What is Cerebral Perfusion Pressure?

Cerebral perfusion pressure, or CPP, is the net pressure gradient which carries oxygen to brain tissue. It is measured by the difference between the mean arterial pressure, or MAP, and the Intracranial Pressure, or ICP, which is measured in millimeters of mercury (mm Hg). Regulating CPP is fundamental in the treatment of patients with intracranial pathology, including shock, hemodynamic distress, and traumatic brain injury. 

Although the average CPP is generally between 60 and 80 mm Hg, these values may change to the left or to the right depending on individual physiology. MAP and ICP has to be measured together because CPP is a calculated measure. Regulating CPP at hemodynamically unstable conditions with abnormal ICP or in cases of intracranial pathology will reduce the chance of ischemic brain injury. 

• CPP = MAP - ICP

Cerebral Perfusion Pressure Physiology

CPP and ICP

At its own average range of 60 to 80 mm Hg, the CPP is determined by the ICP and the mean arterial pressure. Under regular standards, the ICP is between 5 and 10 mm Hg which has a reduced effect on the CPP than the MAP in clinical circumstances not associated with intracranial pathology. ICP is generally measured through intracranial pressure transduction.

Physiologically, the ICP is a function of intracranial compliance. Intracranial compliance is the relationship between the ICP and the volume of the intracranial cavity including cerebrospinal fluid, or CSF, brain tissue as well as arterial and venous blood volume. Because the skull is a fixed and rigid anatomic space, the ICP can increase if the intracranial volume increases while intracranial compliance decreases. As the ICP increases or intracranial compliance decreases, CPP also decreases. Several processes determine that ICP continues to stay within the average range for the longest extended period of time possible, especially throughout periods of affected intracranial volume and compliance. As volume adds to the intracranial space, CSF can shift into the spinal subarachnoid space, causing the ICP to continue significantly unchanged. As volume increases due to a growing space-occupying lesion, brain tissue edema or blood, this process ultimately becomes overwhelming, and ICP begins to increase substantially. 

Cerebral blood flow, or CBF, is also a fundamental factor in ICP homeostasis. Cerebral auto-regulation makes sure that steady blood flow is maintained in the brain over a wide range of physiologic alterations. When blood pressure decreases, auto-regulation causes cerebral vasodilation and an increase in CBF and cerebral blood volume, maintaining ICP and CPP. However, when blood pressure increases, auto-regulation causes cerebral vasoconstriction and a decrease in CBF with a decrease in cerebral blood volume, also regulating ICP and CPP. Too many changes outside of average CBF ranges can cause brain ischemia and injury. 

CPP and MAP

Because ICP in its average ranges is a considerably small number, the CPP generally depends on the mean arterial pressure. MAP is the normal blood pressure during one cardiac cycle which can be measured through invasive hemodynamic monitoring or calculated by the systolic blood pressure, plus two times the diastolic blood pressure, divided by three. The average range of MAP is 70 to 100 mm Hg. 

The average arterial pressure can be affected due to everyday activities, such as rest, stress, and exercise or physical activities. However, if the ICP continues to stay the same, the average arterial pressure can change across its significantly wide range without tremendously decreasing or increasing the CPP. As a matter of fact, CPP and CBF will continue to stay considerably unchanged across a wider range of MAP (50 – 150 mm Hg) than normal due to cerebral auto-regulation and vasoconstriction or vasodilation of cerebral vasculature. 

For patients with hypertension, the auto-regulation setpoint changes, decreasing the average arterial pressure associated with the patient’s normal arterial pressure, which causes vasodilation to increase CBF. Patients with lower than normal average arterial pressure at baseline will have auto-regulatory vasoconstriction as a reaction to an increase in their significant average MAP, to return CBF to baseline. When looking at CBF and CPP in the context of the patient’s average MAP, it is clinically significant based on the regulation of intracranial pathology and hemodynamic derangements. 

Cerebral Perfusion Pressure Complications

Diagnosing and treating cerebral perfusion pressure complications necessitates measuring both the ICP and the MAP. The MAP may be quantified through the utilization of invasive hemodynamic processes, most frequently cannulation of a peripheral artery such as the radial or femoral artery. The MAP may also be measured with a non-invasive blood pressure cuff by applying the formula mentioned above utilizing the systolic and diastolic blood pressures. 

