Neuroanesthesia

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CHAPTER 21 Neuroanesthesia

Preoperative Evaluation

Deepak Sharma, Arthur M. Lam

The neuroanesthesiologist’s goals can be divided into three periods—the preoperative period: (1) prepare the patient and family for the planned surgical procedure both physiologically and psychologically, (2) evaluate and optimize the patient’s preoperative condition, (3) plan the anesthesia and elaborate the risks entailed to the patient and family; the intraoperative period: (4) render patients unconscious and insensitive to surgical and psychological trauma, (5) minimize the stress response to the surgical procedure, (6) maintain and optimize intraoperative physiologic function, (7) provide optimal operating conditions to facilitate surgery; and the postoperative period: (8) ameliorate pain after surgery without compromising patient safety, and (9) provide reassurance to the patient and family. The relative importance of these goals varies according to the patient and the nature of the surgical procedure. This chapter addresses general and neurosurgical procedure–specific considerations in the preoperative evaluation.

The preanesthetic evaluation is defined as the process of clinical assessment that precedes the delivery of anesthesia care for surgical and nonsurgical procedures. The primary aim of preanesthetic evaluation is to minimize the overall patient morbidity associated with surgery and anesthesia. This goal is achieved by assessing the patient’s medical condition and the balance between anesthetic risk and surgical benefit, optimizing the medical condition within the limitations of the surgical circumstances, and formulating the best possible anesthesia plan. Other benefits may include improved safety of perioperative care, optimal resource utilization, improved outcomes, and patient satisfaction.1 Hence, the objectives of preanesthetic evaluation include the following:

1 Establish rapport with the patient and immediate family members/significant others to minimize their anxiety and have a cooperative and relaxed patient
2 Provide information to the patient and family regarding anesthetic techniques and procedures and the associated risks and benefits and postoperative management issues, including pain control and possible need for postoperative mechanical ventilation in a major procedure
3 Evaluate the past medical, surgical, and anesthetic history and current medications to establish a baseline profile
4 Review the relevant personal, family, and social history and history of allergies
5 Perform a general physical examination, including recording of vital signs and examination of individual systems, particularly the nervous and cardiopulmonary systems
6 Interpret the relevant laboratory data and arrange for further investigations and consultations if deemed necessary to eliminate unnecessary preoperative standing “screening tests” and limit investigations to only appropriate ones, with obvious economic savings
7 Optimize the patient’s physiologic condition
8 Stratify patient risk regarding morbidity and mortality based on the considerations just presented
9 Formulate an anesthesia plan and organize resources for perioperative care and postoperative recovery
10 Document informed consent for the proposed anesthetic technique and procedures

The preanesthetic evaluation may be performed well in advance of the planned surgery for most elective procedures during a visit to the preanesthetic evaluation clinic. Otherwise, it may be performed at the bedside in the hospital ward or intensive care unit the “night before” for inpatients or on the day of surgery for morning-admission patients. For urgent and emergency procedures, this evaluation may of necessity take place just before surgery. The consensus of the American Society of Anesthesiologists (ASA) Task Force on preanesthesia evaluation is that an initial record review, patient interview, and physical examination should be performed before the day of surgery for patients with high severity of disease.1 Of patients with low severity of disease, those undergoing procedures with high surgical invasiveness should have the interview and physical examination performed before the day of surgery, whereas those undergoing procedures with medium or low surgical invasiveness may be interviewed and examined on or before the day of surgery.1 Although the task force cautions that the timing of such assessments may not be practical with the limitation of resources, it recommends that at a minimum, a focused preanesthetic examination should include assessment of the airway, lungs, and heart and documentation of vital signs.1

Preanesthesia clinics are ideally run by anesthesiologists with or without the assistance of trained nurses. These clinics have been shown to improve operating room efficiency and minimize unexpected delays and cancellations because of poorly prepared patients.2,3 To be able to run smoothly, however, good organization, concise guidelines and protocols, and adequate medical support are required. Additional staffing issues are also important considerations.

