Cerebral Perfusion Pressure

Issues of Concern

Monitoring cerebral perfusion pressure requires measuring both the MAP and the ICP. The MAP can be measured directly using invasive hemodynamic techniques, most often by cannulating a peripheral artery, such as the radial or femoral artery. The MAP can also be measured indirectly using a noninvasive blood pressure cuff and the previously mentioned formula, based on the systolic and diastolic blood pressures.

Intracranial pressure is usually measured invasively using an intracranial pressure transducer. The most common and most accurate method is with an intraventricular monitor. As such, intraventricular ICP measurement is the current gold standard.[2] An intraventricular catheter is inserted through a hole drilled in the skull and into the lateral ventricle to measure CSF pressure directly. The advantage of an intraventricular catheter is that CSF can be removed, if needed, to decrease ICP in the acute setting. Disadvantages include the risk of bleeding, infection, and difficulty with proper placement if the ICP is very high. Other options include intra-parenchymal and subdural monitors.

ICP can be measured noninvasively by several methods, most commonly transcranial Doppler ultrasonography (TCD). TCD uses a temporal window to measure blood flow velocity in the middle cerebral artery. Systolic and diastolic flow velocity and mean flow velocity are used to calculate a pulsatility index. The pulsatility index has been reported to correlate strongly with ICP in some studies and poorly correlate with ICP in others. Therefore, using TCD alone as a substitute for direct ICP measurement is not recommended.[3][4] Ideally, invasive monitoring of the MAP through an arterial cannula and of the ICP through an intraventricular catheter provides continuous, accurate CPP measurements.

Clinical Significance

Two general categories of pathologic derangement exist wherein management of CPP is vital: intracranial pathology, where ICP management is most important, and hemodynamic instability/shock, where MAP management is most important. Intracranial pathology includes space-occupying lesions such as tumors, epidural and subdural hematoma or acute intraparenchymal hemorrhage, and cerebral edema as seen after ischemic injury, traumatic brain injury, or severe hepatic encephalopathy. In these cases, adequate CPP is dependent on lowering the ICP back to a reasonably normal range as quickly as possible while maintaining an adequate MAP. While CPP is within the normal range, it is important to remember that each patient’s brain tissue has a CPP that is “normal” in the context of that individual’s physiology, which may be influenced by other medical problems such as hypertension or vascular disease. There is a push towards more dynamic management of CPP using the patient’s auto-regulatory capacity.[5] These methods involve more frequent and sophisticated monitoring and may not be widely available.

For example, in the setting of substantial traumatic brain injury, significant cerebral edema can decrease intracranial compliance and overcome initial CSF shunting measures, producing an elevated ICP (intracranial hypertension). Auto-regulatory mechanisms may or may not function normally, and if ICP remains elevated, CPP decreases, causing further injury via ischemic mechanisms. In this case, in addition to initiating ICP-lowering measures, it is important to avoid hypotension (MAP – ICP = CPP), and, in some cases, allowing hypertension is reasonable.

In cases of hemodynamic instability, the ICP is relatively stable as cerebral autoregulation is intact. In the setting of hypotension, the MAP decreases due to blood loss (hemorrhagic shock), intravascular leak (distributive shock), or decreased cardiac output (cardiogenic shock), and the CPP decreases as well. The relationship between MAP and CPP drives resuscitation guidelines to recommend maintaining a MAP of at least 65 mm Hg. Assuming a normal ICP, this threshold should guarantee a CPP of 55 to 60, the minimum needed to prevent cerebral ischemic injury. As with ICP and cerebral autoregulation, MAP goals should be set within the context of an individual patient’s baseline hemodynamic function. Patients with untreated hypertension should have higher MAP goals to maintain appropriate CBF and CPP.

Nursing, Allied Health, and Interprofessional Team Interventions

Clinicians and other medical/ancillary staff who care for patients with conditions in which CPP is an important vital sign bear responsibility to understand the implications of this value and to articulate these issues to members of the interprofessional healthcare team. This encourages accurate diagnosis, prompt intervention, and improved patient monitoring, resulting in the best possible outcomes.