6.1 Traumatic Brain Injury & Intracranial Pressure (ICP) Management

6.1 Traumatic Brain Injury & Intracranial Pressure (ICP) Management

Critical care transport of patients with severe Traumatic Brain Injury (TBI) demands an aggressive, physiology-driven approach aimed at preventing secondary brain injury. While primary brain injury occurs at the instant of mechanical impact (e.g., contusions, lacerations, diffuse axonal injury), secondary brain injury develops over subsequent hours to days as a result of hypoxemia, systemic hypotension, hypercapnia, cerebral edema, intracranial hypertension, and hypoperfusion. For the Critical Care Paramedic (CCP-C), maintaining adequate cerebral perfusion pressure and controlling intracranial pressure (ICP) are the cornerstones of neurocritical resuscitation.

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The Monro-Kellie Doctrine & Intracranial Compliance

The skull is a rigid, non-expansile container. Under physiological conditions, the total intracranial volume is fixed and divided among three distinct structural compartments:

$V_{\text{intracranial}} = V_{\text{brain}} + V_{\text{blood}} + V_{\text{CSF}} = \text{Constant}$

The Monro-Kellie Doctrine states that because the total volume of the intracranial vault is constant, an increase in the volume of any single compartment (or the introduction of a pathological space-occupying lesion, such as an epidural or subdural hematoma) must be offset by an equal reduction in the volume of another compartment.

Spatial Compensation Mechanisms

1. CSF Displacement: CSF is displaced from the cerebral ventricles and subarachnoid space into the spinal subarachnoid space, and CSF production decreases while absorption increases.
2. Venous Blood Displacement: Venous blood is compressed and shunted out of the dural sinuses into the internal jugular veins.

Once these spatial compensatory mechanisms are exhausted, intracranial compliance drops precipitously. At this inflection point on the pressure-volume curve, even minute additions of mass or volume (such as 1–2 mL of blood or cerebral swelling) trigger dramatic, life-threatening spikes in ICP.

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Intracranial Pressure (ICP) & Cerebral Perfusion Pressure (CPP)

Intracranial Pressure Normal Values & Thresholds

• Normal Resting ICP: $5 - 15 \text{ mmHg}$ (in adults seated or supine).
• Intracranial Hypertension: Defined as a sustained elevation of ICP $> 22 \text{ mmHg}$. Sustained pressures above $22 \text{ mmHg}$ are associated with significantly increased mortality and warrant immediate therapeutic intervention.

Cerebral Perfusion Pressure (CPP) Formula

Cerebral Perfusion Pressure is the net pressure gradient driving blood flow through the cerebral vascular bed, supplying oxygen and substrates to brain tissue. It is governed by the following fundamental equation:

$CPP = MAP - ICP$

Where Mean Arterial Pressure (MAP) is calculated as:

$MAP = \frac{SBP + 2 \times DBP}{3} \quad \text{or} \quad MAP = DBP + \frac{1}{3}(SBP - DBP)$

Note: If Central Venous Pressure (CVP) exceeds ICP, CVP is substituted for ICP in the equation, though in brain-injured patients ICP is almost universally higher than CVP.

Clinical Perfusion Targets & Autoregulation

• Target CPP: $60 - 70 \text{ mmHg}$ in adult TBI patients.
• Avoid CPP $< 60 \text{ mmHg}$: Inadequate perfusion leads to cerebral ischemia, cellular hypoxia, cytotoxic edema, and secondary neuronal cell death.
• Avoid CPP $> 70 \text{ mmHg}$: Excessively high CPP causes hyperperfusion, breakdown of the blood-brain barrier, vasogenic edema, and increases the risk of acute respiratory distress syndrome (ARDS) from systemic fluid overload.
• Cerebral Autoregulation: Healthy cerebral vasculature automatically constricts or dilates to maintain constant cerebral blood flow (CBF) across a MAP range of $50 - 150 \text{ mmHg}$. In severe TBI, autoregulation is frequently impaired or completely abolished. The cerebral circulation becomes pressure-passive, meaning cerebral blood flow rises and falls directly with systemic blood pressure. Systemic hypotension ($SBP < 100 - 110 \text{ mmHg}$) in severe TBI doubles patient mortality.

