Hyperthyroid anaesthesia in felines

Hyperthyroidism is characterised by excessive production of thyroid hormones (T3 and T4), which have detrimental effects in several body systems. Many hyperthyroid patients have other concurrent diseases and may require anaesthesia for diagnosis and treatment. Anaesthesia may also be needed to remove one or both thyroid glands.

Anaesthesia for hyperthyroid cats is associated with increased morbidity and mortality. This article aims to explain how a careful study of each individual patient to design an anaesthetic protocol may minimise the anaesthetic risks.

Pathophysiology and clinical presentation

Around 70 per cent of cats diagnosed with hyperthyroidism present with bilateral involvement of the thyroid gland, while in the remaining 30 per cent, the problem is unilateral.

The majority of cases are benign thyroid adenomas, but a small percentage are diagnosed with malignant carcinomas (one to two per cent).

Genetic and environmental factors have been linked to the development of hyperthyroidism, but the aetiology is still unclear¹. The excessive production and release of thyroid hormones into the circulation causes a multisystemic syndrome.

• Central nervous system: behavioural changes, such as hyperactivity, aggression, sleep disorders and vocalisation.

• Muscular system: weakness and muscle wastage.

• Gastrointestinal system: weight loss, polyphagia, vomiting and diarrhoea.

• Renal system: polyuria and polydipsia.

• Respiratory system: dyspnoea and tachypnoea.

• Cardiovascular system.

The direct effect of thyroxine on the myocardium, plus the increased plasma catecholamines, usually leads to mild to moderate hypertrophic cardiomyopathy.

Clinical signs are tachyarrhythmias, heart murmurs and gallop rhythms. In severe cases, pleural effusion and pulmonary oedema develop as a result of heart failure. Hypertension (systolic blood pressure above 170mm Hg) is also frequently diagnosed.

Three treatment modalities are available for hyperthyroidism:

• medical (methiazole or carbimazole);

• surgical (unilateral or bilateral thyroidectomy); and

• radioactive iodine.

If anaesthesia is required, the patient should ideally be stabilised previously. Normally, two to six weeks of medical treatment with methimazole at 2.5mg to 5mg twice a day will return the cat to a euthyroid state, or at least reduce the circulating levels of thyroid hormones. Only in emergency cases should an unstable hyperthyroid cat be anaesthetised.

Anaesthetic considerations

When anaesthetising a hyperthyroid cat, it is of paramount importance to collect some data and perform a thorough physical examination. Initially, age, gender and weight should be noted, and the cat should be kept and examined in a quiet room separate from dogs. Restraint should be minimised as far as possible during physical examination and the collection of blood for analysis. If the patient struggles and signs of respiratory distress appear, it is wise to postpone any handling and let the cat rest in a quiet and, ideally, oxygenenriched environment. The situation can get even more complicated as, very often, these are hyperactive and nervous cats that cope very badly with stress. Any struggling will result in an increase in oxygen demand, leading to potentially fatal arrhythmias.

An accurate record of current medication is also essential. All thyroid (such as methimazole) and cardiovascular (such as atenolol) drugs should be given on the morning of the anaesthetic.

Any abnormal haematology and biochemistry results will have to be taken into account when designing the anaesthetic protocol. The most common abnormalities found are elevation of alanine aminotransferase and alkaline phosphatase, although they do not usually represent significant changes in hepatic function. Some cats present with mild azotaemia, indicating dehydration and impaired kidney function. A chronic increase in plasma levels of thyroxine promotes changes in the myocardium, leading to hypertrophy – mainly of the left ventricular free wall and ventricular septum.

Hyperthyroid cats also have chronically elevated catecholamine plasma levels that, together with myocardial disease, promote tachyarrhythmias (sinus tachycardia and supraventricular and ventricular arrhythmias). ECG, thoracic radiographs and echocardiography are usually indicated before anaesthesia, to assess the cardiovascular function. Anaesthetic management aims to decrease the myocardial oxygen demand, maintain cardiac output and prevent arrhythmias.

