In all animals, clinical symptoms of renal dysfunction are not evident until 65 to 75% of renal tissue has been destroyed (Lister and Rand, 2006), and many veterinary practices instigate renal screening for older patients as part of senior clinics and prior to the start of pharmaceutical regimens (normally osteoarthritis treatments). Nutritional management can affect many consequences of renal failure, and is the cornerstone of its management. Chronic renal failure (CRF) has many physiological effects, these include the decreased ability to excrete nitrogenous waste (and thus build up of azotaemia), sodium and phosphorus, and an increased loss of potassium. Other clinical symptoms may include systemic hypertension, secondary hyperparathyroidism and non-regenerative anaemia (Lane, 2005), and therefore these aspects should be monitored as part of the nursing clinic.
The veterinary nurse clinic
Attendance at nurse clinics for animals that have been newly diagnosed or with suspected renal disease should be instigated as soon as possible (Box 1). All patients should be referred to a veterinary nurse with appropriate knowledge, in order to discuss diet, medications (if required), compliance (McLeod, 2008) and any future requirements for diagnostics and reviews of the patient. The nurse is well placed to answer any questions that the owner may have. This is a role that should be performed by a veterinary nurse for a number of reasons:
At each clinic the animal's weight, body condition score (Figure 1), muscle condition score (MCS) (Figure 2) and hydration level needs to be assessed and recorded. MCS is important as the animal will start to lose lean muscle mass as it can become cachexic if anorexic. Assessing hydration levels is vital as the animal will start to become dehydrated as its urine concentrating ability becomes diminished. Parameters such as blood pressure and haematology and blood biochemistry tests need to be performed on a regular basis as set out by the veterinary surgeon.
Diagnostic monitoring
The International Renal Interest Society (IRIS) renal scoring index identifies the progression of the disease in order to facilitate appropriate treatment and monitoring of the patient (Chakrabarti et al, 2014; IRIS, 2015). The initial staging is based on a fasted plasma creatinine level, and then sub-staged dependent on proteinuria levels, and arterial blood pressure (Table 2). Fasted blood samples must always be used as even a moderately high protein meal prior to sampling can elevate blood plasma creatinine levels. Repeat blood sampling should occur, as required, but should be performed more regularly if urinalysis shows changes in proteinuria levels.
| Plasma creatinine Level umol/l | Stage 1 | Stage 2 | Stage 3 | Stage 4 |
|---|---|---|---|---|
| Canine | <125 | 125–179 | 180–439 | 440+ |
| Feline | <140 | 140–249 | 250–439 | 440+ |
| Maintain phosphate within specific ranges in relation to stage: | ||||
| Serum phosphate target mmol/l | N/A | 0.81–1.45 | 0.81–1.61 | 0.81–1.94 |
| Action | Not required | Renal diet or phosphate binder | Renal diet +/− phosphate binder | As for stage 3 |
Owners may need guidance on methods for obtaining urine samples, as these are the most useful diagnostic tools for monitoring the progression of renal failure. Urine concentration should be routinely measured through refractometery, as should the urine protein:creatinine (UP/C) ratio (Table 3). Medications such as ACE inhibitors should only be instigated when proteinuria is present. It should be noted that proteinuria can present at any stage of the renal failure, and is not directly linked to the level of azotaemia.
| Cats | Dogs | |
|---|---|---|
| Non-proteinuric | <0.2 | <0.2 |
| Borderline | 0.2–0.4 | 0.2–0.5 |
| Proteinuric | >0.4 | >0.5 |
All newly diagnosed renal patients should have their blood pressure monitored. Renal function is directly affected by an increase in blood pressure, and as the kidneys play a role in blood pressure secondary hypertension can result (Table 4). Clinical signs of hypertension may be absent, but where hypertension is identified through monitoring it should be treated, as the effects of hypertension are ultimately negative.
| Systolic BP mmHg | Diastolic BP mmHg | Adaptation when breed specific reference range is available | Arterial pressure substage (AP) |
|---|---|---|---|
| <150 | <95 | <10 mmHg above reference range | 0 Minimal risk |
| 150–159 | 95–99 | 10–20 mmHg above reference range | 1 Low risk |
| 160–179 | 100–119 | 20–40 mmHg above reference range | 2 Moderate risk |
| ≥ 180 | ≥ 120 | ≥ 40 mmHg abovereference range | 3 High risk |
Pharmaceuticals
The most commonly used pharmaceutical in animals with chronic renal failure is benazepril, an angiotensin converting enzyme (ACE) inhibitor. Inhibition of ACE leads to reduced conversion of inactive angiotensin I into active angiotensin II therefore reducing the effects mediated by angiotensin II, including vasoconstriction of both arteries and veins, retention of sodium and water by the kidney and modelling changes (including pathological cardiac hypertrophy and degenerative renal changes). In cats with chronic renal insufficiency benazepril reduces the protein loss in urine and reduces systemic and intraglomerular blood pressure. Benazepril also helps to increase the appetite, quality of life and survival time of the cats, particularly in advanced disease (Mizutani et al, 2006). Benazepril is indicated when proteinuria is present, and therefore proteinuria should be tested for. As ACE inhibitors decrease the intraglomerular blood pressure there will be a refractory increase in nitrogenous waste products.
