We anaesthetise patients with renal disorders all the time. Whether that’s for an ultrasound scan, an oesophagostomy tube or a central line to support them during illness, or for a routine procedure like a dental, veterinary nurses are heavily involved in supporting these patients under GA. 

In order to anaesthetise these patients safely, we need to understand how renal disease affects our patients, and how this is impacted by anaesthesia. And in this post that’s exactly what we’ll chat through! By the end of this post, you’ll have a better understanding of what things to specifically consider when monitoring renal patients under GA.

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Renal disease and anaesthesia

Renal disorders have a variety of effects on our patients, from altering drug metabolism to shifts in fluid balance, acid-base status and electrolyte levels.

We can see different types of renal disease, in varying severity. Patients can present with severe systemic illness significantly increasing anaesthetic risk, or have little or no clinical signs.

In general, we divide renal disease into two main types: chronic kidney disease and acute kidney injury.

Chronic kidney disease

Chronic kidney disease is defined as a reduction in renal function that has persisted for at least 3 months. It is a permanent, progressive condition where more and more nephrons in the kidneys become non-functional.

The signs of CKD can vary from no signs or very mild signs (e.g. a little PU/PD), to severe systemic signs including:

• Weight loss

• Anorexia

• Nausea

• Vomiting

• PU/PD

• Dehydration

• Lethargy

CKD is staged according to the patient’s creatinine (and/or SDMA) level, blood pressure and urine protein:creatinine ratio.

Acute kidney injury

Acute kidney injury patients usually present as emergencies and have intensive treatment and nursing care requirements. They have a rapid onset, potentially reversible, severe decline in renal function due to a specific underlying cause. These causes include:

• Toxins (e.g. ethylene glycol, lilies, grapes/raisins)

• Infections (e.g. leptospirosis)

• Obstructions (e.g. urethral or ureteral obstruction)

AKI treatment is aimed at managing the underlying cause of the renal injury (e.g. relieving a urethral obstruction) and correcting fluid, electrolyte and acid-base abnormalities, alongside supportive care.

Consequences of renal disease include central nervous system and myocardial depression, coagulopathies, pneumonitis and pericarditis, and increased susceptibility to infection.

In addition, these patients often present with dehydration, metabolic acidosis, potassium disorders (hypokalaemia in CKD; hyperkalaemia in AKI), anaemia and systemic hypertension - all of these things will significantly impact anaesthetic risk!

The exact anaesthetic considerations will vary depending on the individual patient and the type of renal disease present, as well as their presenting signs and concurrent diseases (if applicable). This is why performing a thorough pre-anaesthetic assessment and creating a tailored anaesthetic plan is vital in any medical patient.

Pre-anaesthetic planning

Prior to anaesthesia, the patient should receive a full physical examination, particularly looking for signs of concurrent disease such as cardiac disease or hyperthyroidism, which will also affect anaesthetic planning.

If not performed recently the following diagnostic tests should be performed to assess and stage/grade the patient’s renal disease:

• Biochemistry

• Electrolytes

• Haematology

• Urine analysis

• Blood pressure

• +/- Blood gas analysis if appropriate

Following this, an appropriate anaesthetic plan is formulated.

Before we begin…

Before formulating our anaesthetic plan, we want to correct any fluid, acid-base or electrolyte imbalances present.

This means:

• Administering a fluid bolus to a hypovolaemic patient;

• Calculating percentage dehydration and fluid deficit, and correcting this in a dehydrated patient

• Treating hyperkalaemia if present - e.g. with fluid therapy, calcium gluconate, neutral insulin and/or glucose

• Treating severe hypokalaemia with supplemented intravenous fluid therapy

Following initial stabilisation (or stabilising the patient as best as we possibly can if the procedure is an emergency), we can then begin formulating our anaesthetic plan.

Which drugs do we use?

There is no one anaesthetic drug combination that is best when it comes to anaesthetising renal patients. Instead, our main focus is on maintaining renal perfusion and preventing worsening of renal function due to low renal blood flow.

The kidneys receive approximately 25% of a patient’s cardiac output, and consume large amounts of oxygen. Adequately perfusing them is therefore a vital consideration.

Renal blood flow determines glomerular filtration rate; this is the rate by which blood is filtered through the kidney (and therefore the rate at which waste products are removed). Hypoperfusion and a drop in GFR can cause an AKI, or worsen existing renal disease.

To minimise this, we need to keep our patient’s blood pressure between 80-180mmHg. At this level, the kidney can autoregulate filtration, maintaining GFR as much as possible.

All anaesthetic drugs will have an impact on blood pressure/cardiac output and therefore renal blood flow to some extent. However, most are well tolerated when used at appropriate doses. Drugs causing significant vasodilation should be used at lower doses or potentially avoided entirely. These include:

• Phenothiazines such as ACP

• Alpha-2 adrenoreceptor agonists such as dexmedetomidine

• Propofol

• Volatile (inhalant) agents

Benzodiazepines and opioids have minimal effects on renal blood flow and are safe choices for patients with renal disease.

