How can uremic anorexia be minimized?

What factors are associated with anorexia? Modification of diets so that they contain reduced quantities of protein, phosphorus, sodium and acid metabolites, and increased quantities of B-vitamins and caloric density is a cornerstone of therapeutic regimes for chronic renal failure (CRF). However, many patients with CRF may refuse to eat some or all of such diets offered to them.

Reduction in the palatability of renal failure diets associated with reduction in dietary protein, sodium and phosphorus is commonly incriminated as a major factor leading to inappetance. That unpalatability of diets is not the only factor involved can be surmised by the fact that patients with renal failure often show little interest in eating diets containing unrestricted quantities of these ingredients.

In fact, anorexia may be the primary abnormality prompting owners of dogs and cats with renal failure to seek the assistance of veterinarians.

Anorexia associated with renal dysfunction is not directly caused by renal lesions, but rather develops as a result of multiple metabolic deficits and excesses that develop as a result of decreased renal function caused by renal lesions. Anorexia, nausea and vomiting are manifestations of an interaction between:

• autointoxication caused by reduction of glomerular filtration and tubular secretion below that required to clear plasma of metabolic waste products.

• impaired tubular reabsorption of vital metabolites in glomerular filtrate.

• impaired renal production of some hormones.

• impaired renal degradation and elimination of some hormones (Table 1, p. 62).

Because nutritional support is the cornerstone of long-term management of patients with renal failure, management must encompass a plan to minimize inappetance.

Minimize inappetance

Table 1: Examples of some metabolic deficits and excesses contributing to anorexia, nausea and vomiting in patients with renal failure

If catabolic patients with renal failure do not consume their daily requirements of dietary nutrients, further catabolism characterized by metabolism of endogenous proteins for energy will follow. Catabolism of endogenous proteins will in turn augment production of protein catabolic wastes, which further contribute to anorexia.

How can veterinarians and clients enhance the desire of anorexic dogs and cats with CRF to consume diets modified to minimize metabolic deficits and excesses associated with renal failure? In addition to strategies designed to enhance the palatability of renal failure diets, our approach to this question places emphasis on recognition and treatment of the underlying causes of uremic anorexia (Table 1, p. 62; Figure 1, p. 68).

Figure 1: Multiple interacting factors may contribute to anorexia in patients with renal falure.

How can underlying abnormalities be corrected?

Dehydration of patients with oliguric, nonoliguric and polyuric renal failure is typically associated with multiple deficits and excesses in electrolyte and acid-base balance. Reduction of renal perfusion as a consequence of dehydration aggravates retention of metabolic waste products.

Fluid therapy

In addition, reduction of blood flow to the gastric mucosa may increase the risk of gastric dysfunction. It is not surprising that various combinations of these abnormalities would contribute to anorexia, nausea and vomiting. Consult standard textbooks for recommendations about deficit, maintenance and continuous components of fluid therapy.

Care must be used not to induce electrolyte disturbances by inappropriate use of replacement and maintenance fluids. Maintenance fluids differ from replacement fluids in that they generally have a much lower sodium concentration (40 mEq/L versus 130 mEq/L in lactated Ringer's solution), and a higher potassium concentration (13 mEq/L versus 4 mEq/L).

The reason for this difference is that daily maintenance requirements of sodium and potassium are not directly related to the serum concentration of sodium and potassium. Inappropriate use of replacement fluids for maintenance fluids could lead to hypokalemia and hypernatremia, which in turn could contribute to anorexia.

Patients that develop recurrent dehydration are candidates for subcutaneous fluid therapy to be administered at home. Owners my review information at www.felinecrf.org for further details.

Approximately 20 percent of untreated cats with polyuric CRF are hypokalemic (serum potassium less than 3.5 mEq/L). Dietary factors incriminated in the development of hypokalemia in uremic cats include inadequate potassium in the diet (it should be 0.6 percent dry matter), diets with acidifying metabolites and diets designed to contain reduced magnesium.

Hypokalemia

Dietary protein may also be a factor since dietary potassium requirement increases as dietary protein content increases. In our experience, hypokalemia is an uncommon manifestation of renal failure in dogs, except as an iatrogenic complication of fluid therapy. Irrespective of underlying causes, hypokalemia may be associated with anorexia, vomiting, weight loss, lethargy, hypokalemic nephropathy, generalized skeletal and smooth muscle weakness, mild cardiac arrhythmias and progression of renal failure.

Dietary potassium may be supplemented orally as gluconate or citrate salts. Potassium gluconate, available as tablets, a flavored gel or palatable powder (Tumil-K, Daniels Pharmaceuticals, Inc.) is commonly used as a potassium supplement for uremic cats with hypokalemia. Initial dosages range from 2 to 6 mEq per cat per day, depending on the size of the patient and magnitude of hypokalemia.

