Kristen Cooley, BA, CVT, VTS (anesthesia & analgesia) highlights the considerations to make when putting a diabetic patient under anesthesia.
Diabetes mellitus is an endocrine disease that is the result of inadequate serum insulin levels or insulin resistance (decreased sensitivity of peripheral tissues to the effects of insulin).1 It is characterized as being either type 1 or type 2. Type 1 diabetes mellitus occurs when immune-mediated destruction of the pancreatic beta cells ensues, leading to hypoinsulinemia, hyperglycemia and the need for exogenous insulin supplementation.1 This type of diabetes is more common in female dogs.1 Type 2 diabetes is likely a combination of the impaired secretion of insulin as well as a reduction in tissue insulin sensitivity leading to hyperglycemia. Type 2 is more common in male cats and is usually managed through diet, although approximately 70% will require exogenous insulin supplementation.1, 2
Insulin is essential for normal cellular function and works to inhibit glycogenolysis (the breakdown of glycogen into glucose), glucogenesis (the generation of glucose from non-carbohydrate substances mainly in the liver and kidney), and lipolysis (fat hydrolysis).1 Insulin also stimulates glucose uptake into cells and potassium transport, and suppresses ketogenesis. An animal with sustained hyperglycemia and subsequent ketonemia may also suffer from a metabolic acidosis, dehydration, endothelial dysfunction, cerebral ischemia and impaired wound healing.3
On physical exam, the diabetic patient may present dehydrated with serum chemistry abnormalities. The abnormalities of interest to the anesthetist may include elevated liver and kidney enzymes, reductions in sodium and potassium, and hyperglycemia.3 Urine should be evaluated for the presence of glucose and/or ketones. These quick and inexpensive tests give insight into the presence and severity of diabetes as stress itself may also cause elevations in blood glucose. When glucose levels surpass the renal tubular threshold, all the filtered glucose does not get resorbed as some remains in the urine; the presence of ketonuria suggests a greater degree of insulin deficiency compared to hyperglycemia alone. A fundic exam should be performed to rule out hypertension-induced changes such as retinopathy and concurrent cerebral vessel abnormalities.4 Other clinical signs like polyphagia, polydipsia and polyuria are common.1,2
Autonomic neuropathy can lead to gastroparesis predisposing the diabetic patient to regurgitation, poor vasomotor tone leading to hypotension, respiratory depression and impaired thermoregulation.5,6 These concerns are common to all anesthetic cases, diabetic or non-diabetic; the difference is that the former is less resilient and prone to hemodynamic instability under anesthesia.6 Impaired autoregulation of organ perfusion may necessitate an overall higher mean arterial blood pressure in order to maintain oxygen delivery to tissues.2,7
Anesthetic plans for the diabetic and non-diabetic patients are similar in the sense that a balanced approach should be used for both. Injectable induction agents have a negligible effect on blood glucose levels with the exception of ketamine. Ketamine is an indirect sympathetic stimulant which has the potential to worsen hyperglycemia.5,8 Opioids and benzodiazepines moderate sympathetic stimulation, thereby potentially reducing the hyperglycemic response to surgery.9,10 Epidural anesthesia using local anesthetics may also inhibit catecholamine release; however, the anesthetist must be cognizant of the potential complications related to epidural use which can be exacerbated in the diabetic patient with preexisting neurologic deficits.12 Inhalants reduce insulin production and their use should be minimized during anesthesia with the concurrent use of minimal alveolar concentration (MAC) reducing adjunctive medications.6,11
Stress reduction through the use of anxiolytic agents is recommended along with adequate analgesic techniques to minimize the effects that pain and stress can have on blood glucose. The use of alpha-2 adrenergic agonists such as dexmedetomidine is controversial. This class of drugs can worsen hyperglycemia through its interaction with alpha-2a receptors which inhibit insulin secretion.13 However, the use of alpha-2 agonists in healthy dogs does not cause hyperglycemia and actually reduces the stress response through a reduction in sympathetic nervous system activity.13 The diabetic patient may benefit from the pre-operative use of an alpha-2 agonist from this perspective.
Anesthesia in the unregulated diabetic patient should be avoided unless it is emergent. The risks associated with this patient population can manifest as hemodynamic instability and hypotension from changes to vascular endothelium and impaired response to anesthetic drugs secondary to metabolic acidosis.6 The anesthetist should be prepared to monitor and support blood pressure, blood pH and blood glucose levels. The uncontrolled diabetic is minimally classified as an ASA III.
The controlled diabetic that presents for an elective procedure should be scheduled first thing in the morning. This reduces fasting time which can alter insulin requirements and provides time for the patient to recover and resume their normal feeding/insulin schedule. Patients should not be given their morning insulin until their blood glucose levels have been evaluated and insulin needs determined.1 Patients presenting with blood glucose levels less than 100 g/dl should have their insulin withheld and a 2.5-5% dextrose infusion initiated. Pre-operative glucose levels measuring at 300 g/dl and beyond can be given a half dose of insulin. Patients with a blood glucose between 100 and 300 g/dl don't require special pre-operative treatment however, all diabetic patients should have their glucose monitored every 30-60 minutes to avoid broad swings in glucose levels and to help direct necessary therapy.1,7
Diabetic patients should be sufficiently pre-medicated using an opioid and a sedative then rapidly induced with an injectable induction agent. Patients should be intubated with an appropriately sized endotracheal tube and the pilot balloon adequately inflated as needed to create a sealed system prior to positioning. This protects the airway from regurgitation and aspiration which can be seen in the diabetic patient with autonomic neuropathy and gastroparesis.5 Patients should be supported with an intravenous balanced electrolyte solution, inhalant levels kept low with the use of analgesics and adjunctive medications, and glucose levels maintained between 150–250 g/dl as hyperglycemia is associated with increased morbidity and mortality, prolonged hospital stays and decreased wound healing.7 Glucose should be monitored through recovery until the patient is up and eating. Hypoglycemia in the recovery phase may be seen as the muscle activity from shivering consumes glucose. It should also be noted that stress and inflammation common in the post-operative period can increase insulin requirements.7
As a threat to homeostasis, diabetes can negatively affect surgical and anesthetic outcomes. Elective procedures should only be performed on patients who are well-regulated and steps should be taken to ensure that pain and stress are adequately controlled. Patients with an immediate surgical need must be approached with a full understanding of the potential negative sequela of the disease and preparations made to deal with complications should they arise.
Kristen Cooley, BA, CVT, VTS (anesthesia) is an instructional specialist at The University of Wisconsin School of Veterinary Medicine and a consultant in Veterinary Anesthesia Support and Training.