Introduction to Diabetes
Normally the pancreas produces 50U insulin daily.
Complications of Diabetes
Complications of diabetes mellitus include coronary artery disease, cerebrovascular and peripheral vascular disease, autonomic and sensory neuropathies [cardiovascular and GI effects], and joint stiffness, all of which can affect the anesthetic. TMJ and cervical joint stiffness (due to glycosylation of tissue proteins) should be assessed, as 30% of diabetics have difficult intubations.
The most feared complication of diabetes is diabetic ketoacidosis (DKA), which is most commonly precipitated by an infection. DKA should be treated with fluid resuscitation, IV insulin (0.2 U/kg initially, 0.1 U/kg/hr after, goal is to decrease by 75–100 mg/dL or 10% per hour), and K+ supplementation (some, but not all, authors recommend bicarbonate if pH is 7.2).
Hyperosmolar hyperglycemic non-ketotic coma tends to occur in elderly patients with thirst disturbances. Interestingly, most patients will not carry a history of diabetes. NIDDM patients undergoing CABG are particularly vulnerable.
Autonomic neuropathy develops in ~ 1/3 of diabetics (50% of those with coexisting hypertension) and may manifest as orthostatic hypotension, reduced HR variability, baseline tachycardia (decreased inhibitory input), deep breathing, and prolonged QT interval. Patients with the above symptoms may be at increased risk for sudden death [Charlson ME et al. JACS 179: 1, 1994], and bradycardia non-responsive to atropine has been described in diabetic surgery patients. [Burgos et al. Anesthesiology 70: 591, 1989]
The gastrointestinal system may be affected as well, and patients with autonomic symptoms (ex. gastroparesis) should be considered at risk for pulmonary aspiration.
Diabetic patients may also be at increased risk for perioperative nerve injuries [Stoelting p. 439], although this has not been definitively established.
Preoperative Evaluation and Treatment of the Diabetic
Because coronary artery disease is the most common cause of perioperative mortality in diabetic patients, the preoperative evaluation should not neglect the cardiovascular system in favor of the endocrine system. In particular, information about chest pain (often absent), exercise tolerance, etc. should be sought.
Diabetics are often scheduled for first-start. Oral hypoglycemic doses are often held the morning of surgery, as patients are NPO and the risk of hypoglycemia is real. Metformin and sulfonylureas are often held for 24-48 hours before surgery. Metformin also carries with it the risk of metabolic acidosis [Mercker SK et al. Anesthesiology 87: 1003, 1997], although the chances of this are remote.
Insulin-dependent diabetics should receive insulin preoperatively, although there is no standardized dose – most commonly, ½ the daily dose is given as an intermediate acting agent after an IV is placed and glucose is checked.
Note that many of these patients will be on ACE-inhibitors (because of their renal sparing effects) or ARBs and may be at risk for severe hypotension at induction.
Intraoperative Management of the Diabetic
No anesthetic technique has been shown to be superior. IV lidocaine may help facilitate glycemic control [Durieux et al., in press]. Insulin-dependent diabetics should have glucose monitored every 1-2 hours, with a goal of keeping glucose < 180.
Do not rely on SQ or IM insulin intraoperatively, as these routes rely on adequate tissue perfusion. Consider starting a dedicated IV for D5W and IV insulin infusions (which could potentially interfere with other medications).
Most commonly caused by Graves’ disease, most-often in women 20-40 years of age (0.2% incidence in parturients). Elective procedures should be deferred until the disease is well-controlled (B-blockers [inhibit peripheral conversion of T4 to T3 in addition to controlling heart rate, goal rate is 85 bpm], antithyroids [methimazole and propylthiouracil, both of which block synthesis of new thyroid hormones but do not affect release of existing hormone. Methimazole works more quickly and has less side effects], +/- iodine [blocks the actual release of thyroid hormone]).
Preoperative evaluation should always include a neck exam, and in some cases, a CT scan.
Hyperthyroid patients can be chronically hypovolemic and vasodilated and are prone to an exaggerated hypotensive response during induction of anesthesia, thus pay attention to volume status preoperatively.
Some authors advocate for a thiopental induction, as it has antithyroid properties at high doses, but the advantage is theoretical and has not been established in human studies.
Despite animal evidence suggesting that volatile anesthetics are associated with hepatic necrosis in hyperthyroid rats [Berman ML et al. Anesthesiology 58: 1, 1983], volatile anesthetics are the ideal maintenance agents as they effectively blunt sympathetic nervous system responses. Some practitioners believe that MAC requirements are increased, but this is not supported by animal (dog) data [Babad AA and Eger EL. Anesthesiology 29: 1087, 1969]. The common clinical impression of increased MAC is likely due to a misunderstanding of increased volume requirements (secondary to increased cardiac output and metabolism) in the face of identical partial pressure requirements.
