Anesthetic Considerations

Because diabetes affects multiple organ systems, the perioperative impact can be profound. Several clinically relevant issues should be considered during perioperative anesthetic management:

1. Diabetes affects oxygen transport by causing glucose to covalently bind to the hemoglobin molecule and alters the allosteric interactions between β chains. This alteration in normal hemoglobin has been shown to decrease oxygen saturation and RBC oxygen transport in pregnant diabetic patients.^[81]
2. A common complication of diabetes is autonomic dysfunction. Patients in whom diabetes has been poorly controlled for many years often have damage to the autonomic nervous system. One study^[82] showed that diabetic patients with previously diagnosed autonomic dysfunction are at increased risk for intraoperative hypothermia. The pathogenesis may be related to inappropriate regulation of peripheral vasoconstriction to conserve body heat.
3. Autonomic dysfunction also affects the body's ability to regulate blood pressure, leading to significant orthostatic hypotension. This underlying defect is caused by a lack of appropriate vasoconstriction. This denervation may also involve vagal control of the heart rate. The changes in heart rate seen with atropine and β-blockers are blunted in patients with significant autonomic dysfunction.^[83]
4. Damage to the autonomic nervous system can significantly affect the choice of anesthetic technique. Patients are at significantly increased risk of hypotension caused by induction agents such as thiopental or propofol. For this reason, etomidate may be a better induction agent because of its considerably lower incidence of cardiovascular side effects.
5. Diabetes has well-defined adverse effects on the cardiovascular system. Men who suffer from diabetes have twice the age-adjusted risk for coronary artery disease. The risk for women is tripled, indicating that they may be even more sensitive to the cardiovascular effects of diabetes.^[84] Data show that patients with diabetes may be at even greater risk for coronary artery disease than was previously suspected. One study revealed that patients with type 2 diabetes had as great a risk for myocardial infarction as nondiabetic patients who already had a previous myocardial infarction.^[85] This information reinforces the point that diabetic patients must be very carefully evaluated preoperatively for coronary artery disease. It must also be remembered that diabetic patients are more likely to have silent ischemia. They may not experience the classic chest pain and tightness associated with ischemic heart disease. Questions regarding exercise tolerance and shortness of breath with exertion may provide important information regarding underlying heart disease or the degree of compensation.
6. Diabetes affects the gastrointestinal tract in several ways. First, it damages the ganglion cells of the gastrointestinal tract, inhibiting motility, which delays gastric emptying and overall transit time through the gut. This increased transit time has significant impact on the practice of anesthesia in that all diabetic patients should be treated as if they have a full stomach. Preoperative treatment with agents that inhibit acid secretion and neutralize stomach acid is essential. Rapid-sequence induction is commonly employed to try to minimize this risk of aspiration.

Perioperative and intraoperative glycemic-control regimens depend on several factors. First, differentiating type 1 from type 2 diabetes is extremely important. Patients with type 1 diabetes are at risk for ketonemia if they are without insulin. The risk of ketosis is amplified when the patient undergoes the stress of surgery. Second, the degree to which blood sugar levels are chronically controlled affects management. Glycosylated hemoglobin (hemoglobin A_1c ) is the most accurate way to assess glucose control over the previous 2 to 3 months. As levels of

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Successful perioperative glucose management depends on careful monitoring. Perioperative management of blood glucose during a brief surgical procedure in the diet-controlled diabetic generally involves only monitoring of blood glucose immediately perioperatively and every 3 hours until oral intake is resumed.^{[86] [87]} The preoperative physical examination and history may reveal extensive diabetic neuropathy, which may be seen as orthostatic hypotension, syncopal episodes, mononeuropathies or polyneuropathies, erectile or bladder dysfunction, and an electrocardiogram showing a loss of R to R variability. Patients may present with a number of additional findings, including nonfamilial short stature, cerebrovascular disease, renal dysfunction, microalbuminemia, and tight, waxy skin. In an estimated 30% to 40% of diabetic patients, glycosylation of the atlanto-occipital joint may limit joint mobility and cause difficulty with airway management (i.e., stiff-neck syndrome).^{[88] [89]} Laboratory evaluation of hemoglobin A_1c is an accurate measure of the severity of hyperglycemia and has been shown to correlate directly with increasing rates of complications. Conversely, lower hemoglobin A_1c values are associated with decreased risk and can be considered a measure of the quality of the diabetic care. Hemoglobin A_1c provides the best evidence of overall blood glucose control over the past 1 to 2 months and should replace the oral glucose tolerance test as the gold standard for diagnosing diabetes.^[90] Basic electrolyte and renal function tests should be evaluated, especially if the patient has frequent urinary tract infections or renal impairment.^[91]

