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At What Level Should I Start Treating Elevated Intracranial Pressure?

Juan Sahuquillo

BRIEF ANSWER

Background

ICP monitors, per se, do not save lives. The people who judiciously use the data obtained from ICP monitoring can save lives and alter outcomes in patients with severe head injuries.

—T.G. Saul and T.B. Ducker, 1982¹

The goals of the clinical management of severe head injury consist of controlling ICP and maintaining sufficient cerebral perfusion pressure (CPP) and cerebral blood flow (CBF) to avoid brain ischemia. However, the best way to achieve these goals is still debated because most of the management of ICP and CPP takes place in those gray areas of practice that still lack class I evidence that would provide answers to many questions.

Ever since the beginning of neurointensive care, the main targets of the management of severe head injury have generally been the prevention of high ICP and the prompt and aggressive treatment of its elevations; that is, ICP-based management. ICP has traditionally been defined as elevated if it crosses a threshold that is variously described as 15 to 25 mmHg.^2,3

Because of the emphasis on CPP as the main force driving blood flow to the brain, many clinicians focus on that parameter as the main target of the management of severe head injuries; that is, CPP-based management.^4–6 For many practitioners of this approach, achieving an adequate CPP is much more important than maintaining tight control of ICP. CPP has been empirically considered to be sufficient when the difference between mean arterial blood pressure (MABP) and ICP exceeds 70 mmHg. It is assumed that keeping CPP above this threshold indicates that brain perfusion, and consequently CBF, is adequate. This approach is based on the unproven notion that a higher CPP can maintain an adequate CBF even if ICP is high. Consequently, the MABP-ICP gradient is considered more important than the absolute values of both ICP and MABP.

Following this line of thought, the second edition of the Guidelines for the Management of Severe Traumatic Brain Injury recommends (as an option), that ” cerebral perfusion pressure should be maintained at a minimum of 70 mmHg.“7 However, this recommendation is based on class III data and, thus, a CPP threshold of 70 mmHg cannot be considered a better therapeutic option than any other level. Despite the lack of strong evidence, word has been spread in the international neurocritical care community that maintaining CPP below 70 mmHg cannot be considered good clinical practice. Ninety-seven percent of a group of board-certified North American neurosurgeons who participated in a recent survey on the management of severe head injuries felt that CPP should be maintained above 70 mmHg (class III data).⁸ Another survey of neuroanesthesiologists in Germany showed that 69% of patients with a severe head injury were managed with a CPP between 70 and 90 mmHg (class III data).⁹

To add more fuel to the fire, much has been written in the last decade on a completely different and almost opposite approach for managing CPP that is advocated by investigators working at the University of Lund.¹⁰ The Lund concept is based on the strict prevention of increases in—or even the reduction of— MABP to maintain CPP at 50 to 60 mmHg. The aim is to reduce capillary hydrostatic pressure and thus avoid edema in a brain with impaired autoregulatory mechanisms and an altered blood—brain barrier.

As is clear from the above discussion, there exist no universally accepted guidelines for manipulating CPP. Consequently, the concept of so-called CPP-based management has different meanings for different authors. Several other important questions in the ICP-CPP debate also remain unanswered. This chapter investigates one such question by reviewing available data about the threshold at which an increase in ICP should trigger therapeutic interventions.

Literature Review

Is Increased Intracranial Pressure a Good Predictor of Outcome?

Several authors have reported a strong correlation between sustained high ICP and high morbidity and mortality. The relationship between high ICP and poor outcome has been very consistent in both single-center and multicenter studies, and the ability to bring elevated ICP under control has long been considered a requirement for improving outcome of severely head-injured patients. Since the earliest work of Miller et al in the late 1970s, the threshold most frequently used in outcome studies has been 20 mmHg.^2,11–13 In an analysis of data from the Traumatic Coma Data Bank (TCDB), Marmarou et al¹⁴ found that poor outcome after severe head injury was strongly related to high ICP (>20 mmHg) (class II data). In that study, age, admission motor score, and the presence of pupillary abnormalities correctly explained 46% of the observed outcomes when modeled alone. Beyond these three factors, the proportion of ICP measurements greater than 20 mmHg was the factor that was most indicative of outcome. This threshold was identified by testing critical ICP levels from 0 to 80 mmHg in increments of 5 mmHg.

Is There an Optimal Threshold for Treating Intracranial Pressure?

If we accept that the ” gold standard“ method for answering questions in medicine is the randomized, controlled trial, one of the most important problems in practicing evidence-based care in the management of severe head injuries is the lack of unambiguous evidence justifying many routinely used treatments or even basic neuromonitoring methods.¹⁵ One of the most obvious examples is choosing the most appropriate threshold at which to start treating elevated ICP.

However, debate on thresholds seems paradoxical because of the lack of class I evidence that ICP monitoring improves the outcome of patients with severe head injuries. No trials have randomized patients with severe head injury to ICP monitoring versus no ICP monitoring. The negative side of using a dogmatic approach to evidence-based medicine is that strict adherence to evidence-based care management (i.e., only accepting unambiguous evidence) may result in the modern treatment of severe head injuries being subjected to a moratorium that will completely paralyze currently accepted protocols used in clinical practice, as is happening in other areas of critical care. Such a situation might provoke such undesirable effects as flipping the coin to the other side; that is, all therapeutic strategies are valid because nothing has been adequately proved. If scientific fundamentalism can be avoided, however, a positive aspect that may emerge from the current emphasis on evidence-based care is that every single treatment is placed under careful scrutiny. Such a process favors a more rigorous and scientific approach to the management of traumatic brain injuries. Refraining from ” expert opinions“ and unproven hypotheses promotes scientific progress and improves clinical practice.

Although for many the last word on the subject of ICP monitoring has not been said, there is a general consensus that the low risk:benefit ratio of this monitoring technique is obvious and that a randomized trial on this topic could be considered unethical, not to mention very difficult to implement methodologically.3 One of the several problems the clinician would confront in conducting such a trial is similar to that faced by participants in a consensus conference on the use of pulmonary artery catheters (PAC) in critical care. To summarize the problem, these authors asked, ” Can a clinician ethically enroll patients in such a trial, knowing that the patient may not receive a PAC, despite subsequent hemodynamic (or cardiopulmonary) deterioration?” ¹⁶

The morbidity of ICP monitoring, which is quite low, is overshadowed by the fact that ICP monitoring provides essential and continuous cerebral hemodynamic information that cannot be supplied by comprehensive neurologic examination or even by serial computed tomography (CT) scans. Furthermore, ICP is used to calculate CPP and therefore to protect the brain from ischemia, evidence of which is observed very commonly in postmortem examination of patients who die after head injury.¹⁷ For all these reasons, ICP monitoring was recommended by the Brain Trauma Foundation’s (BTF’s) Guidelines as a guideline (i.e., between a standard and an option) for the management of all patients with a severe head injury and an abnormal CT scan.¹⁸ However, in practice, ICP monitoring is considered a ” standard of care“ in the majority of centers managing patients with severe head injuries.