It has been shown that the patient’s preoperative condition predicts postoperative mortality and morbidity.4–8 In one study, preoperative evaluation of patients led to a change in the proposed anesthesia plan in up to 15% of healthy individuals and 20% of ill patients.9 Although these changes in plan do not necessarily reduce patient morbidity, they can lead to delays caused by the need to obtain different drugs and equipment and further specialist consultations and result in increased operating room downtime and cost. Establishment of a preanesthesia assessment clinic streamlines the process and obviates this potential source of delay. In a study from Stanford University, implementation of a preanesthetic evaluation clinic produced an 87.9% reduction in day-of-surgery cancellations.3 It is estimated that $30 to $40 billion is spent annually on preoperative testing and subsequent follow-up in North America alone, 50% of which could be saved by the appropriate and selective ordering of tests.10 In one study, implementation of a preoperative clinic, in which tests were ordered at the anesthesiologist’s request, resulted in a savings of $112.09 per patient. This equated to an annual potential saving of more than $1.01 million at one institution.3

To ensure smooth transition from patient referral to surgical intervention, preoperative communication among neurosurgeons, anesthesiologists, neurophysiologists, and the laboratory is essential. To the extent that the anesthesiologist must be acquainted with the surgical and neurophysiologic monitoring needs of the procedure, the neurosurgeon equally needs to be aware of the anesthetic concerns. Good communication between the surgical and anesthesia teams allows exchange of ideas and an opportunity to address concerns, to the benefits of the surgeon, the anesthesiologist, and most of all, the patient.

General Preanesthetic Evaluation

Optimal preoperative status may not be the same as optimal daily living status, and thus when reviewing the patient, the effects of anesthesia, positioning, surgery, and postoperative pain must be considered in relation to the patient’s surgical state and medications. The patient’s preexisting medical condition, unrelated to the proposed surgical procedure, may require more intense scrutiny than the pathologic process being treated.

The ASA classification of physical status is a universally accepted system used for stratification of a patient’s preexisting health status (Table 21-1). Although it does not take into account surgical risk and is not primarily designed for prediction of outcome, it has been found to correlate with perioperative morbidity and mortality.11–13 In fact, ASA physical status 3 to 5 has been found to independently predict perioperative cardiovascular complications in intracranial surgical patients and is also a risk factor for perioperative mortality.8

TABLE 21-1 ASA Classification of Physical Status

ASA PHYSICAL STATUS DISEASE STATE
1 A normal healthy patient
2 A patient with mild systemic disease
3 A patient with severe systemic disease
4 A patient with severe systemic disease that is a constant threat to life
5 A moribund patient who is not expected to survive without the operation
6 A patient declared brain-dead whose organs are being removed for donor purposes

ASA, American Society of Anesthesiologists.

Excerpted from the Relative Value Guide 2008 of the American Society of Anesthesiologists. A copy of the full text can be obtained from ASA, 520 N. Northwest Highway, Park Ridge, IL 60068-2573.

History and Physical Examination

Careful evaluation of the patient will rate the severity of the medical problems and detect risks for asymptomatic disease. A good starting point is the primary disease process requiring surgical intervention, which may alert the anesthesiologist to potential problems such as trauma and a full stomach, head injury and the development of coagulopathy, or intracranial aneurysm and the need for blood pressure control. Thereafter, the evaluation should focus on each system. The specific neurosurgical aspects are discussed separately later in this chapter. The general approach is summarized here.

