Why does increased ICP cause INCREASED BP and DECREASED HR? Watch
Cerebral perfusion pressure (CPP) = MAP - ICP.
Normally, the ICP is around 7-15 mmHg. Since the brain is encased within the rigid cranial vault, there is not much space to accommodate swelling or bleeding which occurs after an injury or inflammation, therefore, ICP will increase in response to haemorrhage or oedema. As ICP increases, the CPP and hence blood flow to the brain declines - eventually causing ischaemia, which deprives the brain of oxygen causing hypoxia. The medulla (part of the brainstem) detects the hypoxia and activates the medullary ischaemic response. This increases sympathetic outflow to the heart and periphery causing an increase in cardiac output (by increasing heart rate and stroke volume) as well as peripheral vasoconstriction. This causes the an increase in blood pressure. The peripheral baroreceptors detect the increase in blood pressure and this activates the baroreflex in an attempt to reduce the BP, which causes the HR to decrease (bradycardia).
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