Telmisartan is an angiotensin receptor blocker (ARB) that is also licenced for the reduction of proteinuria in cats that have chronic renal insufficiency. ARBs are a relatively new class of drug in the veterinary market, and have the benefit of blocking the last step of the renin-angiotensin pathway, thereby, inhibiting the harmful effects of angiotensin II in a target specific manor, as opposed to the non-specific action of ACE inhibitors (Burnier and Brunner, 2000).
In some cases additional hypertensive medications may be required in order to return the patient to a normotensive state. In these cases amlodipine may be added into the regimen. Once instigated, blood pressure monitoring is required in order to taper the dose according to the patient's readings; this is an important aspect of the nursing clinic.
If medications are prescribed for patients with CRF it is important that the veterinary nurse discusses with the client in clinics whether they are able to medicate their pet. Owners may need guidance on the administration of medications, and on the importance of compliance.
Clinical nutrition
The diet plays a very important role in aiding in the longevity and quality of life of the animal. As many patients with renal insufficiency are azotaemic it can be very difficult to instigate dietary changes. By understanding the ideal nutrient requirements for an animal with renal failure, guidance can be offered when owners wish to prepare homemade diets or feed a commercial diet, such as a renal diet.
Water
Renal disease causes a progressive decline in urine concentrating capacity. Dehydration, volume depletion, renal hypoperfusion and dietary salt intake stimulate urine concentration. Avoiding dehydration and renal hypoperfusion reduces the kidney's work in concentrating the urine and helps to maintain intrarenal protective mechanisms. Patients with CRF must have unlimited access to fresh water and free choice of consumption. This can be exceptionally important in cats, which do have fastidious drinking habits.
An increase in water consumption can be achieved in a number of different ways: feeding a moist diet rather than a dry diet; increasing the availability of water by placing more bowls around the house and in the garden. Allowing water to stand for a period of time prior to being offered can be beneficial to some animals. This allows the chlorine in the water to evaporate off, which some animals prefer. As part of the nursing clinic, it is important to discuss with the client how to monitor hydration levels. Demonstration of how the skin tents and monitoring mucus membranes can help the client determine whether their pet is dehydrated.
Protein
Renal diets for dogs are staged depending on renal function. Those dogs that are in IRIS stage 4 renal disease need a protein restricted diet, whereas this restriction in protein in the lower stages can be detrimental. This staged management system is recommended in dogs, as early cases can benefit from phosphorous restriction, while maintaining a protein intake level equal to an adult maintenance level. Early cases are defined as those that are azotaemic and not uraemic, and make up 18 to 20% of dogs suffering from CRF (Elliott, 2005).
For dogs that are presented with uraemia, the diet needs to have a lower protein level. This will help to reduce the protein catabolites that are produced. The protein that is present in these diets needs to be of a high biological value to minimise the risk of essential amino acid deficiency. If protein is too restricted in both cats and dogs with CRF, hypoalbuminaemia, anaemia and metabolic acidosis can occur. It is, therefore, important to achieve the correct balance of protein levels. If protein levels are too high, protein can act as a source of calories through deamination in the liver, the nitrogenous waste products will then exacerbate the azotaemia.
Dietary protein has been implicated as a direct possible cause of CRF, or responsible for a decrease in renal function in already dysfunctioning kidneys (Robertson et al, 1986). However, the majority of studies have been conducted in rats. Subsequently, studies have identified that dogs (and possibly cats, horses and rabbits), behave differently from rats in response to dietary protein (Allen et al, 2000); thus restriction in protein does not need to occur in early stages of renal failure.
Proteinuria of glomerular origin was traditionally considered to be a consequence of damage to the glomerular barrier (Elliott, 2005); proteinuria observed in association with an increased protein intake, may be related to haemodynamic alteration and physiological changes in glomerular permselectivity, rather than damage to the glomerular filtration barrier (Elliott, 2005). An increase in protein excretion in urine is attributed to the passage of time (wear and tear) rather than protein and phosphorus levels in the blood, and underlines that urinalysis is a valuable, and often under-utilised diagnostic tool in detecting CRF and its severity.