Medical History

1 Medical history related to the intended surgical procedure
2 Medical history related to other constitutional diseases that may or may not have a bearing on the intended procedure
3 History of previous surgery and anesthesia (insight into problems with airway management, intravenous access, postoperative pain, nausea/vomiting, and other problems)
4 Current medications (anticonvulsant therapy is associated with increased resistance to nondepolarizing muscle relaxants and hence an increased requirement for them; steroid administration might be associated with hyperglycemia and adrenal suppression) and allergies (e.g., allergy to latex, antibiotics, adhesive tape)
5 Personal history (smoking, alcohol, recreational drug use—all of which have bearing on anesthesia and intraoperative care)
6 Relevant family medical history and social/religious background (e.g., Jehovah’s Witnesses, which would limit blood transfusion by adherence to faith)
7 Physical examination and assessment of organ system function
8 Ancillary laboratory investigations

General Physical Examination

Before proceeding to examination of individual systems, a general physical examination should be conducted and take into account the patient’s level of consciousness, mental status, build, nutrition, and vital parameters. Patients with malignant tumors and those with high cervical lesions might be emaciated with significantly reduced muscle mass. Conversely, obesity might be coexistent in many patients. Obese individuals have an increased likelihood of associated diabetes, hypertension, coronary artery disease, restrictive lung disease, sleep apnea, and gastroesophageal reflux, which might warrant alteration of the anesthesia plan. Difficulty with tracheal intubation may be encountered more frequently in obese than in lean individuals,14 and the pharmacologic profile of anesthetic agents may also be altered.15 Some neurosurgical patients might be dehydrated because of reduced intake of fluids (as a result of impaired consciousness), vomiting, or the use of diuretics and contrast agents. Correction of significant dehydration before induction of anesthesia can prevent postinduction hypotension in such patients. Significant blood loss is a possibility with surgery for intracranial aneurysms, arteriovenous malformations (AVMs), vascular tumors, craniosynostoses, and extensive spine problems. Preanesthetic evaluation should look for preexisting anemia and attempt to correct it preoperatively or arrange for intraoperative transfusion on a case-by-case basis. Recording of preoperative vital parameters (heart rate, blood pressure) provides baseline values for intraoperative management, which is particularly important in surgeries requiring strict hemodynamic control (e.g., aneurysms and AVMs).

Perhaps the most crucial aspect of the general examination is assessment of the patient’s airway. Although the primary neurosurgical problem may be responsible for potential difficulties in intubation and airway management, inadequate management of the airway may adversely affect the neurological outcome. Routine maneuvers used for airway management may worsen spinal instability in patients with cervical lesions and lead to increased intracranial pressure (ICP) with potentially devastating consequences in patients with decreased intracranial compliance. Hence, the patient’s airway should be assessed carefully for ease of ventilation and difficulty of tracheal intubation, in conjunction with specific surgical needs such as hemodynamic stability and spine immobilization. Mallampati scoring16 thyromental distance, presence of overbite or underbite, and the range of neck flexion-extension collectively provide an estimate of the risk for difficult intubation.17 Some specific situations in which a difficult airway should be anticipated include patients who have recently undergone supratentorial craniotomy, in whom mouth opening might be significantly reduced secondary to ankylosis of the temporomandibular joint,18 acromegalic patients undergoing pituitary surgery,19 and patients with cervical spine lesions. Recognition of potential airway difficulty allows proper planning with the availability of accessory equipment and resources, as well as formulation of a back-up plan, and results in improved patient safety and efficient use of operating time.

Assessment of System Functions

Neurological System

The importance of complete assessment of the neurological system of a patient scheduled for a major neurosurgical procedure cannot be overemphasized. Although most patients would have received a complete assessment by the attending neurologist or neurosurgeon by the time that they are evaluated for anesthesia, careful documentation would nonetheless facilitate planning for anesthesia and anticipation of potential perioperative complications. Moreover, because the signs and symptoms may change in the interim as a result of progression of the pathologic process, the preanesthetic examination will provide a baseline for postoperative comparison.

