Endocrine disorders are some of the most common diseases we face, especially in our senior patients.

Whether you need to anaesthetise your patient to investigate their disease, or they’re a stable patient who needs an unrelated procedure (e.g. a dental), knowing how to anaesthetise and monitor them safely is vital.

In order to do this, we need to understand how each endocrine disease affects our anaesthetic considerations, and how to avoid the common complications seen when anaesthetising endocrine patients.

In today’s post, we’ll chat through the 5 most common endocrine disorders we encounter in practice, and how they impact anaesthesia - as well as what you need to know when planning anaesthetics in these patients.

PS. If you want to know more about managing endocrine patients, check out the guide to endocrine nursing here.

Hyperthyroidism

Hyperthyroidism is a condition causing the excessive secretion of thyroid hormones (T3 and T4). It’s common in our senior cats, and is caused by a benign or malignant tumour on one or both thyroid glands.

When anaesthetising patients with newly-diagnosed hyperthyroidism, we need to stabilise them first (unless it’s an emergency). 

Cardiovascular function can be severely affected by hyperthyroidism, significantly increasing the anaesthetic risk these patients face. By getting treatment on board for a few weeks before anaesthetising them, we minimise this risk.

Pre-GA Considerations

Hyperthyroidism causes:

• Cardiac hypertrophy, cardiac murmurs and gallop rhythm 

• Hypertension

• Tachypnoea

• Vomiting

• Poor BCS/MCS

• Behavioural challenges

• Increased potential for release of catecholamines (e.g. adrenaline)

Many of these effects improve or are reversible entirely after treatment begins.

Hyperthyroidism can also mask renal disease, by increasing the glomerular filtration rate (GFR) and therefore maintaining renal blood flow. However, after treating hyperthyroidism and therefore reducing heart rate and blood pressure, the GFR then drops.

This means after treatment for hyperthyroidism has started, renal function should be reassessed as the treatment of hyperthyroidism can reveal concurrent renal disease. If chronic kidney disease is present, this will impact anaesthetic planning.

Hypertension that persists after treatment for hyperthyroidism should be treated before anaesthetising the patient - so we want to reassess blood pressure after the T4 levels improve.

We also need to consider the behavioural status of these patients. We know they can be a little spicy, even after their disease has been stabilised. Using good, fear-free/cat friendly practices is vital for a smooth anaesthetic period.

So keep handling minimal, and consider IM premedication where an IV catheter cannot be placed consciously. We may also need to get pre-hospital medications (e.g. gabapentin) on board to provide anxiolysis on the way to the hospital.

Monitoring and Maintenance

Additional considerations when maintaining/monitoring hyperthyroid patients under GA include temperature and cardiovascular status.

As our hyperthyroid cats have often marked reductions in body and muscle condition scores, they are at a higher risk of developing hypothermia under anaesthesia. So we want to get pre-warming on board before GA if possible, continue careful active heating throughout the procedure, and monitor the temperature closely. Options for this include:

• Using baby socks and/or bubble wrap around the limbs and tail to minimise heat loss from extremities

• Using active heating aids (e.g. careful use of heat pads, forced air warming devices or ‘Hot Dog’ warming blankets)

• Using insulating/reflective material such as blankets, bubblewrap and foil blankets to trap heat / reflect lost heat back towards the patient.

We also need to minimise cardiac work as much as possible throughout anaesthesia, so avoiding stress, or medications that cause tachycardia, should be an important anaesthesia consideration. The goal is, where possible, to keep the patient’s heart rate similar under GA to the resting/pre-anaesthetic rate.

Hypothyroidism

Hypothyroidism is the inadequate secretion of thyroid hormones, causing a decrease in the body’s metabolic rate. Patients often present with non-specific and varied clinical signs but can be severely unwell on presentation, including in myxoedema coma (more on that specific complication here).

Pre-GA Considerations

The slowing of our hypothyroid patient’s metabolism affects multiple body systems - including a particularly important system for us to consider: the heart.

These patients usually experience:

• Lethargy

• Hypothermia

• Reduced metabolism of drugs

• Impaired cardiac contractility

• Hypotension

• Bradycardia

• Obesity

These patients may need lower doses of sedative agents/maintenance agents, since they have a reduced ability to metabolise them.

