A CPP of around 40-60 mmHg (40-50mmHg in 0-5 year-olds and 50-60mmHg in 6-17 year-olds) is considered ideal. Achieving an adequate CPP can be done by increasing MAP or reducing ICP (using the above equation). Hence it is necessary to have a good understanding of what good target values for MAP and ICP are.
A good target value for MAP is the upper end of ‘normal’ for the child’s age. Reaching this can be done by using fluids (if fluid deficient) or by use of inotropes. The recommended ICP target is < 20mmHg (normal is between 5-15 mmHg and raised ICP is regarded as values over 20mmHg).
When thinking about ICP, it’s useful to remember a mass in the brain; a mass being possible haemorrhage or any other space-occupying lesion. In TBI, oedema is most prominent at around 24-72 hours post-injury. As a result of increased mass, the initial consequence is a displacement of cerebrospinal fluid (CSF) into the spinal cord. Following this, venous blood in the cranium will also be displaced.
If ICP is further elevated, herniation can result – which is serious and often fatal! Signs of uncal herniation can present as unilateral fixed and dilated pupil. Signs of raised ICP can include pupillary dilatation and series of responses known as the ‘Cushing’s Triad’: irregular, decreased respiration (due to impaired brainstem function), bradycardia, and systolic hypertension (widened pulse pressure). Cushing’s triad results from the response of the body to overcome increased ICP by increasing arterial pressure.
Using the Monroe-Kellie Doctrine as a guide, we can predict how to reduce ICP. One management is head positioning. Head-of-bed should be elevated to 30˚, with the head in mid-line position, to encourage cerebral venous drainage. The EVD can also be used to drain CSF.
Commonly, intravenous mannitol and hypertonic saline are used to manage intracranial hypertension in TBI. Mannitol is traditionally used at a dosage of 20% at 0.25-1.0 g/kg – this is repeatedly administered. The plasma osmolality of the patient needs to be kept a close eye on; it should be ≤ 310 mOsm/L. 3% NaCl can be used to raise sodium levels to 140-150 mEg/L – this is slightly higher than normal sodium levels as a higher blood osmolarity will pull water out of neurons and brain cells osmotically and reduce cerebral oedema (Kochanek et al., 2019). Mannitol works in the same manner, however, use with caution as mannitol, being an osmotic diuretic, can cause blood pressure drops and compromise CPP! In last-resort emergency cases, where ICP need to be immediately reduced, a decompressive craniotomy can be performed.