Patients with a depressed level of consciousness preoperatively are likely to have a reduced anesthetic need for induction and more likely to have a slow or delayed emergence postoperatively and need for postoperative mechanical ventilation. Such patients should not receive any sedative or narcotic agents unless they are under continuous supervision, preferably in the operating room itself with vigilance for respiratory depression. Moreover, in patients with previous motor deficits, exacerbation of focal neurological signs may develop after sedative doses of benzodiazepines and narcotics.20 The presence of brainstem lesions or lower cranial nerve dysfunction, or both, predisposes patients to an increased risk for aspiration postoperatively. Finally, life-threatening hyperkalemia secondary to succinylcholine administration may develop in patients with preexisting motor deficits.21 Succinylcholine has also been reported to cause hyperkalemia in patients with ruptured cerebral aneurysms independent of the presence of motor nerve disturbances,22 although this appears to be uncommon. Elevated ICP is often manifested as headache with nausea and vomiting, but it can also lead to olfactory nerve dysfunction with loss of the sense of smell. Unilateral uncal herniation would result in a dilated unresponsive ipsilateral pupil, which should be distinguished from incidental anisocoria, or a unilateral third nerve palsy resulting from compression by a space-occupying lesion. Field of vision might be significantly limited in patients with pituitary and other suprasellar tumors and should be documented for postoperative comparison. Dysfunction of the trigeminal and facial nerves may interfere with mask ventilation and tracheal intubation. A patient with a damaged vagus nerve may have a hoarse voice secondary to vocal cord paralysis and may be at increased risk for airway obstruction.

Respiratory System

Risk for perioperative respiratory complications is increased in patients with preexisting obstructive or restrictive pulmonary disease. Perioperative hypoxemia or hypercapnia is more likely to occur and in turn can further aggravate an already compromised cardiorespiratory status. Patients with a history of pulmonary disease require an assessment of their baseline status, and any element of potential reversibility should be addressed.23–25 Smoking is a common important risk factor for both cardiovascular and pulmonary disease and is associated with a threefold increase in perioperative morbidity. Cessation of smoking for 6 to 8 weeks is recommended for reactivation of mucociliary clearance, but as little cessation as 24 hours can reduce carboxyhemoglobin levels and improve oxygenation.26 The presence of reactive airway disease indicates an increased risk for bronchospasm with airway manipulation and tracheal extubation and an increased risk for coughing and laryngospasm during emergence.

In patients with symptomatic obstructive pulmonary disease, preoperative pulmonary function testing, including flow-volume loops before and after the administration of bronchodilators, and arterial blood gas sampling allow assessment of reversibility and determination of preoperative optimization. An abnormally high PaCO2 or low PO2 preoperatively is predictive of postoperative respiratory complications. In patients with significant impairment, elective postoperative mechanical ventilation after a major neurosurgical procedure may be indicated. Some patients with sleep apnea might be using continuous positive airway pressure devices at home, and it is important to ensure that the same device is available postoperatively.

Management of upper respiratory tract infection preoperatively in children is controversial because the effects on the airway last for 2 to 4 weeks after clinical resolution. The patient is at increased risk for perioperative respiratory morbidity during this period.27 Postponement of elective surgery must be balanced against the risk for progressive neurological disability or the occurrence of a potentially catastrophic complication during the waiting period.

Patients with decreased levels of consciousness because of intracranial pathology and those with high spinal lesions or lower cranial nerve paralysis might have preexisting atelectasis preoperatively, which puts them at increased risk for postoperative mechanical ventilation. Aspiration pneumonitis or superimposed pneumonia, or both, can also develop. A restrictive pattern of lung disease often occurs in patients with craniovertebral junction anomalies preoperatively and persists in the postoperative period.28 Some patients, such as those with head injury, spinal cord injury, or subarachnoid hemorrhage (SAH), might be intubated and mechanically ventilated preoperatively and usually remain intubated postoperatively as well.

