Mean Arterial Pressure (MAP) and Intercranial Pressure (ICP) cycle:
The brain requires adequate blood supply in order to maintain its’ homeostatic balance. For this, Cerebral Perfusion Pressure (CPP) needs to be maintained to ensure normal brain physiology and prevent oedema. CPP is managed by the interactions of Mean Arterial Pressure (MAP) and Intercranial Pressure (ICP). MAP is the mean arterial blood pressure that the body creates during the cardiac pumping cycle. ICP is the pressure that is present within the cranium. For the brain to be adequately perfused and to maintain CPP, MAP needs to be greater than ICP. This means, MAP is able to push blood into the brain against the resistance of ICP.
Within a healthy adult, CPP averages around 60 to 80 mmHg, with ICP around 10 to 15 mmHg. MAP can be calculated by taking diastolic pressure and adding a third of the sum of systolic minus diastolic pressures:
MAP = Diastolic + 1/3(Systolic – Diastolic).
Altogether the sum for CPP is MAP minus ICP:
CPP = MAP – ICP
During trauma, there are 2 scenarios that can arise where this CPP cycle gets disrupted. The first is through hypovolaemic disruption. If the body suffers a loss of blood volume and the MAP isn’t enough to overcome ICP or provide a consistent supply, then the brain will be affected as it is unable to have adequate perfusion. Without oxygenation of cells, brain tissues become hypoxic, and cells begin to die on mass. If perfusion is not quickly restored, then irreversible brain injury can occur, followed eventually by brain death.
The second scenario is through pathophysiology within the cranium itself causing an increase in ICP. There are a number of possibilities which can cause ICP to increase, these can include; trauma causing intercranial haemorrhage, haematoma, swelling, and medical cases such as tumour, abscess, cerebral oedema, etc. Using the example of trauma, as this space-occupying bleeding and swelling increases, due to the rigidity of the skull there is nowhere for pressure to be relieved, meaning ICP rises. This can start a deadly cycle as the body recognises the increase in ICP.
If ICP is too great, then blood supply can’t enter the brain as it pushes against MAP and constricts the supplying arteries. In a sympathetic response, the body increases blood pressure to overcome the ICP. This is successful in countering the ICP and blood perfuses the brain, however, whatever haemorrhage, haematoma, or swelling is present becomes exacerbated as blood is being forced in at a higher pressure. This causes a further increase to ICP, causing the body to react by increasing blood pressure again, and so the deadly cycle continues.
As ICP becomes too great, brain tissue becomes compressed and damaged. Parts of the brain begin to herniate and deform under the pressure. As this continues, the event called ‘coning’ can happen. This is where the brain is forced through the hole at the bottom of the skull, the Foramen Magnum.
At any stage where brain tissue is being compressed, herniation is happening, or coning begins, the patient will likely be in a very poorly state with systemic physical deficits evident due to the injury being administered to the brain.
When intercranial pathology occurs causing an increase in ICP, Cushing’s Reflex can be used to help identify this. There are a triad of symptoms which make up Cushing’s reflex, and they are:
Hypertension, Bradycardia, Altered/Irregular breathing
Hypertension occurs as discussed in the previous section where the body’s sympathetic response increases blood pressure in order to maintain CPP, and allow MAP to overcome ICP.
There are 2 suspected reasons why bradycardia occurs. One is where increased blood pressures lead to activation of baroreceptors in the aortic arch, triggering parasympathetic activation and bradycardia to occur. The other is where bradycardia occurs from compression of the intracranial vagal nerve due to the increase in ICP.
Altered/Irregular breathing occurs through brainstem dysfunction and disruption to the respiratory centre. This can happen through the pressure being exerted on the brainstem area as ICP increases.
Bradycardia and altered breathing are considered later signs of severe ICP. The patient will likely display many symptoms of red flag major head injury as Cushing’s Reflex pathology develops.