Intracranial pressure is generally measured through an intracranial pressure transduction device. The most common and most accurate method or technique is utilizing an intraventricular monitor. The intraventricular dimension of ICP is the normal standard. An intraventricular catheter is inserted into a hole drilled in the skull and into the lateral ventricle to gauge the pressure of the CSF. The benefit of an intraventricular catheter is that CSF could be eliminated, if needed, to decrease ICP. Considerable complications for the ICP include a possibility of bleeding, infection, and difficulty with proper placement. Options include sub-dural and intra-parenchymal monitors. 

The ICP can be measured non-invasively through several methods and techniques, including transcranial Doppler ultrasonography or TCD. TCD utilizes a temporal window to evaluate the speed of blood flow through the middle cerebral artery. Systolic and diastolic average flow velocity is utilized to determine a pulsatility index. The pulsatility index was determined to be closely associated with ICP in several research studies as well as be associated with ICP in other research studies. Therefore, it is not suggested to use TCD as a substitute for direct ICP dimension. Invasive diagnosis and treatment of the MAP through an arterial cannula and the ICP through an intraventricular catheter will give a continuous and accurate calculation of CPP. 

Cerebral Perfusion Pressure Clinical Significance

Two general types of pathologic health issues can ultimately occur where the regulation of the CPP is fundamental, such as intracranial pathology, where ICP regulation is essential and hemodynamic instability/shock where MAP regulation is the most essential. Intracranial pathology involves space-occupying lesions, such as tumors, epidural and subdural hematoma or severe intraparenchymal hemorrhage and cerebral edema as seen after ischemic injury, traumatic brain injury or acute hepatic encephalopathy. In these circumstances, average CPP depends on decreasing the ICP into a normal range as soon as possible while regulating the MAP. When CPP is normal, it's fundamental to keep in mind that every individual's brain tissue has a CPP that is "normal" in the context of that individual patient's physiology, which may be affected by other health issues, such as hypertension or cardiovascular disease. Moving towards a more dynamic direction of the average CPP utilizing the patient's personal auto-regulatory capacity. These diagnosis and treatment approaches involve more frequent and sophisticated monitoring and might not be readily available for widespread utilization. 

In the instance of considerable traumatic brain injury, significant cerebral edema can decrease intracranial compliance and CSF, developing an increased ICP or intracranial hypertension. Auto-regulatory mechanisms and techniques may or may not function normally and when ICP continues to be elevated, CPP will decrease causing further injury through an ischemic process. In circumstances such as these, together with starting the measures for decreasing the ICP, it is essential to prevent hypotension (MAP - ICP = CPP) and in some instances, allowing hypertension to reasonably occur. 

In circumstances of instability, the ICP is considerably stable as cerebral auto-regulation is undamaged. In the instance of hypotension, the MAP decreases due to blood loss, or hemorrhagic shock, intravascular leak, or distributive shock, and decreased cardiac output, or cardiogenic shock, and the CPP also decreases. It's the association between MAP and CPP which carries resuscitation guidelines to recommend regulating a MAP greater than or equal to 65 mm Hg. With a normal ICP, this threshold must make sure that a CPP of 55 to 60, the minimum necessary to stop cerebral ischemic injury, is ultimately maintained. As in the circumstance of ICP and cerebral auto-regulation, the goal of MAP is to be within the context of an individual patient's evaluation hemodynamic function. Patients with untreated hypertension must have increased MAP goals to maintain proper CBF and CPP.  

As previously mentioned in the following article, cerebral perfusion pressure, or CPP, is the net pressure gradient which affects cerebral blood flow to the brain, also known as brain perfusion. According to healthcare professionals, the CPP, or cerebral perfusion pressure, must be constantly regulated within a specific limit because too little pressure or too much pressure could potentially cause a variety of brain health issues. Cerebral perfusion pressure may be associated with a variety of neurological diseases. - Dr. Alex Jimenez D.C., C.C.S.T. Insight

The purpose of the article is to discuss cerebral perfusion pressure and its association with neurodegenerative diseases. Neurological diseases are associated with the brain, the spine, and the nerves. The scope of our information is limited to chiropractic, musculoskeletal and nervous health issues as well as functional medicine articles, topics, and discussions.

Curated by Dr. Alex Jimenez