In their systematic review of preoperative pulmonary risk stratification for noncardiothoracic surgery for the American College of Physicians, Smetana and colleagues found good evidence to support the following patient-related risk factors as being predictive of postoperative pulmonary complications: advanced age, ASA class 2 or greater, functional dependence, chronic obstructive pulmonary disease, and congestive heart failure.29 They also found fair evidence indicating increased risk in patients with impaired sensorium, abnormal findings on chest examination, cigarette use, alcohol use, and weight loss.29 Although asthma is not a risk factor if well controlled, perioperative risk may be increased if it is poorly controlled.29 Important procedure-related risk factors include neurosurgery, emergency surgery, and prolonged surgery.29 The value of preoperative testing in estimating pulmonary risk is controversial. Even though an abnormal chest radiograph does indicate increased risk for postoperative pulmonary complications and spirometry may provide some risk stratification, among potential laboratory tests for stratifying risk, a serum albumin level of less than 35 g/L is the most powerful predictor.29

Cardiovascular System

Anesthesia, surgical positioning, and surgery itself put additional demands on the cardiovascular system. Moreover, intraoperative maintenance of hemodynamic stability is important to avoid adverse neurological effects in neurosurgical patients. The presence of cardiovascular disease significantly increases the risk associated with anesthesia, and optimizing the patient’s condition can significantly improve outcome. The overall risk of cardiac patients undergoing a noncardiac procedure has traditionally been assessed with the Goldman index.4 However, it has now been superseded by the Revised Cardiac Risk Index.5 According to this index,5 the presence of three or more of the following factors is associated with a cardiac morbidity rate of 9%: (1) high-risk surgery, (2) history of ischemic heart disease, (3) history of congestive heart failure, (4) history of cerebrovascular disease, (5) preoperative treatment with insulin, and (6) preoperative serum creatinine level greater than 2.0 mg/dL. Because most patients undergoing intracranial procedures often have two or more of these risk factors, careful evaluation of the other organ systems is important to quantify risk for cardiac morbidity. Preanesthetic evaluation should also be focused on detecting and assessing the physiologic effects of cardiovascular conditions known to be associated with specific neurosurgical conditions, such as hypertension and coarctation of the aorta in patients with aneurysms.

Coronary artery disease is associated with diabetes mellitus, hypertension, smoking, hypercholesterolemia, and peripheral vascular disease. The presence of angina is a significant risk factor—although unstable or resting angina predicts the highest risk for postoperative cardiac complications, the risk with angina on exertion can be minimized with appropriate intraoperative management. Left ventricular dysfunction with symptoms of cardiac failure (dyspnea on mild exertion, orthopnea, peripheral edema) is indicative of significantly reduced cardiac output, which can worsen with general anesthesia. Mannitol must be used carefully and judiciously or not at all in patients with left ventricular failure. Hypertension is a common preexisting condition and is frequently inadequately controlled. These patients often have reduced plasma volume, thus making them more susceptible to the systemic vasodilatory effects of anesthetic agents, which can result in cardiovascular instability and labile blood pressure intraoperatively. Moreover, in patients with chronic hypertension, increased cerebrovascular resistance causes the lower and upper limits of cerebral blood flow (CBF) autoregulation to shift to higher pressure levels, and such patients consequently have poor tolerance of acute hypotension.30,31 However, adaptive hypertensive changes in CBF autoregulation may be reversible with adequate control of blood pressure.30,31 Patients with evidence of myocardial ischemia or myocardial infarction (MI) are at increased risk for postoperative MI, congestive heart failure, malignant arrhythmias, and death.

Preoperative cardiac evaluation must be carefully tailored to the circumstances and nature of the surgical illness. Given an acute surgical emergency, preoperative evaluation might have to be limited to simple and critical tests (such as rapid clinical assessment, hematocrit, electrolytes, renal function, and electrocardiography [ECG]), with a more extensive evaluation conducted after surgery. In patients in whom coronary revascularization is not an option, it is often not necessary to perform a noninvasive stress test. In general, preoperative tests are recommended only if the information obtained will result in a change in the surgical procedure performed, a change in medical therapy or monitoring during or after surgery, or postponement of surgery until the cardiac condition can be corrected or stabilized. A cardiology consultation should be sought if deemed necessary and surgical circumstances allow. Preoperative evaluation by a cardiologist may involve changes in medications, additional preoperative tests or procedures, or recommendations for higher levels of postoperative care.