The maintenance dosage should be titrated on the basis of serially monitored serum potassium concentration. Acidosis is a major risk factor for hypokalemia and therefore should also be managed. Potassium citrate is a logical choice for acidotic hypokalemic cats (see section on acidosis).

Oral potassium chloride is not recommended since it may contribute to metabolic acidosis and is less palatable.

Metabolic acidosis is estimated to occur in two-thirds to three-quarters of dogs and cats with renal failure. It is associated with impaired ability of surviving nephrons to excrete hydrogen ions and reabsorb bicarbonate. Consequences of metabolic acidosis associated with renal failure include anorexia, nausea, vomiting, weight loss, lethargy, protein malnutrition, intolerance to dietary acids, hypokalemia, muscle weakness, bone demineralization, cardiac arrhythmias and possibly progression of renal failure.

Metabolic acidosis

Oral sodium bicarbonate has been the most commonly used alkalinizing drug to correct metabolic acidosis caused by renal failure. Because the effect of gastric acid on orally administered sodium bicarbonate is unpredictable, the dosage must be individualized. A commonly used initial dosage is 8 to 12 mg/kg body weight given every eight to 12 hours. The patient's response should be monitored by serially monitoring blood bicarbonate or serum TCO2 concentrations. Potassium citrate (Polycitra-K Syrup, Baker Norten; Urocit-K, Mission Pharmacal) may be used for hypokalemic acidemic cats (40 to 60 mg/kg/day divided into two subdoses). Other alkalinizing agents that may be considered for sodium intolerant patients that are not hypercalcemic include calcium carbonate and calcium acetate.

Hypoproliferative anemia is a common sequela to progressive CRF in dogs and cats. It is primarily caused by impaired renal production of erythropoietin. Other factors that contribute to the magnitude of the anemia include blood loss, decreased red blood cell survival time and nutritional deficiencies. Consequences of anemia include anorexia, weight loss, weakness and intolerance to cold.

Hypoproliferative anemia

Treatment of anemia associated with renal failure encompasses correction of nutritional deficiencies (protein, calories, vitamins B6 and B12, folic acid, niacin and iron), control of gastrointestinal hemorrhage (with H2-receptor antagonists and/or sucralfate). Severe anemia may warrant treatment with parenteral administration of recombinant human erythropoietin (Epogen, Amgen, Inc.). Consult appropriate references for specific details.

A PTH-mediated interference with the response of pancreatic insulin-secreting islet cells to dietary glucose has been incriminated as a mechanism of carbohydrate intolerance on uremic patients.

Hyperparathormonemia

Inadequate insulin to facilitate reduction of blood glucose in uremic patients may contribute to inappetance. Lowering PTH in uremic dogs and cats by administration of calcitriol has been reported to minimize anorexia (Rocaltrol, Roche). Consult appropriate references for specific details.

Deficiencies of thiamine and niacin may result in anorexia. Patients with renal failure are at risk for B-vitamin deficiency as a result of decreased appetites, vomiting, diarrhea and perhaps losses associated with polyuria. In these circumstances, administration of supplemental B-vitamins is logical. The daily B-vitamin requirement of normal cats is estimated to be six to eight times greater than that of dogs. If multiple vitamin supplements are used, caution must be used not to give excessive quantities of fat-soluble vitamins. There is no data to support the notion that consumption of B-vitamins in excess of daily requirement stimulates appetite. More is not necessarily better.

B-vitamin deficiency

Uremic vomiting is mediated by local and central factors. Local factors include gastritis, forced foods and fluids and intolerance to some medications. Central factors consist of stimulation of the medullary emetic chemoreceptor trigger zone by circulating uremic toxins (such as methylguanidine) and intolerance to some medications.

Nausea and vomiting

Anorexia, nausea and vomiting associated with renal failure are in part related to gastric hyperacidity induced by hypergastrinemia. Normally, gastrin stimulates receptors located on parietal cells in the stomach mucosa to produce and secrete hydrogen ions. Because up to 40 percent of the circulating gastrin is metabolized by the kidneys, reduction in renal function results in increased and more prolonged stimulation of parietal cells to produce hydrogen ions by retained gastrin.

The resultant gastric hyperacidity leads to mucosal irritation, ulceration and hemorrhage. Back diffusion of hydrochloric acid and pepsin into the stomach wall leads to further hemorrhage, inflammation and the release of histamine from mast cells located in the gastric mucosa.

