Many of the effects of hyperthyroidism can be counteracted by blocking sympathetic nervous system activity. β-blockers (eg, atenolol) may help reduce heart rate when administered several days before anesthesia.2,5 The nonselective β-antagonist propranolol has the added benefit of decreasing conversion of TT4 to TT3 peripherally and has been shown to lower TT4 levels following short-term administration.6 β-blockers decrease myocardial oxygen demand and allow for better ventricular filling, especially in situations of thyrotoxic cardiomyopathy that may result in impaired relaxation of the heart (diastolic dysfunction). Minimizing stressors in the hospital environment can also help subdue excessive sympathetic stimulation. Providing a quiet area specifically for cats, allowing owners to hold their pet during examinations, and premedicating patients before catheter placement may help facilitate this. Drugs that stimulate the sympathetic nervous system (eg, induction agents, ketamine, sympathomimetic drugs [eg, anticholinergics]) should be avoided in hyperthyroid patients.7
The ensuing tachycardia from these agents can shorten diastolic filling time, causing myocardial hypoxia and potentially fatal arrhythmias.
In this case, good preanesthetic sedation was desired to minimize stress and facilitate handling. Combining low doses of oxymorphone, midazolam, and alfaxalone can produce excellent sedation while minimizing adverse effects. Alfaxalone, an induction agent that works on the GABAA receptor, has similar cardiopulmonary effects as propofol (eg, vasodilation, dose-dependent apnea) but can be administered intramuscularly.
Preoxygenation is advised in patients with increased oxygen demand, as in this case, to help prevent hypoxemia until an airway can be established. In addition, placing monitoring equipment before induction allows the anesthetist to observe changes in the patient’s cardiovascular status and identify problems rapidly. In Option 1, use of an anesthetic chamber prevented the ability to monitor anesthetic depth or cardiac status and delayed identification of cardiac arrest.
After induction of anesthesia, Buttercup became hypotensive. Although hypertension is commonly associated with hyperthyroidism, significant hypotension can also occur from the combined peripheral effects of TT3 with anesthetic drugs that cause vasodilation (eg, alfaxalone, isoflurane). In Option 2, once anesthetic depth was determined to be adequate, a crystalloid bolus was administered during surgery preparation. Although a bolus of this size should be tolerated given the mild changes in Buttercup’s echocardiogram, other options could have included starting a fentanyl CRI and decreasing the isoflurane concentration further.
Although uncommon, acute thyrotoxicosis can occur with the patient under anesthesia, and the practitioner should be prepared. Having vascular access and emergency drugs drawn in the event of cardiac arrest can be potentially life-saving. β-blockers (eg, esmolol) and antihypertensives (eg, nitroprusside) are often used to control supraventricular tachyarrhythmias and hypertension, respectively.7 Administration of potassium iodate or iodinated contrast agents reportedly suppresses the thyroid gland from releasing preformed thyroid hormones through an autoregulatory mechanism.2,8 In patients like Buttercup controlled with a low-iodine diet, however, this could backfire, as these agents provide iodine to an iodine-deficient thyroid gland, potentially exacerbating thyroid hormone production and release. Of note, administration of these agents is not common nor without potential adverse effects. Glucocorticoids decrease thyroid hormone levels via multiple mechanisms in dogs and are currently recommended in the therapeutic treatment of humans with acute thyrotoxicosis.1,8 Although not reported—and understanding the potential risks in cats with pre-existing cardiac disease—this may offer another treatment option in cats in which thyrotoxicosis is suspected.
The goal of reducing sympathetic stimulation should continue through the recovery period. Maintaining normothermia, ensuring adequate volume status, and providing excellent pain management can all alleviate stress in the immediate postoperative period. In patients receiving β-blockers, continuing treatment for several days after surgery is advised. Managing the unregulated hyperthyroid patient can be challenging and unpredictable. When faced with a patient that may be experiencing acute thyrotoxicosis, carefully evaluating the situation and having an open conversation with the owner regarding anesthetic risk is prudent. Postponing an elective procedure to allow better preparation may be the best alternative.