Many of these cats have poor body condition due to ongoing weight loss. This could aggravate heat loss during anaesthesia, so good perioperative temperature management should be established.

Pre-anaesthetic medication

The aims of pre-anaesthetic medication are contradictory. Moderate to profound sedation will facilitate a stress-free preanaesthetic handling. However, profound sedation is often associated with marked cardiorespiratory depression.

A combination of a sedative and an opioid (morphine 0.25mg/kg IM or buprenorphine 0.01mg/kg to 0.02mg/kg IM) provides sedation and analgesia, facilitating perioperative management and reducing stress.

Benzodiazepines (midazolam 0.2mg/kg to 0.3mg/kg IM) provide moderate sedation and anxiolysis, with minimal cardiovascular depression. This could be the preferred option in the most unstable and debilitated patients; however, the level of sedation is sometimes inadequate. Cepromazine (0.05mg/ kg IM) is a more potent sedative and also has antiarrhythmic properties. The disadvantages of this treatment are a lack of analgesia, hypothermia, low blood pressure and insufficient sedation in fractious cats.

Alpha-2 agonists (such as dexmedetomidine 2.5µmg/kg to 10m/kg IM) are potent sedatives and analgesics, but are known to cause marked cardiovascular changes. Low doses of dexmedetomidine (plus an opioid) by the intramuscular route help to minimise the abrupt and profound cardiovascular effects of the alpha-2 agonist.

Dexmedetomidine has been shown to decrease plasma catecholamines and myocardial oxygen consumption. The use of dexmedetomidine has the potential to improve cardiac dynamics in hyperthyroid patients by decreasing heart rate, increasing diastolic filling time and improving myocardial oxygen metabolism²,³,⁴. The combination of dexmedetomidine and morphine provides reliable sedation, analgesia and a marked isoflurane sparing effect, contributing to a balanced anaesthetic.

Induction and maintenance

Before induction of anaesthesia, a peripheral venous catheter is placed to allow the administration of fluids and drugs.

A high concentration of oxygen, delivered by a face mask, prevents episodes of hypoxia and associated arrhythmias during induction.

A rapid sequence of induction and endotracheal intubation is desirable, and can be easily achieved with low doses of propofol or alfaxalone. Although ketamine has a reputation for being a cardiovascularly “safe” anaesthetic, it is contraindicated in hyperthyroid patients because it can further increase sympathetic tone and myocardial oxygen consumption, thus promoting arrhythmias.

Finally, the larynx should be desensitised with lidocaine spray prior to the introduction of the endotracheal tube. Anaesthesia can be maintained with isoflurane or sevoflurane on 100 per cent oxygen. All volatile agents cause dose-dependent cardiorespiratory depression, but isoflurane may be preferred due to its anti-arrhythmogenic properties. Nitrous oxide can be added to provide analgesia and reduce the amount of halogenated agent needed.

However, it causes sympathetic stimulation that can lead to ventricular arrhythmias in an already predisposed patient. During anaesthesia, it is advisable to maintain an adequate depth of anaesthesia to prevent catecholamine release, leading to arrhythmias. However, an excessive amount of a volatile agent may lead to undesirable cardiovascular depression.

Many of these patients maintain acceptable ventilation during anaesthesia, but artificial ventilation can be instituted (if required) to maintain normocapnia.

The use of opioid infusions during the anaesthetic period may contribute to a more balanced anaesthetic, with little effect on the myocardial performance and blood pressure. Fentanyl can be used either in intravenous boluses (2µmg/kg to 5µmg/kg) every 10 to 20 minutes or as a constant-rate infusion (5µmg/kg/h to 10mµ/kg/h).

Complications

During anaesthesia, attention has to be paid to anaesthetic depth, cardiovascular variables and temperature. Ventricular and supraventricular tachyarrhythmias occur frequently, so a continuous electrocardiogram is very useful.

