Diabetic animals present unique anesthesia challenges to surgeons due to potential complications including hypovolemia, hyper or hypoglycemia, and autonomic dysfunction. Significant changes in the acid/base status and electrolytes of diabetic animals must be managed before, during, and after anesthesia. Various anesthetic agents can also affect insulin and/or glucose levels. Since many diabetic animals undergo cataract surgery, ophthalmologists may operate on more of them than other clinicians. This article will review the different complications of diabetic animal anesthesia and methods to restrict these outcomes.
When a diabetic animal’s blood glucose level exceeds 180 mg/dl, the glucose filtered by the kidneys exceeds the renal capacity for its resorption. Glucose will then be excreted in the urine and cause an osmotic diuresis with an increased excretion of water and electrolytes. In addition, diabetic animals are typically depleted of potassium, magnesium, and phosphate. The loss of water can also result in dehydration/hypovolemia and predispose an animal to hypotension while anesthetized. Reductions in serum potassium levels influence myocardial excitability by increasing membrane potential, diastolic depolarization, the duration of refractory periods and action potentials, and by decreasing conduction velocity. Low serum magnesium and phosphate levels may also decrease cardiac conduction, produce neuromuscular weakness, and in the case of hypophosphatemia lead to hemolysis.
In healthy animals, glycemic limits of 65 to 70 mg/dl serve as a threshold for the activation of glucagon and the secretion of epinephrine. Neurologic damage can occur when blood glucose levels fall below these limits in a diabetic animal. Conversely, hyperglycemia can cause osmotic diuresis, serum hyperosmolarity, and metabolic acidosis. The resultant electrolyte disturbance can lead to cardiac arrythmias and sudden death. For diabetic dogs, the recommended blood glucose level during anesthesia is less than 250 mg/dl. In humans, tight control of glucose levels during surgery have decreased rates of postoperative infection, recurrent ischemia, atrial fibrillation, and stay lengths.
In mornings prior to cataract surgery, owners should be instructed not to feed their diabetic animals and to reduce their normal dosage of insulin by 50%. Upon admission, the animal’s acid-base status, electrolytes and blood glucose levels should be determined prior to anesthesia. Diabetic patients should then by receive intravenous fluids for at least 2 hours. This will decrease any dehydration of intracellular, interstitial, and vascular fluid. Blood glucose and electrolyte abnormalities can also be adjusted during this period.
The pH of the blood of diabetic animals can also be acidotic. Hypovolemic diabetics have poor tissue perfusion and increased anaerobic glycolysis will increase lactate concentrations lowering blood pH (lactate-associated acidosis). The insufficient insulin production by a diabetic can also increase fat metabolism and subsequent ketones acidifying the blood. Comorbidities of diabetic dogs include hyperadrenocortism and pancreatitis that may also elevate lactate B levels in the blood contributing to acidosis. Acidosis can decrease the sensitization of blood vessels and the heart to endogenous and exogenous catecholamines. This can lead to life-threatening hypotension and bradycardia during anesthesia, which is refractory to vasopressors. Proper diabetic control and control of the associated comorbidities is therefore vital during anaesthesia.
In mornings prior to ophthalmic surgery, owners should be instructed not to feed their diabetic animals and to reduce their normal dosage of insulin by 50%. Upon admission, an animal’s acid-base status, electrolytes and blood glucose levels should be determined prior to anesthesia. Anesthetics (e.g. propofol and isoflurane) can induce insulin resistance while other drugs (e.g. dexmedetomidine) can induce hyperglycemia by decreasing insulin release. Therefore, during anesthesia, blood glucose levels should be monitored hourly, not fluctuate by more than 100 mg/dl per hour, and be maintained between 150 mg/dl and 250 mg/dl via injections of insulin or infusions of 2.5 to 5.0 % dextrose. Post-operatively diabetic animals should be monitored for minimally 12 to 24 hours for electrolyte, glucose, hydration, and blood pH levels. Diabetic animals are appropriately discharged only after these levels are normalized by their typical eating habits and dosages of insulin.
For additional questions regarding any diabetic surgery, please consult with a veterinary criticalist.
Marianna Pardo, MV, BVSc, Dip. ACVECC
Noelle La Croix, DVM, Dip. ACVO
The Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800, fax (631) 587-2006