The American College of Cardiology/American Heart Association (ACC/AHA) 2007 guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery grade clinical risk factors as major, intermediate, and minor.32 The presence of one or more of the major risk factors (active cardiac conditions) mandates intensive management and may require delay or cancellation of surgery unless the surgery is being performed on an emergency basis. Major risk factors include

• Unstable coronary syndromes (e.g., unstable or severe angina)
• Decompensated heart failure
• Significant arrhythmias (e.g., high-grade atrioventricular block, supraventricular arrhythmias with an uncontrolled ventricular rate, symptomatic ventricular arrhythmias/bradycardia)
• Severe valvular disease (severe aortic stenosis or symptomatic mitral stenosis)

According to the guidelines,32 intermediate-risk factors include

• History of ischemic heart disease
• History of compensated or previous heart failure
• History of cerebrovascular disease
• Diabetes mellitus
• Renal insufficiency

A history of MI or abnormal Q waves on ECG is listed as a clinical risk factor, whereas acute MI (defined as at least one documented MI 7 days or less before the examination) or recent MI (more than 7 days but 1 month or less before the examination) with evidence of important ischemic risk by clinical symptoms or noninvasive study is an active cardiac condition. This definition reflects the consensus of the ACC Cardiovascular Database Committee. If a recent stress test does not indicate residual myocardium at risk, the likelihood of reinfarction after noncardiac surgery is low. Despite the lack of adequate clinical trials on which to base firm recommendations, it appears reasonable to wait 4 to 6 weeks after an MI to perform elective surgery.32

Minor predictors are recognized markers for cardiovascular disease that have not been proved to increase perioperative risk independently, such as advanced age (>70 years), abnormal findings on ECG (left ventricular hypertrophy, left bundle branch block, ST-T abnormalities), rhythm other than sinus, and uncontrolled systemic hypertension.

The guidelines recommend the following stepwise approach to perioperative cardiac assessment for noncardiac surgery32:

Step 1 (Need for Emergency Noncardiac Surgery)

Further cardiac assessment or treatment is not warranted, and emergency surgery may proceed with perioperative surveillance, postoperative risk stratification, and risk factor management. The cardiologist may provide recommendations for perioperative medical management and surveillance.

Step 2 (Patients with Active Cardiac Conditions)

The presence of unstable coronary disease, decompensated heart failure, severe arrhythmia, or severe valvular heart disease warrants evaluation and treatment according to the ACC/AHA guidelines, with cancellation or delay of surgery until the cardiac problem has been clarified and treated appropriately. However, depending on the results of investigations and cardiac interventions and the risk associated with delaying surgery, it may be appropriate to proceed to the planned surgery with maximal medical therapy.

Step 3 (Patients Undergoing Low-Risk Surgery)

Interventions based on cardiovascular testing in stable patients rarely result in a change in management, and it is appropriate to proceed with the planned surgical procedure in such patients.

Step 4 (Patients with Good Functional Capacity and No Symptoms)

Functional status is a reliable predictor of perioperative and long-term cardiac events. In highly functional asymptomatic patients, management will rarely be changed by the results of any further cardiovascular testing. It is therefore appropriate to proceed with the planned surgery. In patients with known cardiovascular disease or at least one clinical risk factor (ischemic heart disease, compensated or previous heart failure, diabetes mellitus, renal insufficiency, and cerebrovascular disease), perioperative heart rate control with beta blockade is considered appropriate. However, results from the Perioperative Ischemic Evaluation Study (POISE) indicate that the reduced cardiac morbidity with perioperative beta-blocker therapy in patients not previously taking beta blockers is achieved at the expense of an increased stroke rate and an overall increase in mortality.33

Step 5 (Symptomatic Patients and Patients with Poor or Unknown Functional Capacity)