When selecting (and using) a neuromuscular blocking (or any) drugs, one should be aware of two concerns – first, the potential for muscle weakness (and theoretical possibility of prolonged effect) and second, the potential for tachycardia with reversal.
Temperature monitoring is critical, and these patients may need to be chilled.
Epinephrine and ephedrine should be used with caution.
Regional anesthesia should be considered except in cases of high output heart failure.
Hyperthyroid storm (altered mental status, hyperthermia, tachycardia, CHF, shock, and dehydration) can occur intraoperatively but is most common 6-18 hours post-operatively and has a mortality rate of 10-75%. Treat with chilled crystalloid and an esmolol infusion [Thome AC and Bedford RF. Anesthesiology 71: 291, 1989]. Consider 100-200 mg cortisol for hypotension, as well as dexamethasone (inhibits T4-T3 conversion). Propylthiouracil (250–500 mg q6h PO or NG) and sodium iodide (1 g IV over 12 h) are often added, and charcoal, hemodialysis, and plasmapheresis have also been tried.
Complications of thyroid surgery include damage to laryngeal nerves, tracheal compression, and hypoparathyoidism. The superior laryngeal nerves supply motors to the cricothyroid muscles (only), as well as supraglottic sensation. The recurrent laryngeals supply all other muscles in the larynx as well as infraglottic sensation. As patients to say “eeeee” in order to assess these muscles. Tracheal compression is often due to hematoma formation. Hypoparathyroidism is due to surgical removal, and can manifest early as one hour after surgery (stridor, laryngospasm, tingling).
Most commonly caused by Hashimoto’s thyroiditis, but also caused by medical/surgical treatment of hyperthyroid disease. Subclinical hypothyroidism (elevated TSH with no clinical symptoms) is present in ~ 5% of patients (and 13% of elderly patients) but there is no evidence that these patients are at increased risk. [Benett-Guerrero E et al. Anesth Analg 85: 30, 1997]
Severe, symptomatic hypothyroidism can cause myxedema coma, pericardial effusion, and heart failure – if any of these are present, elective operations should be delayed until thyroid replacement has been initiated and is adequate. That said, mild to moderate hypothyroidism is not an absolute contraindication to surgery.
Hypothyroid patients require less preoperative sedation and are prone to drug-induced respiratory depression, thus consider benadryl for sedation, as well as metoclopramide for gastric-emptying (which is slowed). Euthyroid patients may receive their usual dose of thyroid medication on the morning of surgery (remember, however, the t½ of T4 is about 8 days).
Hypothyroid patients, who have diminished cardiac output, blunted baroreceptor reflexes, and decreased intravascular volume, are at risk for severe hypotension. Induction may be best accomplished with ketamine. Consider pancuronium as a paralytic agent (chronotropic effects).
Intubation may be complicated by a large tongue
Do NOT give volatile anesthetics to hypothyroid patients, as they are at risk for severe myocardial depression. Instead, consider nitrous oxide plus intravenous agents (ex. benzodiazepines). There is no evidence that hypothyroid patients have reduced MAC requirements.
Monitoring should focus on temperature control and cardiac output (i.e. avoiding congestive heart failure). In cases of refractory hypotension, one should always be cognizant of the possibility of acute adrenal failure.
Other potential problems include hypoglycemia, anemia, hyponatremia, and hypothermia from a low basal metabolic rate.
Recovery from general anesthesia may be delayed by hypothermia, respiratory depression, or slowed drug biotransformation, and hypothyroid patients often require prolonged mechanical ventilation. Because of the risk for respiratory depression, opiates may need to be avoided in favor of ketorolac for postoperative analgesia.
Patients with asthma, COPD, autoimmune disease, or dermatologic conditions often take steroids, placing them at risk for adrenal suppression that can last up to a year. The total dose of steroids required in order to induce adrenal suppression is not known, thus any patient who has taken steroids for a month or more is usually replaced for a year [Symreng T et al. Br J Anaesth 53: 949, 1981]. There is some evidence that these patients should be given physiologic doses of steroids in preparation for surgery. [Salem M et al. Ann Surg 219: 416, 1994]
Rare cause of hypertension (~ 0.1%), but if diagnosed, 16% will have other, associated disorders. Initial diagnosis often made by the classic triad of tachycardia/palpitations, headache, and diaphoresis in the setting of hypertension. VMA and metanephrines are the most specific tests. Clonidine (which blocks sympathetic outflow) reduces blood pressure in essential hypertensives, but not in those with pheochromocytoma.
Preoperatively, patients with pheochromocytoma should be fluid resuscitated and have a pre-established alpha-blockade (ex. phenoxybenzamine, prazosin) as well as treat any arrhythmias if present (beta blockers). Note that alpha-blockers will enhance insulin release, decreasing blood glucose levels.