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Clinical Signs of Herniation & Cushing's Triad

When intracranial pressure continues to rise unchecked, brain tissue is forcibly displaced from high-pressure compartments to lower-pressure compartments across rigid intracranial structures (falx cerebri, tentorium cerebelli, or foramen magnum).

Cushing's Triad

Cushing's Triad is a physiological response to life-threatening brainstem compression and impending transtentorial or tonsillar herniation:

Physical Exam Features of Brain Herniation

• Uncal Herniation: Unilateral expansion of the temporal lobe compresses the ipsilateral Oculomotor Nerve (CN III), causing an ipsilateral dilated, non-reactive pupil ("blown pupil"), accompanied by contralateral hemiparesis.
• Posturing: Progression from decorticate posturing (abnormal flexion, reflecting damage above the red nucleus/midbrain) to decerebrate posturing (abnormal extension, reflecting midbrain/upper brainstem destruction).

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Tiered Management Protocol for Intracranial Pressure

Management of elevated ICP follows a stepwise, tiered protocol to minimize secondary brain injury.

Oxygenation & Ventilation Targets

• PaO2 Target: $\ge 80 \text{ mmHg}$ ($SpO_2 \ge 95%$). Hypoxia ($PaO_2 < 60 \text{ mmHg}$ or $SpO_2 < 90%$) independently doubles TBI mortality.
• Normocapnia Target: $PaCO_2 \ 35 - 40 \text{ mmHg}$ ($EtCO_2 \ 35 - 40 \text{ mmHg}$). Carbon dioxide is a potent cerebral vasodilator. Hypercapnia ($PaCO_2 > 45 \text{ mmHg}$) dilates cerebral arteries, increasing intracranial blood volume and spiking ICP.
• Role of Hyperventilation: Prophylactic or prolonged hyperventilation ($PaCO_2 < 35 \text{ mmHg}$) causes profound cerebral vasoconstriction, severe hypoperfusion, and focal cerebral ischemia. Hyperventilation is strictly restricted to acute, transient herniation crises (e.g., sudden unilateral pupil dilation or decerebrate posturing) as a short-term rescue maneuver targeting $PaCO_2 \ 30 - 35 \text{ mmHg}$ while hyperosmolar therapy is prepared.

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Hyperosmolar Therapy: Mannitol vs. 3% Hypertonic Saline

Hyperosmolar agents expand intravascular volume, decrease blood viscosity, and establish an osmotic gradient that pulls water out of edematous brain parenchyma across an intact blood-brain barrier.

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External Ventricular Drain (EVD) Management & Transport Safety

An External Ventricular Drain (EVD), or ventriculostomy, is the gold standard for both therapeutic CSF drainage and continuous monitoring of ICP.

Anatomical Leveling

To obtain accurate pressure readings and regulate CSF drainage, the zero reference point of the EVD transducer must be aligned with the Foramen of Monro. Anatomically, the external landmark corresponding to the Foramen of Monro is the Tragus of the ear (or the external auditory meatus).

Drainage Pressure Settings

The drainage chamber is set at a specific height in centimeters of water ($cm \ H_2O$) above the tragus (e.g., $+10 \text{ cm } H_2O$), as ordered by neurosurgery. When ICP exceeds this hydrostatic pressure threshold, CSF drains into the collection bag.

Critical Transport Precautions

• CLAMP the EVD during transfer and transport movement: Prior to moving the patient from bed to stretcher, raising/lowering the stretcher, or transferring between facilities, the EVD must be clamped. Movement without clamping risks catastrophic over-drainage or under-drainage:
  • Over-drainage: Rapid outflow of CSF can cause ventricular collapse, tearing of bridging veins leading to subdural hematoma, or brain herniation.
  • Under-drainage / Backflow: If the drain is positioned too high during transport, CSF will fail to drain or backflow, causing rapid accumulation of ICP.
• Re-leveling upon Arrival: Once the patient is secured in the transport vehicle and motionless, re-level the transducer precisely to the tragus of the ear, verify line patency, unclamp the drain, and confirm baseline ICP waveforms.