- Intracranial pressure (ICP) = pressure exerted by the contents of the skull on the dura mater
- Normal ICP varies with age, body position, and clinical condition. Normal ICP is 5–15 mm Hg in a supine adult, 3–7 mm Hg in children, and 1.5–6 mm Hg in term infants
- Skull content includes brain tissue (compressible, ˜83% by volume), blood (incompressible, ˜6% by volume), and CSF (incompressible, ˜11% by volume). In the event of increased ICP, compensation can occur by shifting either CSF or blood out of the intracranial compartment
- Intracranial hypertension over a critical threshold of 20 mm Hg is an independent predictor of poor neurological outcome after severe head injury
- Pathological CT scan of the head with a Glasgow Coma Scale Score (GCS) of <9. Example of a pathological CT scan is one that shows hematomas, contusions, edema, herniation, or compressed basal cisterns (Figure 98-1)
- Normal CT scan and GCS <9 accompanied by two of the following: age >40, unilateral or bilateral motor posturing or systolic blood pressure < 90 mm Hg
Figure 98-1. Devices for ICP Monitoring
Various anatomic sites to monitor intracranial pressure: Extradural/epidural, subdural, intraparenchymal, intraventricular, lumbar subarachnoid. Reproduced from Hall JB, Schmidt GA, Wood LDH: Principles of Critical Care, 3rd Edition. Figure 65-8. Available at: www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
NB: A large body of clinical evidence supports the use of ICP monitoring to guide therapeutic interventions, detect intracranial mass lesions early, and assess prognosis in the setting of TBI.
- Intraventricular catheter/drainage (IVD)
- Gold standard for ICP monitoring. Thought to be the most accurate method
- Requires placement of a catheter through a burr hole into the lateral ventricle; non-dominant hemisphere placement preferred (bleeding complications from insertion less likely to cause language disturbance)
Pros and Cons of Intraventricular Catheter
- Direct pressure measurement
- Measures global pressure
- Accurate and reliable
- Allows therapeutic CSF drainage
- In-vivo calibration possible
- Allows adminstration of drugs (i.e., antibiotics)
- Relatively inexpensive
- Rate of infection ˜ 5%
- Risk of bleeding ˜ 2%
- Placement can be difficult if the ventricles are shifted out of place or compressed due to increased ICP
- Potential for leaks/blockages in the system → underestimates ICP
- Invasive method
- Transducer adjustment needed with head movement
NB: Transduction of ICP while the CSF chamber is open to drainage is meaningless. The ICP should be transduced with the ventricular drain closed to drainage at least every 30–60 minutes for a 5- to 10-minute period with a recording of the ICP and CPP at the end of that time.
- Subdural, subarachnoid screw/bolt
- Indicated if insertion of intraventricular drainage is difficult or impossible
- Hollow screw inserted via a burr hole into the subarachnoid/subdural space
- Extradural transducer/Epidural catheter:
- Indicated if insertion of intra-ventricular drainage is difficult or impossible
- Pressure sensor placed directly in contact with the dural surface
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