Arrhythmias are prevented or treated by maintaining good oxygenation and normocapnia in the first instance. Beta blockers like esmolol (0.05 to 0.1mg/kg slow IV bolus) can be used as a second-line treatment if the tachyarrhythmia persists.

Haemoglobin saturation and pulse rate is monitored by means of a pulse oximeter. Capnography provides complementary information about metabolic status and cardiovascular, ventilatory and breathing system function. Blood pressure can be easily measured by non-invasive methods. Oscillometric methods tend to be less accurate in cats, so Doppler blood pressure testing is generally preferred. The cuff (normally size two) is placed just under the elbow. A small patch of skin is clipped on the ventral carpus and ultrasound gel and the Doppler probe are placed on top of the artery. Measures of systolic blood pressure, as well as information about pulse rate and rhythm, are readily obtained with this method.

Invasive blood pressure can also be measured where facilities are available. Mean blood pressure should be ideally maintained around 70mmHg to 80mmHg (or systolic blood pressure more than 120mmHg) to promote adequate blood supply to the main organs (including the brain, heart and kidneys).

The risk of hypothermia is high in hyperthyroid patients, due to their small size (increased body surface to mass ratio) and poor body condition. Temperature can be constantly monitored by means of an oesophageal temperature probe or, alternatively, taking the rectal temperature at regular intervals. Prevention of heat loss is always more effective than re-warming the patient.

The environmental temperature has to be controlled, ideally from pre-anaesthetic medication. If surgery is required, a good insulation from the table has to be provided. The area has to be clipped, with scrubbing solutions used (restricted to the minimum necessary).

Active warming devices, such as waterbeds and hot air pumps, are very effective in these cases. Normothermia helps to speed up recovery and prevents shivering, which increases oxygen demand.

Postoperative care

In the postoperative period, analgesia has to be continued. If renal function is normal, a NSAID is indicated (such as meloxicam at 0.3mg/kg IV/SC). Buprenorphine (0.01mg/kg to 0.02mg/kg) has demonstrated efficacy and versatility by different routes of administration in cats. It also has a prolonged duration of action, with minimal side effects, so it is the author’s first choice.

Recovery from anaesthesia should happen in a warm and calm environment. After thyroidectomy, attention must be paid to the surgical area in case swelling and haematoma develop, as this could obstruct the upper airway. Hypocalcaemia occurs frequently after bilateral thyroidectomy, due to parathyroid gland or blood supply damage during the surgical procedure. Ionised calcium should be measured at least twice a day for five days, with calcium and vitamin D supplements provided if necessary.

Hyperthyroid cats are challenging anaesthetic patients that demand careful management from the pre-anaesthetic physical examination to recovery from anaesthesia.

• The author would like to thank his colleagues for their assistance in writing this article.

References

• 1. Peterson M E (2000). Hyperthyroidism. In Ettinger S J and Feldman E C (eds). Textbook of Veterinary Internal Medicine (5th edn), Saunders: 1,556-1,573.
• 2. Murrell J C and Helebrekerst L J (2005). Medetomidine and dexmedetomidine: A review of cardiovascular effects and antinociceptive properties in the dog, Veterinary Anaesthesia and Analgesia, 32(3): 117-127.
• 3. Lamont L A et al (2001). Cardiopulmonary evaluation of the use of medetomidine hydrochloride in cats, American Journal of Veterinary Research, 62(11): 1,745-1,749.
• 4. Lawrence C J and others (1996). The effect of dexmedetomidine in nutrient organ blood flow, Anaesthesia and Analgesia 83: 1,160-1,165.

Further reading

• Fox P R, Broussard J D and Peterson M E (1999). Hyperthyroidism and other high output states. In Fox P R, Sisson D and Moise N S (eds), Textbook of Canine and Feline Cardiology (2nd edn), Saunders: 781-789.
• Johnson C and Norman E (2007). Endocrine disease. In Seymour C and Duke-Novakovski T (eds). BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2nd edn): 281-282.