Arterial line placement should proceed IV induction (avoid ketamine). Sevoflurane can be added prior to laryngoscopy in order to blunt the SNS response – DL should not occur until a surgical level of anesthesia has been obtained, and both fentanyl and IV lidocaine should be considered prior to intubation. Pancuronium should be avoided (because of its SNS stimulating effects). Nitroprusside should be available at induction.
Maintenance is accomplished with volatiles, which are ideal as they blunt the SNS. If MAC 1.5-2.0 does not adequately control blood pressure, nitroprusside infusion should probably be started. Note that when the veins are ligated, circulating catecholamines will drop rapidly, necessitating a reduction in volatile anesthesia as well as fluid resuscitation. Hyperglycemia (due to reduced alpha-induced insulin production) may ensue, thus glucose should be checked with regularity.
Droperidol should be avoided, as it has been shown to cause a paradoxical hypertensive response (thought to be due to inhibition of norepinephrine reupdate. [Oh TE et al. Anaesth Intensive Care 6: 322, 1978])
Tend to be volume overloaded and have hypokalemic metabolic alkalosis due to the mineralocorticoid activity of excess glucocorticoids.
BMI = weight (kg) / [height (m)]2. Overweight is defined as a BMI of 24 kg/m2, obesity as a BMI of 30, and extreme obesity (old term “morbid obesity”) as a BMI of 40. The BMI > 30 cutoff is somewhat arbitrary, as morbidity/mortality/longevity are affected at lower BMI (~ 28).
Obesity is associated with HTN, CAD, and cardiomegaly. Cardiac output has to increase 0.1 L/min/kg for adipose tissue (mostly accomplished through increased SV). Importantly, VO2 is increased, as is CO2 production.
Obesity predisposes to OSA (and accompanying hypersensitivity to opiates as well as sedatives), obesity-hypoventilation syndrome (in 8% of patients), and increases pulmonary blood pressure, but reduces FRC, expiratory reserve, and vital capacity. FRC may fall below closing capacity, producing a baseline V/Q mismatch and hypoxemia. Hypercapnea is rare in obese patients and suggests hypoventilation syndrome (hypercapnea, cyanosis-induced polycythemia, RH failure, somnolence). Chest wall rigidity increases, further complicating mechanical ventilation.
Obese patients are allegedly at risk for aspiration (increased incidence of GERD and hiatal hernia), although Harter’s study of 256 fasted surgical patients showed that lean patients had a higher incidence of high-volume and low-pH (HVLP) gastric contents (40%) compared to obese (BMI > 30) patients (26.6%, p < 0.05) [Harter RL et al. Anesth Analg 86: 147, 1998]. Many practitioners use a rapid sequence technique, and many authors recommend premedication with H2 blockers and metoclopramide.
Preoperative evaluation of extremely obese patients undergoing major surgery should attempt to assess cardiopulmonary reserve. IV and intraarterial sites should be checked in anticipation of technical difficulties. Particular attention should be paid to the airway exam (limited mobility, shortened distance between mandible and sternal fat, lower FRC reduces time to desaturation).
Induction and Anesthesia
Decreased FRC leads to more rapid changes in lung concentrations of volatile anesthetics. IV anesthetics are difficult to predict, as plasma volume is increased (decreases drug concentrations), but adipose tissue is not well-perfused. Some authors advocate dosing based on ideal body weight, others spilt the difference between ideal and actual weight. Rarely does one need to dose based on more than 80 kg for females or 100 kg for males [Bouillon T and Shafer SL. Anesthesiology 89: 557, 1998]. Many drugs accumulate in adipose tissue, further complicating management, although obese patients may not be at increased risk for delayed emergence [Cork RC et al. Anesthesiology 54: 310, 1981]. PEEP should be strongly considered intraoperatively.
Theoretically, obese patients have larger volumes of distribution for opiates and benzodiazepines (fat-soluble), thus some authors recommend larger loading doses and lower/less-frequent maintenance doses. It is difficult to accurately predict the dosages needed, however. Interestingly, obese patients usually require 20–25% less local anesthetic (epidural fat, distended veins) and are at particular risk for respiratory compromise, but again, actual dosage requirements are difficult to predict.
Respiratory failure is the major fear. The 45° modified sitting position unloads the diaphragm and improves ventilation/oxygenation. Consider routine post-operative supplemental oxygen for days, as PaO2 bottoms out at 2-3 days post-op. Incentive spirometry and early ambulation are helpful. Wound infection, deep venous thrombosis, and pulmonary embolism are also more likely in this population.
Many authors advocate postponing elective surgery in patients who have lost 20% of their body weight. Enteral nutrition should be administered if possible, as outcomes are improved [Lin L and Cohen NH. Cont Crit Care 2: 1, 2005]. TPN is only used when absolutely necessary, as it adds an unnecessary fluid load, can lead to hyperchloremic metabolic acidosis, excess CO2 production, and in the pre-glucose control era, increased infection rates.