Hypothyroidism should, wherever possible, be treated/stabilised before anaesthesia (unless the procedure is an emergency), allowing the metabolic rate to normalise and minimising the effects on organ systems such as the heart.

Patients with hypothyroidism can also present with laryngeal paralysis, and if this is the case, the risk of airway occlusion, stridor and hypoxia is high. This further increases anaesthetic risk in these patients, and can make intubation and recovery challenging. If your patient has laryngeal paralysis:

• Prepare a range of ET tubes including small sizes, tube stylets and a laryngoscope

• Prepare rigid urinary catheters with ET tube ends, in case an ET tube cannot initially be passed

• Consider having a bronchoscope on hand (if you have one) to allow easier visualisation of the larynx, and to guide intubation

• Have an emergency airway kit on standby during recovery

Monitoring and Maintenance

Hypothyroid patients have a reduced ability to thermoregulate due to their suppressed metabolism. This means they are at a higher risk of hypothermia and can take longer to return to normal body temperature during/after an anaesthetic.

These patients should be pre-warmed where possible. Active heating should be continued throughout anaesthesia and in recovery, until the patient is persistently normothermic.

Hypercortisolism (Cushing’s Disease)

Our adrenal glands are responsible for secreting hormones such as cortisol and aldosterone. Cortisol is a hormone heavily involved in the stress response. This is the body’s way of dealing with things like illness, pain and physiological stress - therefore, our bodies rely on this system heavily during anaesthesia/surgery.

In Cushing’s disease, there is an overproduction of cortisol (glucocorticoids). The excessive levels of this steroid hormone can cause:

• Muscle weakness

• Thinning and fragile skin (worth noting for incisions, clipping and catheter placement!)

• Abnormal coagulation function

• Abdominal distension with abdominal fat deposits

• Hypertension

Pre-GA Considerations

Maintaining normal blood pressure is an important consideration in cushingoid patients - so we ideally want to avoid medications that will cause significant vasodilation or hypo/hypertension. 

Monitoring, Maintenance and Recovery

Regular blood pressure monitoring is essential throughout anaesthesia, so that any hypotension can be detected and corrected at an early stage.

Respiratory function can also be impacted by Cushing’s disease, with weakness of the respiratory muscles and abdominal distension the culprits for this. Monitoring SpO2, ETCO2, respiratory rate and effort are important considerations - our patients may need positive-pressure ventilation or repositioning. This will help avoid the distended abdomen compressing the diaphragm, and ensure the lungs have room to expand.

In recovery, we want to get our patients up, mobilising and recovering well, to avoid issues associated with coagulation (e.g. pulmonary thromboembolism). 

Hypoadrenocorticism (Addison’s Disease)

Hypoadrenocorticism is characterised by low levels of circulating cortisol +/- aldosterone. This significantly increases anaesthetic risk, since our patients lack the hormones needed to deal with physiological stress. In addition, where aldosterone levels are low, significant abnormalities can be seen, including:

• Hypovolaemia

• Hypotension

• Azotaemia

• Hypoglycaemia

• Hyponatraemia

• Hyperkalaemia

• Cardiac arrhythmias

• Bradycardia

Pre-GA Considerations

A patient in an Addisonian crisis should not be anaesthetised unless it is a life-threatening emergency. Procedures should, if possible, be delayed until the patient has been stabilised.

After stabilisation, patients should be admitted as normal for their procedure, with their steroid medications given as normal (we may increase the dose of these in the days leading up to hospitalisation in some cases).

We also need to check electrolytes before making an anaesthetic plan, to ensure these are normal.

Following this, most anaesthetic considerations are normal for these patients. We do, however, give them steroids with their premedication. This is because of the stress anaesthesia will place on the body, and their impaired ability to respond to this. Common medications used include dexamethasone, hydrocortisone and prednisolone. These may be considered after surgery if the procedure is major, or the patient requires prolonged hospitalisation.

Diabetes Mellitus

Diabetes mellitus is the impaired release of, or action of, insulin in the body. It differs between dogs and cats, but the net result is the same - our patients are hyperglycaemic, PU/PD, polyphagic and have weight loss.

Where DM goes undiagnosed or treated diabetic ketoacidosis results. This is the combination of hyperglycaemia, ketosis and metabolic acidosis, and DKA patients are usually severely unwell, with marked hydration, acid-base and electrolyte disturbances. 