Regardless of the protocol used, multimodal anaesthetic techniques (including local or regional techniques if appropriate) should be employed to reduce doses of volatile agents required, and minimise hypotension.

Anaesthetic monitoring

When monitoring renal patients, particular attention should be paid to blood pressure, heart rate, temperature, oxygenation and ventilation.

Blood pressure

Mean arterial pressure should be maintained above 80mmHg throughout anaesthesia to maintain normal renal blood flow. This is achieved using a combination of intravenous fluid therapy and minimising vasodilation under anaesthesia.

If necessary, medications to increase blood pressure (e.g. those causing vasoconstriction and/or tachycardia) should be administered to keep MAP above that 80mmHg cutoff value.

Temperature

Patients with renal disease often have low body condition, predisposing them to heat loss under anaesthesia. Continuous temperature monitoring should be used, if possible, or core body temperature measured every 5-10 minutes throughout anaesthesia.

Active warming aids should be used from the time of premedication to minimise heat loss, and insulation provided to retain body heat as much as possible.

Heart rate

Hyperkalaemia is especially common in AKI patients, and if present, causes significant effects on the heart. Progressive hyperkalaemia results in bradycardia and cardiac arrhythmias which can be fatal.

Hyperkalaemia should be managed as much as possible prior to anaesthesia, and alongside this the patient’s heart rate and rhythm should be carefully monitored. If available, a continuous ECG should be placed, and the trace assessed for evidence of:

• Tall or spiked T waves

• Bradycardia

• Absent P waves

• Wide and bizarre QRS complexes

Where tachycardia is present, patients should be assessed for signs of pain, hypercapnia, hypoxaemia or hypovolaemia, and this corrected as necessary.

Recovery considerations

Recovery in our renal patients is mostly routine. 

Active warming should continue until the patient can maintain their body temperature within normal range.

Intravenous fluid therapy should continue until hydration status normalises, and any ongoing losses are accounted for. 

Blood pressure assessment should be performed at appropriate intervals for the patient - if hypotension has been noted under anaesthesia, this should be assessed in recovery to ensure blood pressure remains normal. If the patient is hypertensive, blood pressure should be regularly reassessed to determine whether antihypertensive medications are needed, and if so, the effect these are having.

Pain should be assessed regularly in recovery, particularly if the patient has undergone surgery or has evidence of abdominal pain (e.g. in a cat with a urethral obstruction). Appropriate opioids should be administered as needed; in some cases, the use of non-steroidal anti-inflammatory drugs (NSAIDs) will be contraindicated (e.g. if significant azotaemia is present). Untreated - or undertreated - pain can cause vasoconstriction, resulting in changes to renal blood flow.

So as you can see, anaesthesia doesn’t have to be scary or particularly complex in our renal patients!

We can make our anaesthetic safer for our patients, and smoother for us, by performing a full pre-anaesthetic assessment, and creating a tailored anaesthetic plan. 

During the anaesthetic our main consideration is blood pressure and renal blood flow - so avoid medications that significantly affect this, use balanced and multimodal techniques and add in things like local anaesthetic to reduce your doses even further.

As your patient recovers, keep an eye on blood pressure, temperature and pain especially, and continue with intravenous fluids where indicated to support hydration.

What anaesthetic plans do you use for your renal patients? I’d love to know - drop me a DM on Instagram and let’s chat about it!

References

1. American Animal Hospital Association, 2019. AAHA Guidelines - Renal Diease [Online] AAHA. Available from: https://www.aaha.org/aaha-guidelines/aaha-anesthesia-guidelines-for-dogs-and-cats/anesthesia-with-comorbidities/renal-disease/

2. Cooley, K. 2019. No more kidney around: anaesthesia and the renally impaired cat [Online] DVM360. Available from: https://www.dvm360.com/view/no-more-kidney-around-anesthesia-and-renally-impaired-cat 

3. Mama, K. and Rezende, M. 2015. Anaesthesia for patients with renal disease [Online] Clinician’s Brief. Available from: https://www.cliniciansbrief.com/article/anesthesia-patients-renal-disease

4. McNerney, T. 2020. Filter & flow: anaesthesia for the feline with kidney disease [Online] UPenn. Available from: https://www.vet.upenn.edu/docs/default-source/penn-annual-conference/pac-2019-proceedings/companion-animal-track-2019/nursing-track-tue-2020/tasha-mcnerney---kidney-disease-anesthesia.pdf?sfvrsn=9ef6f2ba_2

5. Robertson S. 2015. Anaesthetic risks and management of patients with chronic renal failure [Online] VIN. Available from: https://www.vin.com/apputil/content/defaultadv1.aspx?pId=14365&id=7259246&print=1 

6. Schroeder, C A. 2015. Renal disease. In: Snyder, BC. and Johnson, RA. 1st ed. Canine and feline anaesthesia and co-existing disease. Iowa: Wiley-Blackwell.

7. Veterian Key, 2017. Anaesthesia for small animal patients with renal disease [Online] Veterian Key. Available from: https://veteriankey.com/anesthesia-for-small-animal-patients-with-renal-disease/