Introduction
The collective burden of critical neurologic illnesses is overwhelming. Stroke occurs in about 800,000 individuals each year in the United States, with case fatality rates from acute ischemic stroke (AIS) being close to 15% to 20%. Although primary intracerebral hemorrhage (ICH) and aneurysmal subarachnoid hemorrhage (SAH) are less common than ischemic stroke, their case fatality rates are nearly two to three times higher than AIS. Traumatic brain injury (TBI), according to the World Health Organization, will surpass many diseases as the major cause of death and disability worldwide by the year 2020. The number of patients with hypoxic-ischemic encephalopathy following resuscitation is between 100,000 and 200,000 per year in the United States. The mortality in this particular cohort of critically ill neurologic patients has been decreasing steadily, in part because of advances in scientific understanding of the clinical disorder but perhaps more so because of the impact of neurologically oriented critical care. In addition, patients with AIS have better outcomes when admitted to a dedicated stroke/neurocritical care unit rather than a general medical intensive care unit (ICU). In addition, studies have shown that admission of ICH patients to specialized neurocritical care units is associated with improved clinical outcome. The institution of such dedicated neurocritical care units has improved resource utilization, introduced more efficient patient care protocols, established neuroprotective measures, reduced the impact of comorbid illness, and allowed for prevention and management of poststroke complications.
Principles of Neurocritical Care
A Brief History of Neurocritical Care
Historians date the beginning of neurocritical care back to the 16th century, at the time resuscitation practices and attempts at artificial ventilation first appeared. Modern neurocritical care probably started with the polio epidemics and evolved thereafter with the introduction of the iron lung in the early to mid-20th century. At that time neurologists were the primary treating physicians for these patients and probably laid the groundwork for the first large-scale use of mechanical ventilation. In 1923, a neurosurgeon, Dr. W.E. Dandy, created a three-bed unit for postoperative neurosurgical patients. World War II further expanded the field of intensive care with the designation of “shock wards.”
The advent of polio vaccines and few important developments in neurosurgical techniques led to a somewhat a nihilistic approach to critically ill neurologic and neurosurgical patients in the 1950s. By 1960, health care witnessed a rapid spread of resuscitation and surgical techniques, including the creation of recovery rooms. Toward the late 1960s almost 95% of all acute care hospitals in the United States had some sort of critical care unit. Advances in neuroanesthesia and the organization of critical care protocols allowed for a more comprehensive care model for the critically ill. In the 1960s, techniques to measure cerebral blood flow (CBF) and intracranial pressure (ICP) were introduced and evolved. During the 1970s, neurologists David Jackson and Alan Ropper, and an anesthesiologist, Sean Kennedy, spearheaded the concept of a “neuro-ICU” in North America. Later pioneers included Dr. Daniel Hanley and Dr. Thomas Bleck. In 2003, the Neurocritical Care Society was formed with Dr. Bleck as its first president. The scope of neurocritical care has further evolved with the advent of more sophisticated monitoring techniques and informatics and computer systems and guidelines for program requirements for neurocritical care training and a core curriculum and competencies for training.
Scope of Practice of the Modern Neurointensivist
In day-to-day clinical practice, neurointensivists focus on subtle changes in the neurologic exam and physiologic or pathophysiologic interactions between the brain and other organ systems. The main argument in favor of staffing neuro-ICUs with full-time neurointensivists stems from the notion that these individuals are specially trained to recognize the specific interactions between the intracranial physiology and systemic derangements of the neurocritically ill patient. The members of a neurocritical care team are more likely to be aware of secondary physiologic insults to the brain that can include fever, hyperglycemia, anemia, hyponatremia, and delirium. A neurocritical care team typically will care for patients with AIS, ICH, SAH, intracranial neoplasms, TBI and spinal cord injury, status epilepticus, neuromuscular respiratory failure, postoperative neurosurgical care, hypoxic-ischemic brain injury after cardiac arrest, postprocedure neurovascular care, and acutely ill medical patients with neurologic injury requiring critical care, for example, hepatic encephalopathy. Neurointensivists have intimate knowledge of acute circulatory, respiratory, and metabolic disturbances and general skills required for advanced cardiac life support, cardioversion, and intubation and ventilator management, and for the insertion of invasive hemodynamic monitoring. In addition, given their knowledge with nervous system function, neurointensivists have significant experience in end-of-life questions, prognosis in severe brain injury, and defining brain death. Consequently neurointensivists lead the way at the interface for organ procurement and donation.