The preoperative hemoglobin A_1c level gives the anesthesiologist a reasonable idea about the patient's blood glucose level over the past several months, and this information can be used to evaluate the need for preoperative and intraoperative insulin requirements. The regimen selected to manage diabetics undergoing surgery depends on the severity of the diabetes and the magnitude of the surgery. Frequent glucose monitoring and preparation for insulin administration are essential for the diabetic patient. Recommendations include discontinuation of long-acting insulin or oral hypoglycemic agents 1 to 2 days preoperatively. Short-acting insulin should be administered every 4 to 6 hours subcutaneously, with the dose adjusted according to glucose levels just before administration. Metformin, a biguanide oral hypoglycemic, also possesses a low risk of lactic acidosis (0.03 cases per 1000 patient-years). Despite these low figures, metformin should not be used in patients with even mild renal dysfunction (serum creatinine >1.5 mg/dL in male patients or >1.4 mg/dL in female patients), congestive heart failure, recent myocardial infarction or any other condition producing a hypoxic state, current alcohol abuse, and impaired hepatic function. Metformin should be discontinued 24 hours before and for at least 48 hours after any procedure using intravenous contrast dye. It should be reinstituted only after renal function has been re-evaluated and found to be normal.^{[92] [93]}

The typical "sliding scale" is destined to fail because it involves the administration of a fixed dose after documentation of hyperglycemia. A small modification improves control. The selected dose should be administered every 4 to 6 hours, based on response. If the glucose level is below 60 mg/dL, the dose should be held for at least an hour and 50% dextrose given intravenously (0.01 to 0.02 mL/kg/min), with blood glucose monitored hourly. When the blood glucose is above 125 mg/dL without supplemental dextrose infusion, the next insulin dose should be 20% to 40% lower. If the glucose is less than 100 or is less than 125 mg/dL and falling, the scheduled dose should be maintained until the hourly measured glucose is above 125 mg/dL, followed by resumption with a 10% to 20% lower dose. If the glucose level is 100 to 200 mg/dL and stable, the current dose and interval are continued. If the glucose level is 200 to 350 mg/dL, the scheduled dose is increased by 10% to 20%. If the glucose level is more than 350 mg/dL, the dose is increased by 20% to 40%.

On the day of surgery, a dextrose infusion (2 mg/kg/min) is started at the time a meal would have been ingested, and glucose is measured preoperatively. For patients currently receiving insulin, an insulin infusion (0.25 units/mL) is started at a rate of 0.5 to 1.25 units/hour, depending on the amount of insulin normally administered and the current glucose level. Blood glucose is monitored hourly, and the infusion rate is adjusted to maintain glucose at 100 to 200 mg/dL. After the blood glucose level is stable, urine glucose levels and ketone bodies are checked to ensure that glycosuria due to a low threshold does not confuse interpretation of urine output.

A more intense monitoring and treatment regimen is recommended for patients requiring more than 50 units/day for control, diabetics in poor control, or the insulin-treated diabetic undergoing major surgery. Long-and intermediate-acting insulin is discontinued, and the patient is managed with an intravenous insulin infusion or scheduled subcutaneous insulin perioperatively. Oral intake must be stopped 12 hours before anesthesia because some element of gastroparesis exists in these patients. Typically, patients are administered a histamine (H₂ ) blocker along with a gastric-emptying drug such as metoclopramide the night before and the morning of surgery. Because the gastrointestinal tract is a prime target for autonomic neuropathy, there may be esophageal dysfunction with difficulty swallowing, constipation, or diarrhea. When oral intake stops, maintenance fluids containing dextrose at 2 mg/kg/min are started and should be continued throughout the procedure. Glucose is measured before induction and hourly until stable postoperatively. Urinary ketones are measured every 6 hours. An insulin infusion is started with an initial rate of 1 to 2 units/hour or to match the amount administered hourly the previous day if good control was achieved. Patients with obesity, liver disease, steroid therapy, or severe infection require higher doses. Glucose levels should be maintained at 100 to 200 mg/dL, and test results for urinary

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Provided a patient has reasonable glucose control (<250 mg/dL), an alternative to an infusion would be to hold all short-acting insulin and give one half of the intermediate- or long-acting insulin the morning of the surgery. It is imperative to provide close preoperative, intraoperative, and postoperative glucose and electrolyte monitoring. Careful titration of a D₅ W drip with an initial intravenous rate of 75 mL/hour can prevent hypoglycemia or hyperglycemia. Diabetic ketoacidosis, dehydration, impaired wound healing, and electrolyte imbalance are minimized with the proper use of exogenous insulin. There is no clear consensus about the method of insulin therapy or the exact range of blood glucose that can affect morbidity or mortality.^[95] If a general anesthetic is used, clinical consideration should include a rapid-sequence induction because of the high rate of gastroparesis. Cerebrovascular accidents, peripheral vascular disease, and cardiovascular infarction are 2 to 10 times more common in diabetics. Strategies designed to reduce the risk of labile blood pressures and myocardial ischemia related to autonomic or vascular disease may include β-blockade to blunt the stress of induction, a narcotic-based anesthetic to minimize cardiopulmonary depression, and prophylactic nitroglycerin in these patients with their significant risk of coronary artery disease. Commonly associated conditions include obesity and stiff cervical joints, which may make airway management challenging. Associated cardiovascular conditions often result in the need for additional invasive monitoring.