From the Guidelines
Intracranial pressure (ICP) is primarily determined by the volume of three main components within the rigid skull: brain tissue, cerebrospinal fluid (CSF), and blood, as stated in the Monro-Kellie doctrine 1.
Factors Affecting ICP
Several factors can alter these volumes and affect ICP, including:
- Brain tissue volume increases with edema, tumors, abscesses, or hemorrhage
- CSF volume changes with altered production, obstruction of flow, or impaired reabsorption at arachnoid granulations
- Blood volume fluctuates with cerebral blood flow, which is affected by arterial CO2 levels, blood pressure, and autoregulation mechanisms
- Systemic factors like hypoxia, hypercapnia, and severe hypertension can increase cerebral blood flow and subsequently raise ICP
- Venous outflow obstruction can increase cerebral blood volume and elevate ICP by preventing blood from leaving the cranial vault
Clinical Considerations
According to the most recent evidence, monitoring of ICP and cerebral perfusion pressure (CPP) is considered fundamental to the care of patients with acute brain injury (ABI), particularly those in coma 1. The threshold that defines intracranial hypertension is generally considered to be greater than 20–25 mmHg, although both lower and higher thresholds are described 1.
Management of ICP
The management of ICP involves a balanced approach, starting with simple and less aggressive measures, such as head positioning, analgesia, and sedation, and progressing to more aggressive measures as clinically indicated 1. A balanced approach to ICP makes use of any of the approaches detailed, with appropriate monitoring safeguards in a critical care unit.
From the Research
Factors Determining Intracranial Pressure
Intracranial pressure (ICP) is determined by the volumes of intracranial blood, cerebrospinal fluid, and brain tissue 2, 3. The dynamics of ICP are more important than the absolute value at any given time point, but mean ICP exceeding 20-25mmHg is usually treated aggressively 3.
Key Components
- Brain tissue
- Blood
- Cerebrospinal fluid (CSF) These components exert pressure on the dural envelope, and an increase in any of these volumes can trigger compensatory changes in the other compartments, resulting in initially limited change in ICP 3.
Pathophysiology
The pathophysiology of raised ICP involves the expansion of any of the intracranial volumes, which can lead to increased ICP 3. This can be caused by various factors, including traumatic brain injury, stroke, neoplasm, Reye's syndrome, hepatic coma, or other pathologies 2.
Measurement and Monitoring
ICP can be measured invasively using techniques such as direct ventricular manometry or measurement in the parenchyma with electronic or fiberoptic devices 2. Monitoring ICP is crucial in managing patients with intracranial hypertension, and displaying the time course of pressure (high-resolution ICP tonoscopy) allows assessment of the validity of the signal and identification of specific pathological findings 2.
Treatment
Treatment of intracranial hypertension typically involves a three-staged approach, starting with restoration of systemic physiology, sedation, and analgesia 3. If these measures are insufficient, surgical options, such as drainage of cerebrospinal fluid or evacuation of mass lesions, are considered 3. In the absence of surgical options, stage 2 treatments are initiated, consisting of either mannitol or hypertonic saline 3, 4.
CSF Dynamics
CSF dynamics, including CSF formation rate, resistance to reabsorption of CSF, and brain tissue water content, play a crucial role in determining ICP 4, 5, 6. The pressure-volume index (PVI) is a measure of the volume that theoretically will increase the ICP tenfold when injected into the CSF space, and it can be used to evaluate the cranio-spinal volume buffering capacity 6.