The grades of evidence (I-III) and levels of recommendations (I-III) are defined at the end of the "Major Recommendations" field.
Level I
There are insufficient data to support a Level I recommendation for this topic.
Level II
Aggressive attempts to maintain cerebral perfusion pressure (CPP) above 70 mm Hg with fluids and pressors should be avoided because of the risk of adult respiratory distress syndrome (ARDS).
Level III
CPP of <50 mm Hg should be avoided.
The CPP value to target lies within the range of 50–70 mm Hg. Patients with intact pressure autoregulation tolerate higher CPP values.
Ancillary monitoring of cerebral parameters that include blood flow, oxygenation, or metabolism facilitates CPP management.
Summary
It is important to differentiate physiologic thresholds representing potential injury from clinical thresholds to treat. Much of the definition of the former can come from simple physiologic monitoring; the latter requires clinical evidence from controlled trials using outcome as their dependant variable. With respect to CPP, it appears that the critical threshold for ischemia generally lies in the realm of 50 to 60 mm Hg and can be further delineated in individual patients by ancillary monitoring.
At this time, it is not possible to posit an optimal level of CPP to target to improve outcome in terms of avoiding clinical episodes of ischemia and minimizing the cerebral vascular contributions to intracranial pressure (ICP) instability. It is becoming increasingly apparent that elevating the CPP via pressors and volume expansion is associated with serious systemic toxicity, may be incongruent with frequently encountered intracranial conditions, and is not clearly associated with any benefit in terms of general outcome. Based on a purely pragmatic analysis of the randomized, controlled hypothermia trial, one set of researchers noted that a CPP target threshold should be set approximately 10 mm Hg above what is determined to be a critical threshold in order to avoid dips below the critical level. In combination with the studies presented in the original guideline document, this would suggest a general threshold in the realm of 60 mm Hg, with further fine-tuning in individual patient based on monitoring of cerebral oxygenation and metabolism and assessment of the status of pressure autoregulation. Such fine-tuning would be indicated in patients not readily responding to basic treatment or with systemic contraindications to increased CPP manipulation. Routinely using pressors and volume expansion to maintain CPP at >70 mm Hg is not supported based on systemic complications.
Definitions:
Grades of Evidence
Class I - Good quality randomized controlled trial (RCT)
Class II - Moderate quality RCT, good quality cohort, or good quality case-control
Class III - Poor quality RCT; moderate or poor quality cohort; moderate or poor case-control; or case series, databases, or registries
Levels of Recommendation
Levels of recommendation are Level I, II, and III, derived from Class I, II, and III evidence, respectively.
Level I - Recommendations are based on the strongest evidence for effectiveness, and represent principles of patient management that reflect a high degree of clinical certainty.
Level II - Recommendations reflect a moderate degree of clinical certainty.
Level III - Recommendations for which the degree of clinical certainty is not established.