Unless it’s really needed, we want to avoid anaesthetising DKA patients - instead waiting until they have been stabilised and are on a slower-acting insulin (usually given either every 24 or 12 hours).

Pre-GA Considerations

Diabetes has many significant effects on our patients, including:

• Decreases in oxygen transport, due to glucose molecules interfering with haemoglobin and oxygen binding

• Increased risk of hypothermia, as patients cannot vasoconstrict as easily due to autonomic nervous system dysfunction

• Increased risk of hypotension, due to lack of vasoconstriction

• Hypoglycaemia (where present e.g. due to starving but still giving a full dose of insulin) - if seen, this can cause seizures, bradycardia, respiratory failure and hypotension. All of these signs can be hidden when the patient is anaesthetised!

Insulin and Feeding

Timings and doses of meals and insulin injections are vital pre-anaesthetic considerations. We need to know:

• Which type of insulin the patient normally receives, how much, and at what times

• When insulin was last given and the dose used

• How much food the patient normally eats, which food, and what times

• When the patient was last fed and how much they ate

• Whether the patient receives any other medications, and when they were last given

Our goal is to return our patient’s diabetic control to normal as soon as possible after their anaesthetic. Even if a patient’s diabetes is stable, we’re likely to cause some destabilisation for a short period.

As a general rule:

• Feed the patient and give insulin as normal the night before

• Do not feed the patient or give any insulin prior to admission the morning of the GA

• Check the patient’s blood glucose on admission to the hospital

• Depending on the BG result, give either 0, 25 or 50% of the patient’s normal dose

• If the patient is on twice daily insulin and their procedure is in the afternoon, they can have their normal insulin and a meal in the morning, then be fasted for at least 6 hours before their GA.

During the GA, monitor blood glucose regularly (every 30 minutes). We may need to administer a dextrose CRI (2.5-5% in Hartmann’s solution) if the patient’s blood glucose level drops under anaesthesia.

After the procedure, the patient should be fed and a half dose of insulin given (if they have skipped their morning dose). Their glucose levels should then be monitored regularly throughout the afternoon as needed, and at the insulin nadir.

Alpha-2s or Not?

Dexmedetomidine and medetomidine are insulin antagonists - which means they can cause hyperglycaemia. For that reason, we tend to avoid them in our diabetic patients where possible. 

The rest of our anaesthetic drug choices in diabetic patients are unchanged, and a protocol should be selected based on the patient’s temperament and pain level.

Monitoring and Maintenance

If your diabetic patient also has pancreatitis, we may need to avoid propofol for anaesthetic induction, since it is lipid-based. As pancreatitis can cause issues with diabetic control, we want to avoid further flare-ups in chronic pancreatitis patients, for example.

During anaesthesia, we need to keep an especially close eye on blood pressure and respiration/oxygen saturation. Keeping our arterial blood pressure between 100-150mmHg (systolic) or 60-100mmHg (mean) will help maintain perfusion to vital organs, including the pancreas. 

So that’s an overview of the top nursing considerations when anaesthetising endocrine patients! We need to consider the impact of our patient’s disease, stabilise any endocrine emergencies before anaesthesia, and maintain perfusion, blood pressure and temperature as much as possible. In our diabetics and Addisonian patients, we also have specific protocols regarding their medications - so advising our clients on this in advance is vital.

Is this what you’re currently doing when anaesthetising these patients? DM me on Instagram and let me know!

References

1. Fischer, B. L. 2017. Anaesthetic considerations for endocrine disease. [Online] Veterian Key. Available from: https://veteriankey.com/anesthetic-considerations-for-endocrine-disease/

2. Fischer, B. L. 2021. Endocrine Disease. In: Johnson, R. A., Snyder, L. B. C., and Schroeder, C. A. eds. Canine and Feline Anaesthesia and Co-Existing Disease. 2nd edition. Iowa: Wiley-Blackwell. 

3. Walsh, K. 2016. Anaesthesia for patients with endocrine disease. [Online] Vet Times. Available from: https://www.vettimes.co.uk/app/uploads/wp-post-to-pdf-enhanced-cache/1/anaesthesia-for-patients-with-endocrine-diseases.pdf