Evidence-Based Practice of Neuroanesthesia

Introduction

It was in the year 1992 that evidence-based practice (EBP) was formally introduced into clinical practice. The first EBP was started in medicine as evidence-based medicine (EBM) and later spread to other fields such as nursing, psychology, education, and library sciences. EBP involves conscientious decision making, which is based on the best available evidence as well as preferences and patient characteristics. EBP got its reputation because of its reasoning on all procedures, medicines, and treatment thereby assuring patient safety. EBP is to enhance and promote safe medical practice, to offer guidance for diagnosing, managing, or treating clinical conditions. These EBP parameters can be used to form guidelines or advisories. The components of guideline development include review and evaluation of published scientific evidence, meta-analytical assessments of controlled clinical studies, statistical assessment of expert and practitioner opinion by formally developed surveys, and informal evaluations of opinions obtained from invited or public commentary. Sources of these evidences are based either on literature or on opinion. Literature-based sources include the randomized controlled trials (RCTs), non-RCTs, controlled observational studies, uncontrolled observational studies, retrospective studies, case series, and case reports. However opinion-based sources include views of consultants, survey opinions, invited sources, expert comments, open forum commentary, and the Internet. Because of the vast availability of studies linking professionals, EBP has been successfully incorporated into treatment services. It is now expected that the professionals must be up to date and well informed with advances and achievements in medical field to best serve their patients.

While reviewing a particular topic, it is important to have knowledge of different levels of evidence. However, it needs to be clearly stated that all levels of evidence are important. In scientific and health care field, the four levels of clinical treatment evidence mentioned in Table 54.1 are widely accepted.

Table 54.1

Levels of Evidence

The United States Department of Health and Human Services http://www.ahrq.gov/ .

Level 1	Level 2	Level 3	Level 4
Randomized controlled trials —includes quasi-randomized processes such as alternate allocation	Non–randomized controlled trial— a prospective (preplanned) study, with predetermined eligibility criteria and outcome measures	Observational studies with controls— includes retrospective, interrupted time series (a change in trend attributable to the intervention), case-control studies, cohort studies with controls, and health services research that includes adjustment for likely confounding variables	Observational studies without controls (e.g., cohort studies without controls, case series without controls, and case studies without controls)

To classify levels of evidence for the different categories mentioned earlier, it was necessary to derive the following table of “Classifications of Evidence.” To use these classes of evidence, one must identify the type of study and then classify it according to the following table ( Table 54.2 ).

Table 54.2

Classifications of Evidence

Class 1	Level I studies, studies with longitudinal design with control group, studies related to basic sciences, reliability studies with >30 subjects, validity studies, professional surveys
Class 2	Level II studies, studies with cross-sectional design with control groups, reliability studies with <30 subjects
Class 3	Level III studies, studies with longitudinal design without control group
Class 4	Level IV studies, studies with cross-sectional design without control groups
Class 5	Expert opinions

The EBM pyramid helps us understand different levels of evidence to make best health-related decisions ( Figure 54.1 ). As we ascend through the pyramid, it shows different levels, which represent the types of study design and correspond to increasing quality and reliability of the evidence.

The first level of the evidence-based pyramid is the background information or expert opinion, which is important and helpful. However, these types of evidences are influenced by many factors such as opinions, politics, or traditions. Case series/reports usually include only a few participants and case-control studies are performed in the early stage to identify variables that might predict a condition. Disadvantages of these study designs are that there are less number of participants and they are not randomized. Cohort studies include large group of participants over a period of time. These study design are difficult to blind and are not randomized. The next important level in the pyramid is the RCT. A large RCT provides a most reliable study design. In this study design, individuals are grouped into two or more groups, where one group receives the intervention and another receives no treatment or a placebo. Critically appraised topics are basically the short summaries of the best available evidence. On the top of the pyramid is systematic review. Systematic reviews and meta-analysis are considered the strongest and highest quality of evidence.

Evidence-Based Practice and Neuroanesthesia

Neuroanesthesiology is a rapidly growing superspecialty branch of medicine that has achieved remarkable growth in neuroanesthetic techniques and management. Despite achievements controversies persist. Most of the multicentric trials conducted, provide results that have little clinical significance. Most RCTs focus on surrogate end points rather than on clinical or neurologic outcomes. Some of the common issues related to the practice of neuroanesthesia are discussed in the following sections.