Impact of Specialized Neurocritical Care Team
Several observational studies suggest that the evolution of neurocritical care units (NCCUs) has led to improved outcomes and optimal utilization of resources for patients with severe acute brain injury ( Table 2.1 ). Mortality after TBI has improved because of prehospital advanced life support and specialized neurointensivist-led teams. In patients with AIS and who are critically ill, admission to neurointensivist-led NCCU is associated with improved condition at discharge and shortened hospital length of stay. ICH traditionally has carried the highest mortality rate and been the most disabling form of stroke. Recent data suggest that those patients admitted to the NCCU have improved outcomes, shorter lengths of stay, and lower total cost of care compared with a national benchmark. In addition, studies show that mortality among patients who undergo surgery for ruptured intracranial aneurysms is significantly reduced in high-volume centers that provide access to specialized multidisciplinary care. In addition, aggressive treatment through specialized care is associated with improved outcome and proven to be cost effective in poor-grade subarachnoid hemorrhage patients. Hypoxic-ischemic brain injury after cardiac arrest is one of the most devastating conditions; historically only 15% of patients successfully resuscitated in the field have survived to hospital discharge. Studies have shown that induced therapeutic hypothermia after cardiac arrest is associated with improved survival and better functional outcomes. In many institutions neurointensivists have become the ideal practitioners to care for these critically ill patients. Studies also have shown that availability of neurointensivists is associated with improved documentation in the medical records and an increased rate of organ and tissue donation after brain death declarations.
| Author | Objective | Design | Outcome Measure | Conclusion |
|---|---|---|---|---|
| Diringer et al, 2001 | To determine whether mortality rate after intracerebral hemorrhage (ICH) is lower in patients admitted to a neurologic or neurosurgical (neuro) intensive care unit (ICU) compared with those admitted to general ICUs | Outcomes, cross-sectional with concurrent control, retrospective | Hospital mortality, hospital length of stay (LOS), readmission rates, long-term mortality | For patients with acute ICH, admission to a neuro- vs. general ICU is associated with reduced mortality rate. The presence of a full-time intensivist was associated with a lower mortality rate |
| Patel et al, 2002 | To document the effect of neurocritical care, delivered by specialist staff and based on protocol-driven therapy aimed at intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targets, on outcome in acute head injury | Cohort with historical control, retrospective | Hospital mortality | Specialist neurocritical care with protocol-driven therapy is associated with a significant improvement in outcome for all patients with severe head injury. Such management may also benefit patients requiring no surgical therapy, some of whom may need complex therapeutic interventions |
| Wilby et al, 2003 | To prospectively assess outcome and cost for poor-grade subarachnoid hemorrhage patients presenting to a regional neurosurgical center | Outcomes, cross-sectional | Hospital mortality, hospital length of stay, cost analysis | Poor-grade aneurysmal subarachnoid hemorrhage is associated with a high mortality, but a significant subset of patients can achieve favorable outcomes. |
| Berman et al, 2003 | To examine the impact of hospital characteristics on outcome after the treatment of ruptured and unruptured cerebral aneurysms | Outcomes, cross-sectional | Hospital mortality, hospital length of stay | Hospital procedural volume and the propensity of a hospital to use endovascular therapy are both independently associated with better outcome |
| Suarez et al, 2004 | Length of stay and mortality in neurocritically ill patients: impact of a specialized neurocritical care team | Cohort with historical control, retrospective | Hospital mortality, hospital length of stay, readmission rates, long-term mortality | Introduction of a neurocritical care team, including a full-time neurointensivist who coordinated care, was associated with significantly reduced in-hospital mortality and length of stay without changes in readmission rates or long-term mortality. |
| Helms et al, 2004 | Measuring the effect of uncoupling and removal of the treating physician from organ and tissue donation requests on consent rates for donation in the neurocritical care unit | Cohort with historical control, retrospective | End-of-life care and organ donation | Neurointensivist-led policy change resulted in an increase in consent rates for organ donation. |
| Varelas et al, 2004 | To evaluate the impact of a newly appointed neurointensivist on neurosciences intensive care unit (NICU) patient outcomes and quality of care variables | Cohort with historical control, retrospective | Hospital mortality, hospital length of stay, readmission rates, long-term mortality | The institution of a neurointensivist-led team model was associated with an independent positive impact on patient outcomes, including a lower ICU mortality, LOS, and discharge to a skilled nursing facility and a higher discharge home. |
| Varelas et al, 2005 | Examined the hypothesis that a newly appointed neurointensivist may alter documentation practices in a university hospital setting | Cohort with historical control, retrospective | Length of stay, readmission rates | A major change was implemented in the NICU regarding documentation after a neurointensivist was appointed. |
| Varelas et al, 2006 | Impact of a neurointensivist on outcomes in patients with head trauma treated in an NICU | Cohort with historical control, retrospective | Hospital mortality, hospital length of stay, readmission rates, long-term mortality | Neurointensivist-led team model had an independent, positive impact on patient outcomes, including a lower NICU-associated mortality rate and hospital LOS, improved disposition, and better chart documentation |
| Bershad et al, 2008 | Impact of a specialized neurointensive care team on outcomes of critically ill acute ischemic stroke patients | Cohort with historical control, retrospective | Hospital mortality, hospital length of stay, readmission rates, long-term mortality | Institution of a dedicated neurocritical care team was associated with a reduction in resource utilization and improved patient outcomes at hospital discharge. |
| Varelas et al, 2008 | The impact of a neurointensivist on patients with stroke admitted to an NICU | Cohort with historical control, retrospective | Hospital mortality, hospital length of stay, readmission rates, long-term mortality | The direct patient care offered and the organizational changes implemented by a neurointensivist shortened the NCCU stay and hospital LOS and improved the disposition of patients with strokes admitted to an NCCU. |
Physiologic Parameters in Neurocritical Care Patients
The adult brain (1200-1400 g) comprises 2% to 3% of total body weight and yet receives 15% to 20% of cardiac output. However, it has a high cerebral metabolic rate of oxygen (CMRO 2 ) and uses glucose predominantly as a substrate for its energy needs. Normal CBF in humans averages 50 mL/100 g/min brain tissue per minute. Irreversible neuronal damage occurs when CBF is less than 10 mL/100 g/min. The brain has no significant storage capacity; hence cerebral metabolism, CBF, and oxygen extraction are tightly coupled. This relationship is expressed by the Fick’s equation: CMRO 2 = CBF × AVDO 2 , in which AVDO 2 represents arteriovenous difference of oxygen. Under normal conditions the brain maintains a constant AVDO 2 by responding to changes in metabolism, cerebral perfusion pressure (CPP), and blood viscosity with changes in vessel caliber, a phenomenon referred to as autoregulation. Ischemia occurs if oxygen delivery is below the metabolic demand of the tissue (despite the increased AVDO 2 ). Hyperemia, or “luxury perfusion,” occurs if oxygen delivery is greater than the metabolic demand. These various changes may not always manifest clinical features, and monitoring techniques can help detect and define these parameters before irreversible injury occurs.