Thus, the cycle is perpetuated as mast-cell derived histamine causes further stimulation of parietal cells to produce hydrogen ions. The cycle of hypergastrinemia-induced anorexia, nausea and vomiting may be interrupted by administration of H2-receptor antagonists such as ranitidine (Zantac, Glaxo Wellcome) or famotidine (Pepcid, Merck). Ranitidine is commonly given orally or intravenously at a dosage of 0.5 to 2 mg/kg q12 to 24 hours. Famotidine is commonly given IV at a dosage of 0.5 to 1.0 mg/kg q12 to 24 hours, or 1 to 2 mg/kg orally q12 to 24hr.

Because ranitidine and famotidine are partially excreted by the kidneys, dosage adjustments of 50 to 75 percent for ranitidine, and 10 to 25 percent for famotidine should be considered for patients with marked reduction in renal function.

As an alternative to H2-receptor antagonists, sucralfate (Carafate, Hoechst Marion Roussel) may be given to create a protective layer over the gastric mucosal surface. In an acid environment, sucralfate becomes charged and binds to gastric proteins of the opposite charge, thereby minimizing back diffusion of hydrochloric acid and pepsin into the stomach wall.

An empirically established oral dosage of sucralfate for dogs is 0.25 to 1.0 grams/patient eight to 12 hours. For cats, the empiric oral dosage is 0.25 gm/cat eight to 12 hours.

Because sucralfate may interfere with the action of other orally administered drugs, they should be given approximately 30 minutes prior to the administration of sucralfate. Caution should also be used if consideration is being given to using sucralfate for extended periods, since significant elevations in serum aluminum concentration may occur.

As an alternative or supplement to H2-receptor antagonists and sucralfate, metaclopromamide (Reglan, Robbins) may be given to minimize the action of uremic toxins on the medullary emetic chemoreceptor trigger zone. Metaclopramide is a D2-dopamine receptor antagonist. An empirically established antiemetic oral dose of metoclopramide is 0.1 to 0.5 mg/kg eight hours.

Injectable metoclopramide may be given subcutaneously at the same dose, or by constant rate IV infusion at a dosage of 0.01 to 0.02 mg/kg/hr. Because the kidneys eliminate metoclopramide, the dosage should be reduced by 50 percent in patients with marked renal failure. Signs of metoclopramide overdosage include drowsiness, disorientation, dyskinesia, muscle dystonia and reduction in seizure thresholds.

Discomfort associated with oral erosions and ulcerations may be reduced by application of compounds containing lidocaine (2% lidocaine viscous; Roxane Labs Inc.). Oral rinses with 0.1 to 0.2% chlorhexadine solution (CHX-Guard, VRx Products) may be used to reduce bacteria.

nausea and vomiting

Drug-induced anorexia,

Patients in renal failure commonly are intolerant to side effects of many drugs. Examples include captopril, enalopril, trimethoprim-sulfa, digoxin and tetracyclines. In addition, some drugs may contribute to anorexia by impairing taste or smell. They include ampicillin, sulfas, tetracyclines, aminoglycosides, allopurinol, d-penicillamine and captopril. The manufacturer's recommendations and description of side effects should be reviewed before giving any drug to a patient in renal failure.

Because many renal failure patients are intolerant to drugs, they should not be routinely given with the philosophy that they might help, but will do no harm. If the benefit of administration of drugs dependent on adequate renal function for elimination from the body are likely to outweigh the risks, adjustments in dosage and/or maintenance intervals should be considered.

In general, drugs should not be mixed with the primary source of food or water.

How can diet palatability be enhanced?

Because patients with CRF may be unable to adapt to large and abrupt changes in dietary nutrients (such as reductions in sodium), it is advisable to make dietary changes over one to two weeks. Making gradual changes in diets is also advisable because preference for new flavors and textures of food may be a learned response for some dogs and cats.

Changes in diet

Dogs, and especially cats, accustomed to a specific texture of food (canned or dry) may refuse to eat a diet of a different texture. When obtaining the medical history, special emphasis should be placed on the diet history, especially in relation to the patient's preferred foods. If the patient is hospitalized, it may be preferable to continue with these foods, rather than making a sudden diet change.

In addition to making gradual dietary changes at the appropriate time, it may be beneficial to choose new diets with the same texture and flavor as preferred diets.

In general, extremes in the temperature (hot vs. refrigerated) of foods are poorly accepted. However, warming food to just below body temperature may improve its palatability. If dry foods are selected, addition of warm water may be of benefit.

Offering fresh aromatic food is sometimes helpful, provided the patient's nasal passages are open. If the patient will not accept solid food, they may respond to strained baby foods (i.e. chicken or lamb). Caution: baby foods that list onions in the ingredients should be avoided since onions may have an adverse effect on red cells.

