Chapter 37
Anesthetic Considerations for Endocrine Disease
That gland is not bland
Berit L. Fischer
Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, USA
- Q. What is endocrine disease?
- A. Endocrine organs are responsible for secreting hormones that function on target organs throughout the body to regulate metabolism, growth, cellular function, and reproduction. Excessive secretion, impaired secretion, and/or neoplasia of endocrine organs can result in endocrine disease.
- Q. What are common veterinary endocrine diseases?
- A. Hyperadrenocorticism (Cushing’s disease), hypoadrenocorticism (Addison’s disease), diabetes mellitus, and hypothyroidism are common endocrine disease in dogs. Cats are more commonly afflicted with diabetes mellitus and hyperthyroidism. Other less common endocrine diseases that affect veterinary species include pheochromocytoma, insulinoma, acromegaly (cats), and hyperaldosteronism (cats).
Diseases of the Thyroid Gland
- Q. What role do thyroid hormones play in the body?
- A. The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are synthesized in and released from the thyroid gland in response to thyroid releasing hormone (TRH) and thyroid stimulating hormone (TSH) from the hypothalamus and anterior pituitary respectively.
Once released, T3 and T4 act at the cellular level to regulate metabolic rate, influence protein synthesis, and assist in normal fetal development. These actions help to maintain rate and contractility of the heart, thermoregulation, and proper ventilatory responses to hypercapnia and hypoxemia.
- Q. What is hypothyroidism and who does it affect?
- A. Hypothyroidism is a deficiency in thyroid hormones caused by destruction of thyroid tissue through autoimmune disease, atrophy, or neoplasia. It is relatively common in older (>7 years) dogs and is over-represented in English Setters, Giant Schnauzers, Golden Retrievers, Dobermans, Boxers, Shetland Sheepdogs, and Cocker Spaniels [1]. It is rare in cats but, when present, occurs following radioiodine therapy or surgical thyroidectomy for hyperthyroidism.
- Q. What are the effects of hypothyroidism on various body systems?
- A. See Table 37.1.
Table 37.1 Effects of hypothyroidism on various organ systems.
Body system
Effects
Clinical result
Cardiovascular system
Decreased number/affinity of beta receptors on heart
Atherosclerosis from altered lipid metabolism
Decreased contractility and heart rate = decreased cardiac output (CO)
Bradyarrhythmias
Hypotension
Decreased end-organ perfusion
Respiratory system
Decreased sensitivity to increases in carbon dioxide and decreases in oxygen
Hypoventilation
Hypoxemia
Respiratory acidosis
Gastrointestinal tract
Delayed gastric emptying/ileus
Increased risk of regurgitation/aspiration
Metabolism
Decreased hepatic metabolism
Obesity
Prolonged drug action
Hypothermia
Hypoventilation
Nervous system
Peripheral neuropathies
Motor/sensory deficits
- Q. Prior to anesthesia, what tests should be performed?
- A. Prior to anesthesia, a thorough physical exam should be performed with particular emphasis on the cardiovascular system. Standard biochemical tests may reveal mild anemia, and elevations in cholesterol and triglycerides; however these findings rarely impact anesthesia.
- Q. What are appropriate premedications to use in patients with hypothyroidism?
- A. Anesthetic drugs may have a more profound effect in the patient with hypothyroidism. Use of drugs that are reversible or titratable is advised. Opioids, such as morphine, hydromorphone, or methadone, in combination with a benzodiazepine, may provide sufficient premedication to allow catheter placement and pre-oxygenation prior to anesthetic induction. Doses should be tailored to represent lean body weight in obese patients.
Acepromazine is long-lasting and irreversible, can promote the development of hypothermia, and may exacerbate hypotension. When deciding to use acepromazine, the anesthetist should use low doses (0.01–0.02 mg/kg), ensure the patient is normovolemic and aggressively prevent and treat hypothermia.
An alpha-2 agonist, such as dexmedetomidine, can exacerbate bradyarrhythmias. It can also promote hypothermia and hypotension. These drugs are reversible, however, and in stable hypothyroid patients, low doses (1–1.5 mcg/kg IV; 2–3 mcg/kg IM) may be used if sufficient sedation is not achieved with opioids and benzodiazepines alone.
- Q. What are appropriate induction agents to use in patients with hypothyroidism?
- A. In patients with mild hypothyroidism, most induction agents are acceptable. Because ketamine stimulates the sympathetic nervous system, it may cause a beneficial increase in heart rate, contractility, and cardiac output.
- Q. Adjunct analgesia?
- A. Additional analgesia should be considered in the hypothyroid patient. Dose-dependent vasodilation and hypotension from inhalant anesthetics can be minimized through the administration of additional opioids as a constant rate infusion (fentanyl 5–10 mcg/kg/h) or intermittent boluses.
Loco-regional techniques, such as nerve blocks and epidurals, are equally beneficial in reducing doses of other drugs. Be careful employing these techniques in patients who have evidence of a peripheral neuropathy since deficits could worsen post procedure. It is also worth noting that hair regrowth can be slow or absent in dogs with hypothyroidism. If an epidural is planned, forewarning owners that their pet may have a bald spot can prevent angst later.
- Q. Are there specific concerns regarding anesthetic management in the hypothyroid patient?
