Intracranial pressure is an important domain that is asked in the exam. Whether it is your spotter in the exam or whether it is a short answer type of question. Questions on intracranial pressure do make to the exam that is very imperative to know at least few important facts for measurement and rule of intracranial pressure in management of neurotrauma and neurosurgical patient. We will discuss here about intercanal pressure.

What is the normal intercanal pressure? What are the important determinants of intercanal pressure? We will discuss about briefly the current level of evidence supporting or refuting the use of intercanal pressure monitoring and of course a little bit detail about the modalities of measuring intercranial pressure. Now intercranial pressure is defined as the pressure within the intercranial space relative

to the atmospheric pressure. It is difficult to establish a universal normal value for intercal pressure as it depends on age, body posture and also the clinical condition of the patient. Clinically, usual upper limit is thought to be around 15 mmHg and the value varies between 5 to 10 millimeter of mercury usually. There is a physiological increase in intracranial pressure that comes back quickly to a normal value which is associated with coughing, sneezing, where the intracranial pressure can go up to 30 or 50 millimeter of mercury but it is not sustained and it

drops down to its normal value once such event is over. Now the physiological determinants of intracranial pressure, this is an important question in the exam. This is governed by the Monroe Kelly Doctrine which says that in adults the rigid and the closed cranial vault forms a fixed brain volume which contains the brain parenchyma constituting around 80% of the whole intracranial mass, cerebrospinal fluid constituting around 10% and cerebral blood and blood vessels which again constitute to 10%.

vault whether the volume in terms of brain parenchyma or in terms of CSF or in terms of blood. Any of these component if it increases then there will be a corresponding decrease in the other components within a physiological limit and beyond this when it reach the tipping point any small increase in volume of any of these three components will lead to a significant increase in the pressure.

So this is called as the Monro-Keyle doctrine. As you can see in this figure, the ICP determinants are three major things, brain volume, CSF and blood volume. Now talking about the brain volume, the brain is the brain matter, the brain volume is the brain matter first of all, also it has the blood inside it and also if there is a pathological condition that leads to increased amount of blood or extravasation of blood or there is edema in the brain. So any new mass or increase in blood or blood outside the blood vessel and any sort of edema can increase the brain volume from its normal range.

Similarly the second component cerebrospinal fluid fluid again there is a tight control between the absorption and production of cerebrospinal fluid if any pathology causes a disruption of this process whether there is increased production or there is decreased absorption, both of this can lead to increase in the CSF volume and if it is beyond the physiological limit, then it can cause an abrupt increase in intracranial pressure.

We know that if the cerebral perfusion pressure is more than 70 mm Hg, there is no effect on the production of the CSF. But if the cerebral perfusion pressure is persistently less than 70 mm Hg, the production of CSF is decreased. Similarly, absorption is normal if the ICP stays normal or low, but the absorption increases as there is an increase in ICP value.

These physiological mechanism compensates for any increase or decrease in CSF production or any increase or decrease of CSF absorption so that there is a volume of CSF that is permissible under physiological limit is maintained in the brain. Going to the third component the blood volume inside the blood vessel of the brain again that is determined by the cerebral blood flow how much blood is coming into the brain

and the cerebral blood flow is again determinant on three major factors. First of all is the cerebral perfusion pressure, second is the cerebral vascular resistance and third is the temperature. We know the cerebral perfusion pressure is dependent on the MAP or mean arterial pressure and the CVP or the ICP whichever is more. Cerebral perfusion pressure is MAP minus the intragonal pressure. So if the MAP is high, then the cerebral perfusion pressure will be high. If the ICP is high, then the cerebral perfusion pressure can have a corresponding decrease.

Again in a physiological condition, simple coughing, sneezing, vomiting or just putting your head low down can cause an increase in CVP and that can determine a low cerebral perfusion pressure because the difference between MAP to CVP or MAP to ICP whichever is greater is the cerebral perfusion pressure. With all these manoeuvres such as coughing, sneezing, vomiting or keeping the head down can increase the CVP beyond the ICP and in that case the cerebral perfusion pressure will be the difference between the mean arterial pressure and the CVP. Okay, so thereby if the CVP is increased

there is decreased blood flow to the brain or decrease in cerebral perfusion pressure. Similarly, the cerebral vascular resistance is dependent on predominantly partial pressure of oxygen and also partial pressure of carbon dioxide. The major determinant is the partial pressure of carbon dioxide where if you have a high PaCO2 then there will be more vasodilatation of the cerebral blood vessels causing there will be more vasodilatation of the cerebral blood vessels causing

increased amount of cerebral blood flow and vice versa.