Target Intracranial Pressure and Cerebral Perfusion Pressure

The management strategy for treatment of raised intracranial pressure (ICP) has usually been either cerebral perfusion pressure (CPP) targeted (Rosner concept), ICP targeted or volume targeted (Lund’s concept). The “Rosner concept” states that lower CPP might precipitate intracranial hypertension, and hence advocates increasing blood pressure to augment cerebral blood flow (CBF) and CPP. Another strategy for raised ICP treatment is the ICP-targeted strategy, which focuses on aggressive reduction of ICP as the primary target. The commonest subcategory of ICP control involves a “volume-targeted” strategy (Lund concept), which is based on physiological principles for brain volume regulation and improved microcirculation. Both concepts have their own pros and cons. However, because of lack of RCTs on this topic, no approach can be considered superior to another. According to a systematic review in 2013, there is no evidence that the Lund concept is a preferable treatment option in the management of severe traumatic brain injury (TBI) and further research is needed in this field. According to the current clinical evidence, the target ICP should be maintained <20 mmHg and CPP should never exceed >70 mmHg. The target CPP should be maintained between 50 and 70 mmHg; however, critical CPP is considered to be between 50 and 60 mmHg.

Effect of Inhalational Anesthetics on Intracranial Pressure

The effect of volatile anesthetic agents on central nervous system vasculature is a depression in the cerebral metabolic rate in a dose-dependent manner. Sevoflurane cause less cerebral vasodilation compared to isoflurane or desflurane. Despite being used for more than 160 years, use of nitrous oxide in neurosurgery is still debatable among neuroanesthesiologists. The initial Evaluation of Nitrous Oxide in the Gas Mixture for Anesthesia trial brought out the query over routine use of nitrous oxide in patients undergoing major surgery. Intraoperative avoidance of nitrous oxide does not affect duration of hospital stay significantly. However, on long-term observation, the risk of myocardial infarction increases when exposed to nitrous oxide, but does not increase the risk of stroke or death. According to subgroup analysis of the General Anesthesia versus Local Anesthesia for Carotid Surgery trial, nitrous oxide use does not increase the risk of stroke, mortality, and myocardial infarction. Although nitrous oxide is a potent cerebral vasodilator, no outcome studies demonstrate its deleterious effect.

Effect of Intravenous Anesthetics on Intracranial Pressure

Total intravenous anesthesia received attention in neuroanesthesia as it avoids cerebral vasodilation. Intravenous agents such as propofol produce cerebral vasoconstriction and reduction in cerebral blood volume, CBF, and ICP secondary to decrease in cerebral metabolic rate of oxygen consumption, however, preserving cerebral autoregulation. According to the Intraoperative Hypothermia for Aneurysm Surgery trial data, administration of thiopental or etomidate does not have any significant clinically demonstrable effect on postoperative neurologic outcomes in patients undergoing temporary clipping. As per the American Heart Association/American Society of Anesthesiologists (AHA/ASA) guidelines for management of aneurysmal subarachnoid hemorrhage (SAH), at present there are insufficient data available to support their routine use, other than their use in those with high risk of prolonged temporary clipping (Class IIb, level of evidence C).

Effect of Hyperventilation and Positive End Expiratory Pressure on Intracranial Pressure

Hyperventilation has a short-term and temporary, but profound effect on CBF. It could be a life-saving measure in the treatment of acute intracranial hypertension. According to a multicentric randomized cross-over trial of hyperventilation and normoventilation in patients undergoing craniotomy for supratentorial brain tumors, intraoperative hyperventilation (PaCO ₂ 25 vs. 37 mmHg) was found to be associated with decreased ICP (12 vs. 16 mmHg) and 45% reduction in surgeon-assessed brain bulk. However, on the other hand, if hyperventilation is used for prolonged period, it has not been certainly shown to be beneficial to the patients. It has been observed that the patients who were hyperventilated had significantly worse outcome than those who were on normal ventilatory rate. At present, there has been no evidence to suggest that it improves clinically relevant outcomes (death or neurologic disability). Mechanical ventilation and addition of positive end expiratory pressure (PEEP) can lead to increase in ICP. However, higher level of PEEP that can be used safely without an increase in ICP is up to 15 cm H ₂ O.

Effect of Hyperosmolar Therapy on Intracranial Pressure

Hyperosmolar therapy including mannitol or hypertonic saline (HS) can rapidly reduce the ICP. Raised ICP can cause global ischemia or even brain death if its value crosses 50–60 mmHg. The brain contains 80% water, so the use of hyperosmolar agents to create an osmolar gradient between the systemic circulation and brain has significant role. HS and mannitol do not cross the blood–brain barrier, hence they draw water out of the injured brain. However, in the condition wherein the blood–brain barrier is disrupted, these hyperosmolar agents will not be effective. The interstitial accumulation of mannitol is most prominent if used in continuous infusion, hence it is recommended to use mannitol as repeated boluses rather than as continuous infusion. There is no conclusive evidence that supports the role of hyperosmolar agents in saving lives or intercept disability. There is no distinct evidence at present saying that either of hyperosmolar agents, mannitol or HS are superior over another at reducing ICP. The overall outright differences in effects between these two agents have been quite less.

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