The possibility that food aversion may develop should also be considered. If a diet designed for long-term management of renal failure is given when CRF patients are nauseated or vomiting, aversion to that food may develop. As a result, they may continue to avoid that food, even after the nausea and vomiting have subsided.

Food aversion

Food aversion is most likely to occur if nauseated patients are force-fed, or if painful sample collection or painful administration of drugs are associated with feeding.

To minimize the possibility of the patient's aversion to renal failure diets designed for long-term use, they should not be offered to renal failure patients until the underlying causes contributing to anorexia, nausea and vomiting are minimized or eliminated.

Consider introducing renal failure diets to CRF patients in the home environment when they are feeling better. If anorexia, nausea or vomiting recur, owners should be advised to return to the hospital so that the patient may be re-evaluated with the goal of making appropriate adjustments in therapy.

A variety of foods may be used as toppings to enhance the palatability of diets. They include poultry fat, butter, dehydrated cottage cheese, yogurt, garlic powder, bullion, clam juice, tuna juice, anchovy paste, brewer's yeast and carnitine. Enteral nutrition liquids designed for renal failure may also be used as flavored toppings.

Flavoring agents

Modifying feeding patterns and environment

For patients with CRF, it is generally best to provide small quantities of food several times each day. Minimizing distension of the stomach may minimize gastric secretions and the feeling of nausea. Likewise, consumption of smaller quantities of food will minimize post-prandial elevation of absorbed nutrients, including protein catabolites.

Placing small quantities of palatable food in the mouth or on the patient's paws may stimulate a licking response. This, in turn, may stimulate neural and humoral mechanisms that normally stimulate appetite.

The feeding environment should also be considered when managing renal patients with anorexia. Timid animals should not be hospitalized in noisy wards with heavy traffic. Loud and persistent barking may be especially stressful to cats. Food should be placed in wide bowls or flat saucers so that tactile whiskers are not adversely stimulated. A comfortable environmental temperature should be maintained.

Rewarding patients at the time of feeding may be helpful. Owners may play an important role by providing a reassuring voice and gentle hand. Partially anorexic cats may begin to eat if gently stroked at the time a meal is offered.

Anabolic steroids

Pharmacologic appetite stimulants

Although manufacturers of veterinary anabolic agents state that these drugs will improve appetite, there is no data to support this claim in dogs or cats with renal failure. In one, six-week study in moderately azotemic dogs with induced renal failure, anabolic agents had no beneficial effect on food intake, body weight, nitrogen balance or lean body mass. Anabolic agents have been reported to cause hepatotoxicity in some cats.

Corticosteroids

There is no data to support a long-term beneficial effect of glucocorticoids as appetite stimulants in uremic dogs or cats. To the contrary, glucocorticoids enhance catabolism, impair the repair of gastric mucosa and enhance glomerular hyperfiltration. Although they may enhance appetite, this effect does not translate into weight gain.

Benzodiazepines

Benzodiazepine derivatives such as diazepam and oxazepam are often used to stimulate the appetite of cats. They have been most effective in patients with partial anorexia. However, they are not generally of long-term benefit. The objective of short-term therapy is to stimulate the appetite of the patient in a "jump-start" fashion, so that licking and chewing will stimulate normal and humoral and neural appetite mechanisms. Diazepam (Valium, Roche) may be given to cats orally, intramuscularly or intravenously but is most effective when given at an intravenous dose of 0.05 to 0.1 mg/kg as needed.

Oxazepam (Serax, Wyeth-Ayerst) is available only for oral administration. It is commonly given to cats at a dosage of 0.2 to 0.4 mg/kg 12 hours, or as needed.

Care must be used to not use excessive dosages of these drugs in depressed patients. Some cats have developed evidence of hepatic dysfunction after being treated with diazepam for several days.

Other agents

Serotonin, a neurotransmitter, inhibits appetite. Cyproheptadine (Periactin, Merck), a serotonin antagonist, has been used to stimulate appetite in cats. It is often given at an empirically established oral dose of 2 mg/cat eight to 12 hours. The liver metabolizes cyproheptidine; renal failure reduces the elimination of these metabolites.

Voluntary consumption of food stimulates digestion and metabolism; therefore it is preferable to enteral feeding. However, if patients do not respond to the aforementioned therapeutic strategies by eating a sufficient amount of food to meet their daily nutrient requirements, tube feeding should be considered.

Enteral nutrition

Liquid diets administered through a nasogastric tube may be used for short-term therapy. For long-term nutritional support, good results may be obtained with a low-profile percutaneous gastrostomy tube. They are usually well tolerated by the patient, and easily managed by owners in the home environment. They facilitate feeding blenderized renal failure diets and oral medications. By fostering oral administration of water, they also circumvent the need for subcutaneous fluid therapy. n