In this scenario, the presence of clinical risk factors determines the need for further evaluation. If the patient has one or two clinical risk factors, it is reasonable to either proceed with the planned surgery, with consideration of beta blockade for heart rate control, or perform cardiac testing if it will change management. In patients with three or more clinical risk factors, the surgery-specific cardiac risk is important, which in turn is related to the degree of hemodynamic cardiac stress (alterations in heart rate, blood pressure, vascular volume, pain, bleeding, clotting tendencies, oxygenation, neurohumoral activation, and other parameters) associated with the surgery. In patients undergoing intermediate-risk surgery (including head and neck surgery and carotid endarterectomy [CEA]), data are insufficient to determine the best strategy.

Gastrointestinal System

Patients at risk for aspiration include those with full stomachs, bowel obstruction, and gastroesophageal reflux. Patients with cranial nerve dysfunction involving the 9th and 10th cranial nerves, as well as those with decreased level of consciousness, are also at risk if they have not been fasting. In these patients, general anesthesia with rapid-sequence induction and cricoid pressure should be undertaken to minimize the risk for aspiration.

Renal System

Patients needing neurosurgical interventions sometimes have coexistent renal dysfunction that might be acute or chronic. Acute renal failure can be prerenal, renal, or postrenal, depending on its cause. In contrast, chronic renal failure is attributable most commonly to hypertensive nephrosclerosis, diabetic nephropathy, chronic glomerulonephritis, and polycystic renal disease. Patients with kidney disease represent an anesthetic challenge because they may have autonomic neuropathy, encephalopathy, fluid retention (congestive heart failure, pleural effusion, ascites) and yet intravascular volume depletion, hypertension, metabolic acidosis, electrolyte imbalance (hyperkalemia, hyponatremia, hypocalcemia), anemia, and delayed gastric emptying, among other manifestations. The generalized effects of azotemia mandate a thorough evaluation of patients in renal failure. Signs of fluid overload or hypovolemia should be sought. Hematocrit, serum electrolytes, coagulation studies, blood urea nitrogen, and creatinine measurements are advisable. A chest radiograph and arterial blood gas analysis might be required in patients with breathlessness, and the electrocardiogram should be examined for signs of hyperkalemia or hypocalcemia, as well as ischemia and conduction blocks. Severely anemic patients may require preoperative red blood cell transfusions. Preoperative drug therapy should be carefully reviewed for drugs with significant renal elimination. Dosage adjustments and measurement of blood levels are sometimes necessary to prevent drug toxicity.

Intravascular volume depletion, injection of contrast dye, and use of aminoglycoside antibiotics, angiotensin-converting enzyme inhibitors, and nonsteroidal anti-inflammatory drugs (NSAIDs) are risk factors for acute deterioration in renal function and must be avoided. Hypovolemia appears to be a particularly important factor in the development of acute postoperative renal failure. The emphasis in management of these patients is on prevention because of the high mortality associated with postoperative renal failure. Optimal management may require preoperative dialysis in certain situations, the usual indications being severe acidosis or volume overload, hyperkalemia, metabolic encephalopathy, and drug toxicity. Patients with nausea, vomiting, or gastrointestinal bleeding should undergo rapid-sequence induction with cricoid pressure. Volume status is often difficult to assess and may necessitate invasive monitoring, including placement of intra-arterial and central venous pressure catheters. Neuromuscular blocking agents not dependent on renal function for elimination should be selected. Mannitol is contraindicated in anuric patients. Postoperative mechanical ventilation is sometimes required in patients with renal failure because inadequate spontaneous ventilation with progressive hypercapnia can result in a respiratory acidosis that may exacerbate any preexisting acidosis, lead to potentially severe circulatory depression, and dangerously increase the serum potassium concentration.

