secondary insults must be prevented because each independently worsens mortality: hypoxia (keep SpO2 >= 90-94%, PaO2 > 60) and hypotension (keep SBP >= 110 mmHg). The injured brain loses autoregulation and becomes pressure-passive, so even a single episode of hypotension can extend injury.

Cushing's Triad and Herniation

When rising ICP outpaces treatment, herniation occurs — brain tissue forced across fixed dural or bony boundaries. The classic late warning is Cushing's triad: hypertension with a widening pulse pressure, bradycardia, and irregular respirations (Cheyne-Stokes or agonal). Paired with a unilateral fixed, dilated (blown) pupil from third cranial nerve (CN III) compression in uncal herniation, this is a neurologic emergency demanding immediate ICP-lowering. Herniation syndromes include uncal, central, subfalcine (cingulate), and tonsillar; tonsillar herniation compresses the brainstem and is rapidly fatal.

Tiered ICP Management

Interventions are layered from least to most aggressive:

• Positioning: head of bed 30 degrees, head and neck midline and neutral; avoid tight cervical collars and hip flexion to promote jugular venous drainage.
• Normocapnia: maintain PaCO2 35-45 mmHg. Avoid hypercapnia, which vasodilates cerebral vessels and raises ICP. Brief, controlled hyperventilation to PaCO2 30-35 is only a short bridge for acute herniation, not a maintenance strategy — sustained hyperventilation causes ischemic vasoconstriction.
• Sedation and analgesia to blunt stimulation-driven ICP surges; treat fever (normothermia) and seizures.
• Osmotherapy: mannitol 0.25-1 g/kg (an osmotic diuretic — monitor for hypovolemia and hypotension, keep serum osmolality <320) OR hypertonic saline (3% infusion or 23.4% bolus), which draws water across the blood-brain barrier while supporting intravascular volume (target Na 145-155, avoid rapid overcorrection).
• CSF drainage via an external ventricular drain (EVD) leveled to the tragus / foramen of Monro and clamped during repositioning.
• Refractory tiers: neuromuscular blockade, barbiturate coma, therapeutic hypothermia, and decompressive craniectomy.

ICP Waveforms, Autoregulation, and Metabolic Control

An intact ICP waveform has three peaks: P1 (percussion), P2 (tidal), and P3 (dicrotic). When P2 rises above P1, intracranial compliance is falling — an early warning that the brain is running out of buffer before the numeric ICP climbs. Plateau (Lundberg A) waves are sustained ICP elevations to 50-100 mmHg lasting 5-20 minutes and are ominous. Cerebral autoregulation normally holds blood flow constant across a MAP of roughly 60-150 mmHg; after severe TBI this range narrows or is lost, making the brain pressure-passive and vulnerable to both hypotension and hypertension. Supportive measures matter: maintain glucose 140-180 mg/dL (both hypoglycemia and hyperglycemia harm injured neurons), keep the patient normothermic because fever raises cerebral metabolic demand, prevent shivering, and treat pain and agitation that spike ICP. Deep-vein-thrombosis prophylaxis, stress-ulcer prophylaxis, and early enteral nutrition round out the bundle once bleeding is controlled.

Nursing Surveillance and Traps

Cluster care to avoid repeated stimulation, keep MAP adequate before manipulating ICP, and never suction longer than necessary. A key trap is giving a fluid bolus to raise ICP — that is wrong; you support MAP judiciously, but the goal is to lower ICP. Another trap is laying a herniating patient flat: keep the head elevated and midline unless you are resuscitating from arrest. When an EVD is used, level it to the tragus, clamp it during repositioning per protocol, and document CSF color and hourly output; a sudden change in drainage volume or waveform demands a provider call. Choose hypertonic saline over mannitol when the patient is hypovolemic or hypotensive, because mannitol's osmotic diuresis can drop the MAP and paradoxically lower CPP.