- A. Many of the concerns surrounding anesthesia in the hypothyroid patient can be addressed in proper pre-operative preparation. The anesthetist should confirm that the patient was fasted prior to anesthesia to minimize risk of regurgitation and aspiration. Likewise, ensuring patients are hydrated prior to anesthesia can help prevent hypotension and improve perfusion of vital organs. Finally, pre-oxygenation benefits hypothyroid patients since they are at risk of desaturation from impaired ventilatory responses.
Intra-operative support should include aggressive prevention and treatment of hypothermia. Hypotension is common and can be treated through good cardiovascular support as is covered in Chapter 17.
- Q. Are there particular concerns in the recovery period in patients with hypothyroidism?
- A. Recovery may be prolonged due to slowed hepatic metabolism of drugs. Patients should be kept warm and oxygenation in the post-anesthetic period should be monitored using pulse oximetry. If SpO2 readings are <94%, provide supplemental oxygen.
- Q. What is hyperthyroidism and who does it affect?
- A. Hyperthyroidism is excessive synthesis and secretion of T3 and T4 from the thyroid gland. It is the most common endocrine disease in cats older than 8 years of age [1]. It is uncommon in dogs, but when present, is often caused by thyroid carcinoma or excessive administration of levothyroxine for treatment of hypothyroidism.
- Q. Is it safe to anesthetize patients with hyperthyroidism?
- A. Unlike hypothyroidism, anesthesia of patients with hyperthyroidism carries significant risk. Every attempt to stabilize T4 levels prior to elective anesthesia should be made to minimize morbidity and mortality.
- Q. What are the effects of hyperthyroidism of various body systems?
- A. See Table 37.2.
Table 37.2 Effects of hyperthyroidism on various organ systems.
Body system
Effects
Clinical result
Cardiovascular system
Increased: O2 demand, CO, myocardial work, sensitivity of heart to catecholamines, number/affinity of beta receptors on heart
Vasodilation
Tachyarrhythmias, systemic hypertension, thyrotoxic cardiomyopathy, tissue hypoxia, cardiac failure
Hypotension (anesthetized)
Respiratory system
Increased O2 demand
Tissue hypoxia
Gastrointestinal system
Decreased transit time, impaired peristalsis
Vomiting/diarrhea with fluid losses
Metabolism
Increased basal metabolic rate
Cachexia
Hyperthermia
Increased/more frequent anesthetic drug dosing
Hypothermia (anesthetized)
- Q. Prior to anesthesia, what tests should be performed?
- A. In addition to a thorough physical exam, blood pressure measurement should be performed. Patients may present with tachyarrhythmias, such as a gallop rhythm, bounding pulses, and a parasternal cardiac murmur. If discovered, a complete cardiac work-up with chest radiographs, echocardiography, and ECG is advised. Complete blood count (CBC) and biochemistry panels may show mild polycythemia, elevations in liver enzymes, and electrolyte changes, particularly hypokalemia. Because of the increase in cardiac output, BUN and creatinine levels can be falsely lowered. This can make diagnosis of concurrent renal disease difficult in affected patients and warrants further investigation if it is suspected.
- Q. What are appropriate premedications to use in patients with hyperthyroidism?
- A. Although it is ideal to postpone anesthesia until patients have undergone treatment for hyperthyroidism, there may be emergent scenarios where one cannot wait. In these situations, drugs causing stimulation of the sympathetic nervous system should be avoided. These include anticholinergics, such as atropine or glycopyrrolate, and ketamine.
Similar to hypothyroidism, titratable drugs that are short-acting are advised. Opioids and benzodiazepines make good premedications for patients with hyperthyroidism, but may not offer enough sedation, particularly in aggressive cats. Alpha-2 agonist use is controversial. It may be beneficial since it causes an overall reduction in sympathetic tone, but it may further increase blood pressure and cardiac work through initial vasoconstriction. Acepromazine should be used with caution since its long-lasting effects and vasodilation could lead to cardiovascular collapse.
- Q. What are appropriate induction agents to use in patients with hyperthyroidism?
- A. Ketamine increases myocardial oxygen demand and can promote tachyarrhythmias. It should be avoided in patients with hyperthyroidism. Propofol, alfaxolone, and etomidate are all suitable choices. Dose-dependent vasodilation from propofol administration can lead to an unexpected drop in blood pressure. This can be minimized by adding a co-induction agent, such as fentanyl (3–5 mcg/kg). Chamber or mask inductions should be avoided in patients with hyperthyroidism. Not only can they cause profound vasodilation and hypotension, but the stress the induction causes can lead to catecholamine release and sudden cardiac arrest.
- Q. Adjunct analgesia?
- A. Additional opioids in the form of intermittent boluses or constant rate infusion (CRI) can help to minimize inhalant concentrations and decrease release of catecholamines during surgery. Loco-regional techniques, such as ring blocks, should be utilized when possible to minimize stress response.
- Q. How should anesthetic management of the hyperthyroid patient be handled?
- A. Pre-operative preparation should include crystalloid administration with stabilization of all electrolytes. Stress should be minimized through good premedication and gentle handling. Pre-oxygenation is especially important in these patients because of high O2
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- A. The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are synthesized in and released from the thyroid gland in response to thyroid releasing hormone (TRH) and thyroid stimulating hormone (TSH) from the hypothalamus and anterior pituitary respectively.
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