No more kidney around: Anesthesia and the renally impaired cat
Pre-planning and basic diagnostics can take the stress out of anesthetizing cats with kidney problems in your veterinary practice.
(benevolente / stock.adobe.com) The kidney is a complex organ with three primary functions: filtration, reabsorption and secretion. The renal system, which the kidney is part of, filters blood by removing nitrogenous waste while preventing various solutes, proteins and blood cells from being excreted. The renal system also maintains the appropriate balance of sodium and water; regulates acid-base balance; upholds and regulates electrolyte homeostasis, bone metabolism and erythropoiesis; participates in controlling blood pressure and is involved in drug metabolism and excretion.1 As such, the kidney is an important organ to manage, preserve and maintain during anesthesia.
The kidney receives approximately 25 percent of cardiac output2,3 and consumes a large quantity of oxygen. Blood flow to the kidney (renal blood flow; RBF) is reflected in the glomerular filtration rate (GFR; the amount of blood passing through the glomerulus each minute). Renal autoregulation stabilizes the GFR despite large fluctuations in systemic blood pressure. Maintaining constant perfusion pressure preserves and protects the kidney during bouts of hyper- or hypotension. The range at which autoregulation occurs approximately 80-180 mm Hg.4 Due to their inherently unique vasculature, the kidneys may experience local tissue ischemia and hypoxia regardless of normal organ blood flow. Blood pressure support by way of fluid therapy is recommended in the renal patient.
There is no single test that is an ideal way of assessing renal function. Instead, performing a variety of tests can offer the most complete picture. When running traditional tests, a 75 percent decrease in renal function must be present before abnormalities are noticed on a blood chemistry profile.5 Even mild increases in serum blood urea nitrogen (BUN) and creatinine concentrations may indicate severe disease. The biomarker test, symmetric dimethylarginine (SDMA-Idexx) can detect as little as a 25 percent decrease in GFR, alerting clinicians to early renal disease. Evaluation of all middle-aged and older patients and those with suspected renal disease is recommended and test trends are more useful than singular measurements.6
Renal patients may present without clinical signs or they may be dehydrated, anemic, azotemic, anorectic or have acid-base abnormalities. A thorough physical exam and detailed history should precede a complete blood count (CBC), serum chemistry profile and urinalysis (at minimum, a urine specific gravity) plus the SDMA biomarker test for all suspect renally impaired patients. These tests provide vital information that can reduce patient morbidity and mortality. For example, azotemia increases the sensitivity of anesthetic drugs by affecting the permeability of the blood brain barrier.7 Having this type of data prior to choosing and delivering drug dosages can help to avoid overdosing a patient and increasing morbidity.
Some anesthetic drugs can have deleterious effects on renal physiology but for the most part tend to be well tolerated at appropriate doses. Phenothiazines (e.g. acepromazine) can cause vasodilation and subsequent hypotension that may dip below the range of autoregulation.8,9 Appropriate doses along with careful monitoring of blood pressure and treatment of hypotension is essential. Alpha-2 adrenergic agonists (e.g. medetomidine, dexmedetomidine) are used cautiously in cats with renal disease because this class of drug has the potential to cause up to a 60 percent decrease in cardiac output.10 In addition, a 50 percent decrease in RBF is seen in dogs alongside an increase in GFR but no definitive evidence exists to support this in cats; however, it would be prudent to assume these mechanisms may be similar in cats as well. Alpha-2 agonists should be avoided in obstructed cats due to their diuretic effect and should be used with caution in all other cases of renal impairment if possible.
