Anesthesia for dogs and cats with endocrine disease (Proceedings)

The endocrine systems throughout the body play crucial rolls in the maintenance and metabolism that are required to maintain health. Perturbations in many of these symptoms occur in dogs and cats and veterinarians are often required to diagnose and treat these conditions that may last throughout the lifetime of a pet. In addition, many of these patients will require anesthesia for diagnostics and treatments that may or may not be related to underlying endocrine diseases. Veterinarians should have an understanding of how these conditions can affect physiology, anesthesia and how anesthetic and analgesic medications can affect these conditions.

Hyperadrenocorticism

Physiologic changes commonly seen in patients with hyperadrnocroticism include poor tissue and wound healing, PU/PD, hypercoagulability, muscle wasting, lethargy, hypoxemia, polyphagia, abdominal enlargement, excessive lipid deposits, and electrolyte abnormalities. This condition can be either from an intrinsic disease or iatrogenic from steroid administration. Stabilization of the disease state should be done prior to any anesthesia for routine or elective procedures. Many of these patients can be hypertensive as well due to increased systemic vascular resistance. The can put an additional stress on the heart that may already have weakened cardiac muscle. Complications can include difficult catheterization, fluid retention, excessive bleeding, pulmonary thromboembolism, and ineffectual ventilation.

Hypoardrenocorticism

Patients with hypoadrenocorticism can have physiologic abnormalities including bradycardia, dehydration, collapse, PU/PD, weight loss, weakness, lethargy, and shock. Stabilization of these patients with intravenous fluid and electrolyte therapy and replacement steroid therapy can be easily achieved and these patients respond quickly to treatment. In most of these patients, additional doses of glucocorticoids, for the stress of anesthesia and surgery, are warranted even if the patients are well regulated.

Diabetes mellitus

Diabetic patients present to the veterinarian for anesthesia for procedures often unrelated to glucose disorders. However, as the prevalence of obesity continues to increase in our population of patients, the prevalence of glucose disorders will also increase. It is therefore essential that veterinarians be able to safely anesthetize these patients. Well controlled patients are usually uncomplicated to get back on their regular insulin and eating schedule. Poorly or uncontrolled patients can be very difficult and should be regulated prior to anesthesia, especially where the anesthesia can be delayed. There are various protocols available for the administration of feed and insulin on the day of the anesthetic event that can be used successfully. It is probably more important that diabetic patients are anesthetized early in the morning to limit the fasting period and allow for the remainder of the day to be recovered and reregulated.

Diabetic ketoacidosis

Unlike simple diabetics, patients with diabetic ketoacidosis are very poor anesthetic candidates and have high complication and mortality rates. These patients are severely dehydrated, hypovolemic, acidotic and have electrolyte abnormalities. They are also often extremely hyperglycemic as well that can result in hyperosmolarity. Animal that are positive for urine or blood ketones should not be anesthetized unless it is for a life saving procedure. Even then, if at all possible stabilization with large volumes of intravenous fluids and insulin should be performed to correct hypovolemia and bring blood glucose levels to less than 300 mg/dL. Repeated monitoring of glucose should be performed to dictate therapy preventing wide swings in glucose.

Hyperthyroidism

Hyperthyroidism is usually seen in older cats and can be accompanied by behavioral changed such as hyperactivity, nervousness, and aggression. These patients may also have dyspnea and become easily stressed when handled or restrained. In cats that have conditions secondary to hyperthyroidism such as hypertrophic cardiomyopathy and renal disease, anesthetic care can becomes extremely difficult. These patients are often in a fragile state and can progress to cardiac arrhythmias and heart failure when overly stressed, sometimes resulting in death. Preanesthetic medical therapy is advised to stabilize patients prior to elective procedures. Anesthetic protocols are based around opioids and drugs that can increase heart or precipitate arrhythmias such as atropine and ketamine should be avoided. Fluid and electrolyte therapy is also recommended throughout the peri-anesthetic period.

Hypothyroidism

Canine hypothyroidism is usually slow in onset and has minimal impact on anesthetic concerns compared to other endocrine diseases. These patients can be obese and have signs consistent with a decreased metabolic rate. Bradycardia, hypotension, decreased stroke volume, decreased drug metabolism, and hypothermia are common concerns. Many of these concerns can be alleviated with thyroid hormone supplementation and regulation of the condition is recommended prior to elective procedures.

Pheochromocytoma

Anesthesia for patients with a pheochromocytoma can be one of the most challenging for the veterinarian. Excessive catecholamine put these patients at risk for wide swings in heart rate and cardiac output as well as blood pressure. These patients are extremely unstable and can have cardiac arrest at any time. Preoperative stabilization for a minimum of two weeks with phenoxybenzamine can help reduce hypertension and restore circulating blood volume. The use of beta blockers such as propanolol to reduce tachycardia can be administered as well. It is critical that beta blockers are not administered until after a minimum of one week of phenoxybenzamine administration. Administration of beta blockers without prior alpha blockade with phenoxybenzamine can result in worsening of hypertension. Fluid and electrolyte therapy should be performed prior to anesthesia as well to ensure a proper circulating volume. During surgery, complications from catecholamine release from tumor manipulation can be managed with administration of nitroprusside for hypertension and esmolol for tachyarrhytmias.