Hematologic System

Postoperative intracranial hemorrhage is a potentially lethal catastrophe. Thus, any bleeding tendency should be investigated thoroughly and corrected preoperatively. If deemed necessary, appropriate clotting factors and platelets should be made available at the time of surgery.34 Patients taking NSAIDs such as aspirin should have their medications stopped for a week before intracranial surgery.35 This decision may have to be modified in patients suffering from transient ischemic attacks, in whom the risk associated with discontinuation may exceed the benefits.

Endocrine System

Patients with diabetes mellitus who are about to undergo surgery require special attention because hyperglycemia is associated with hyperosmolarity, infection, and poor wound healing. More importantly, it may worsen neurological outcome after an episode of cerebral ischemia. Nonetheless, hypoglycemia is also detrimental because the brain depends on glucose for its energy supply. Close monitoring of glucose perioperatively is therefore essential, and treatment with insulin is often required to maintain euglycemia, but sulfonylureas and metformin should not be used for 24 to 48 hours before surgery because of their long half-lives. The perioperative morbidity of diabetic patients is related to their preoperative end-organ damage. Hence, the pulmonary, cardiovascular, and renal systems should be examined carefully. Diabetics have an increased incidence of ST-segment and T-wave abnormalities on ECG, and myocardial ischemia may be evident despite a negative history (silent myocardial ischemia). Diabetic autonomic neuropathy may predispose patients to cardiovascular instability and even sudden cardiac death. Furthermore, autonomic dysfunction contributes to gastroparesis, which may require treatment with H2 blockers or metoclopramide, or both, preoperatively. Chronic hyperglycemia can lead to glycosylation of tissue proteins and a stiff joint syndrome. Diabetic patients, especially those with type 1 diabetes, should be routinely evaluated preoperatively for adequate temporomandibular joint and cervical spine mobility to help anticipate difficult intubation.36

Glucocorticoid excess (Cushing’s syndrome) in neurosurgical settings may be due to exogenous administration of steroid hormones or hypersecretion by a pituitary adenoma (Cushing’s disease) and is characterized by muscle wasting and weakness, osteoporosis, truncal obesity, abdominal striae, glucose intolerance, hypertension, and changes in mental status. Patients with Cushing’s syndrome tend to be volume overloaded and have hypokalemic metabolic alkalosis resulting from the mineralocorticoid activity of glucocorticoids, which should be corrected preoperatively. Patients with osteoporosis are at risk for fractures during positioning, whereas preoperative weakness may indicate an increased sensitivity to neuromuscular blocking agents. Conversely, acute adrenal insufficiency can be triggered in steroid-dependent patients who do not receive supplemental doses during the perioperative period.

Patients with hyperthyroidism or hypothyroidism might undergo surgery for neurosurgical illness. Ideally, all elective surgical procedures should be undertaken when the patient is clinically and chemically euthyroid with medical treatment. However, mild to moderate hypothyroidism is not an absolute contraindication to surgery. Antithyroid medications and β-adrenergic antagonists are continued through the morning of surgery in hyperthyroid patients and thyroid hormone supplements in hypothyroid patients (although most thyroid preparations have long half-lives). If emergency surgery must proceed in a hyperthyroid patient, the hyperdynamic circulation can be controlled by titration of esmolol infusion. The possibility of associated myopathies and myasthenia gravis should be considered in hyperthyroid patients. Hypothyroid patients, in contrast, are very prone to drug-induced respiratory depression and do not usually require much preoperative sedation. However, premedication with H2 antagonists and metoclopramide may be considered because of delayed gastric-emptying times. Other potential problems with hypothyroidism include hypoglycemia, anemia, hyponatremia, difficulty with intubation because of a large tongue, and hypothermia from a low basal metabolic rate. Emergence from anesthesia may be delayed in hypothyroid patients because of hypothermia, respiratory depression, or slowed drug biotransformation, and such patients may require postoperative mechanical ventilation.

Laboratory Investigations

The current literature is not sufficiently rigorous to permit an unambiguous assessment of the clinical benefit or harm associated with routine or selected preoperative tests. According to the ASA Task Force on preanesthesia evaluation,1

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