CPP, defined as mean arterial pressure (MAP) minus ICP, is well maintained in healthy subjects. In pathologic states there can be initial compensatory changes to maintain CPP; for example, small increases in intracranial volume such as an intracranial mass lesion can be accommodated for by translocation of cerebrospinal fluid (CSF) into distensible spinal subarachnoid space with little effect on ICP. This is expressed as the Monro-Kellie doctrine. Exhaustion of this compensatory mechanism can result in large increases in ICP, and so decrease CPP or CBF. Uncorrected, this results in a vicious cycle of further increases in ICP and decreases in CBF. Various monitors can help define and detect this delicate balance.
A large body of evidence suggests that knowledge of normal CBF physiology and alterations caused by specific diseases is paramount in the treatment of critically ill patients with acute brain injury, although not all cellular dysfunction results from altered blood flow. CBF is equal to CPP divided by the cerebrovascular resistance (CVR) (CBF = CPP/CVR). Measurement of CBF in the critically ill can be technically difficult, so CPP (i.e., ICP and MAP) is monitored as a surrogate for the adequacy of CBF. Exactly what optimal CPP is remains debated, but adequate values for CPP generally are considered to be between 55 and 100 mm Hg. This is a wide range, and these values, however, may depend on the specific patient and pathology and so emphasize a need for monitoring. In addition, a normal CPP represents a normal CBF only if CVR is normal. If CVR is high, a normal CPP still may be accompanied by ischemia. Alternatively, if CVR is low, a normal CPP may be associated with hyperemia and increased ICP. Thus it is important to maintain CPP in acute neurologic disorders, preferably in a range that is targeted to the patient especially in situations in which autoregulation is impaired. Although subtle changes in serial neurologic examinations may suggest altered CBF, the sensitivity of these clinical findings can be enhanced by careful attention to other bedside indicators, such as CPP.
Neurocritical Care Delivery and Patient Outcome
The Senate and House of Representatives of the United States designated the decade beginning January 1, 1990, as the “Decade of the Brain.” The goal of this proclamation was to stimulate multidisciplinary efforts of scientists from diverse areas to better understand “the structure of the brain and how it affects our development, health and behavior.” An immediate consequence of this has been a significant increase in the number of new tools available to monitor the central nervous system. A multidisciplinary team comprised of neurologists, neurointensivists, neurosurgeons, and neuroradiologists is frequently seen in the NCCU of the academic medical center of today. The efficiency and core proficiency of any NCCU further depends on a central core of nurses with neurocritical care training.
There is ongoing debate about whether general ICUs require a dedicated, full-time team. Several studies have shown that admission to closed units is associated with a decrease in morbidity and mortality rates, average ventilation time, reduced length of stay, complications, resource utilization, and total hospital charges. For patients with ICH, TBI, and severe AIS, the introduction of a multidisciplinary NCCU team has been associated with decreased mortality, shortened the hospital length of stay, and lowered total cost of care, and has led to better disposition at discharge, that is to say, increased rate of home discharges and a concomitant decrease in nursing home discharges. Differences in care, including the use of more invasive intracranial monitoring, tracheotomy, and less use of intravenous (IV) sedation, may account for some of the observed better outcome in NCCUs. It remains to be seen whether true “closed” or “open” NCCUs provide better outcomes. However, it does appear that patients with neurologic disorders that require ICU admission are better in dedicated NCCUs than general ICUs. Although few studies have directly addressed the value of using information from a monitor in the NCCU, a recent meta-analysis of the TBI literature that included more than 100,000 patients suggests there is a value to using an ICP monitor and aggressive treatment in severe TBI. Similarly clinical studies suggest that use of a brain oxygen monitor and ICP monitor and management based on that may help improve outcome, whereas microdialysis studies suggest that changes in the brain can be observed before altered ICP occurs.
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