Benzodiazepines (e.g. diazepam, midazolam) are well tolerated in renally impaired cats; nevertheless, this class of drugs can cause paradoxical excitement and should be used alone with caution in cats or young, healthy animals. Co-administration with an opioid is recommended. Opioids (e.g. morphine, hydromorphone, fentanyl, buprenorphine, methadone, butorphanol) are generally safe for renally impaired cats11 and their use is often beneficial. Opioid administration may help decrease the sympathetic response associated with pain and surgery, thereby minimizing renal vasoconstriction. The addition of opioids can also decrease the amount of inhalant necessary to keep the patient immobilized, avoiding unnecessary hypotension.12
Nonsteroidal anti-inflammatory drugs (NSAIDs) have the potential to cause ill effects in renally impaired patients but multiple studies evaluating COX-2-specific NSAIDs in healthy patients with moderate hypotension has not been established.13-16 Some evidence exists that supports the careful use of NSAIDs in renally impaired cats.
Induction agents like propofol and alfaxalone are good choices for anesthetic induction since the negative effects associated with these drugs are transient and generally well-tolerated.17-19 The use of premedication is recommended to reduce the dosage necessary to induce anesthesia. Ketamine as an induction agent is often avoided in the renally impaired cat mainly because this drug partially relies on renal excretion. Compromised renal function may lead to impaired drug elimination.20 Subanesthetic doses of ketamine as an adjunctive medication are considered safe.
Inhalants should always be used sparingly, but especially in renally impaired patients. Inhalants are potent vasodilators which may cause hypotension and a reduction in RBF. The effects of inhalants are usually rapidly reversed once discontinued, helping its safety profile.
When anesthetizing a renally impaired cat, it becomes more important to focus on the management of the individual and the sequelae of their disease in order to avoid exacerbation of their illness. The general rule of thumb is to stabilize before you anesthetize. It is also understood that this is not always possible since renally impaired cats do sometimes present an emergency. Weigh the value of the procedure against the risks of the event.
Begin improving hydration by replacing fluid loss with a balanced electrolyte solution, keeping a close eye on electrolytes and blood pH. Intraoperatively, monitoring and supporting blood pressure is essential. Clinicians need to be prepared and efficient to complete any procedure in a timely manner. Positive ionotropic support may be indicated to maintain arterial blood pressure and subsequent renal blood flow well within the range of autoregulation.21
As is true in all surgical procedures, pain should be adequately treated. Untreated or undertreated pain may cause renal vasoconstriction and a reduction in renal blood flow. Sufficiently treating pain using a balanced approach is best practice and can be achieved by utilizing locoregional techniques and adjunctive medications to keep inhalant settings low whilst maintaining adequate blood pressure. In the recovery period, renal patients should be maintained on intravenous fluids to ensure adequate fluid volume and diuresis. Body temperature should be supported and oxygen supplemented when deemed necessary.
A renally impaired cat doesn't need to be a stress-inducing challenge when anesthesia is needed. Basic diagnostics and pre-planning can prove beneficial. Proper dosages, diligent monitoring and fluid support can mean the difference between an animal that just survives the procedure and one that thrives.
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Stepien RL, Bonagura JD, Bednarski RM, et al. Cardiorespiratory effects of acepromazine maleate and burprenorphine hydrochloride in clinically normal dogs. Am J Vet Res 1995;56:78-84.
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Frendin JH, Bostrom IM, Kampa N, et al. Effects of carprofen on renal function during medetomidine-propofol-isoflurance and anesthesia in dogs. Am J Vet Res 2006;67:1967-1973.
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Bostom Im, Nyman G, Hoppe A, et al. Effects of meloxicam on renal function in dogs with hypotension during anesthesia. Vet Anaesth Analg 2006l33:62-69.
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IIlkie JE, Pascoe PJ, Haskin SC, et al. Cardiovascular and respiratory effects on propofol administration in hypovolemic dogs. Am J Vet Res 1992;53:2323-7.
Warne LN, Beths T, Whittem T, et al. A review of the pharmacology and clinical application of alfaxalone in cats. Vet J 2015;203(2):141-148.
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Kristen Cooley, BA, CVT, VTS (anesthesia & analgesia) is an instructional specialist at The University of Wisconsin School of Veterinary Medicine and a consultant in Veterinary Anesthesia Support and Training.