Cerebral Perfusion Pressure - Ensuring Blood Flow In The Brain
Cerebral Perfusion Pressure (CPP) is defined as the difference between the Mean Arterial Pressure (MAP) and the Intracranial Pressure (ICP).
The equation is: CPP = MAP - ICP.
This represents the pressure gradient driving cerebral blood flow (CBF) and hence oxygen and metabolite delivery.
Cerebral Perfusion Pressure (CPP) made easy
COPYRIGHT_SPINE: Published on https://spinal-injury.net/cerebral-perfusion-pressure/ by Dr. Bill Butcher on 2022-10-24T11:17:12.286Z
The normal brain autoregulates its blood flow to provide a constant flow regardless of blood pressure by altering the resistance of cerebral blood vessels.
These homeostatic mechanisms are often lost after head trauma (cerebral vascular resistance is usually increased), and the brain becomes susceptible to changes in blood pressure.
Those areas of the brain that are ischaemic, or at risk of ischemia (ischaemia) are critically dependent on and adequate cerebral blood flow, and therefore cerebral perfusion pressure.
Maintenance of an adequate Cerebral Perfusion Pressure is a cornerstone of modern brain injury therapy.
After brain injury, and especially in the multiply injured patient, cerebral blood flow may be lowered to the ischaemic threshold.
To prevent further neuronal death (the secondary brain injury), this flow of well oxygenated blood must be restored.
There is no class I evidence for the optimum level of CPP, but 70-80mmHg is probably the critical threshold.
In those studies where CPP is maintained above 70mmHg, the reduction in mortality is as much as 35% for those with severe head injury.
Cerebral Perfusion Pressure may be maintained by raising the Mean Arterial Pressure or by lowering the Intracranial Pressure.
In practice, ICP is usually controlled to within normal limits (<20mmHg) and MAP is raised therapeutically. It is unknown whether ICP control is necessary providing CPP is maintained above the critical threshold.
Control of intracranial hypertension is discussed on the pages on intracranial pressure.
There is substantial evidence now that early hypotension (BP < 90mmHg) is associated with increased morbidity and mortality following severe brain injury.
Even patients with one episode of hypotension during their ICU stay have a significantly reduced prognosis.
Maintenance of an adequate MAP requires primarily a normovolaemic patient.
Control of other sites of hemorrhage has the highest priority (with oxygenation).
These patients should NOT be kept “dry” with fluid restriction, but maintained in zero balance.
Further elevation of MAP, once normovolaemia is achieved, is usually accomplished with norepinephrine, though dopamine may be used.
There is little evidence to recommend any one agent over another.
Below are the important things to consider regarding Cerebral Perfusion Pressure:
- Maintenance of CPP reduces mortality in severe head injury.
- CPP should be maintained above 70-80mmHg.
- Systemic hypotension is associated with poor prognosis.