The level of dietary protein restriction however is important in cats. Veterinary diets designed for renal management in cats do not have staged protein restriction levels. This is because cats cannot reduce their enzyme activity in the liver. These enzymes are involved in protein catabolism, and if the dietary protein level is greatly restricted, then protein malnutrition can occur. As with dogs, the protein level should be adequate for the animal's needs, but not too high so that excess is used as an energy source.
Vitamins and minerals
Sodium levels in commercially produced renal diets are reduced, this is because of the reduction in the number of viable nephrons within the kidneys; as a result as serum sodium levels remain the same, each nephron has an increased load delivery. This, in turn, will create hypertension, as the blood pressure rises. Hence, blood pressure monitoring in animals with renal dysfunction is recommended. Sodium levels should, however, not be too restricted, as this can result in a reduced capacity to reabsorb bicarbonate. This will contribute towards metabolic acidosis. If sodium intake is rapidly reduced, dehydration and volume contraction may occur, therefore a gradual change in diets is recommended when changing to a salt restricted diet. In addition, salt is a main determining factor in palatability for all animals. A large decrease in salt content can cause a perceived change in palatability; the food is very palatable, but if pets are used to eating a high salt diet a low salt diet may not seem as appetising. Sodium levels in renal diets are recommended at 0.3% dry matter basis (DMB) in cats and 0.2% (DMB) in dogs (Lane, 2005).
Phosphorus is absorbed from the gastrointestinal tract, and primarily excreted by the kidneys. A restriction in dietary phosphorus has been shown to slow the progression of renal failure in cats and dogs (Elliott et al, 2000). Hyperphosphataemia is a common finding in patients with CRF, and occurs when the glomerular filtration rate (GFR) falls below 20% of the norm (Elliott et al, 2000). This can result in reduced renal phosphate excretion, and hence raised serum phosphate levels. This can result in renal mineralisation, secondary hyperparathyroidism, thereby exacerbating renal damage, and aid in the development of hyperlipidaemia. If dietary means alone do not reduce serum phosphorous levels, then oral phosphorous binding agents can be used. It has been shown that cats that have low phosphate levels within their diets live almost 2.5 times longer that cats that are fed normal diets (Elliott et al, 2005).
Potassium deficiency has been identified in cats with CRF (Plantinga et al, 2005). Hypokalaemia impairs protein synthesis, promotes weight loss, a poor hair coat and contributes to polyuria by decreasing the renal responsiveness to antidiuretic hormone (ADH).
It is important to remember that with any disease or disorder that shows the clinical symptoms of polydipsia (PD) and polyuria (PU), water-soluble vitamins can be lost. Water-soluble vitamin deficiency can further contribute to anorexia, as even higher demands are placed on these vitamins, as the body tries to recuperate. Further supplementation with water-soluble vitamins is often not required as commercially available veterinary diets contain additional quantities of these vitamins.
Fats
As discussed earlier, the renal patient needs to gain calories from a non-protein source. This can be from fats (lipids) and carbohydrates. The addition of fats to the diet is beneficial, as it offers twice the energy per gram as carbohydrate and aids in palatability. The use of fatty acids has also been shown to decrease inflammation in the kidney, lower hypertension and preserve renal function; therefore veterinary diets are supplemented with omega fatty acids or polyunsaturated fatty acids, especially EPA and arachidonic acid (Plantinga et al, 2005). Omega 6 fatty acids appear to be detrimental in dogs with naturally occurring renal disease by acutely increasing GFR.
Carbohydrates
Most manufacturers of veterinary diets do not raise the importance of dietary fibre in the management of renal disease. The animal itself cannot digest soluble fibre in the diet, but the microbes within the intestine can. Soluble fibre is fermented into short-chain fatty acids (SCFA) or volatile fatty acids (VFA). These products are an important energy source for the intestinal cells and can increase blood flow to the intestine.
Nitrogenous waste products in the blood are presented to the intestinal lumen where urease, an enzyme produced by intestinal bacteria, hydrolyses the urea into ammonia and carbon dioxide. The ammonia is then utilised by the intestinal bacteria. This process means that nitrogenous waste products are excreted in faecal matter, rather than in urine by the kidneys. Dietary fibre may also be beneficial for improving gastrointestinal motility in dogs with renal failure. Colonic transit times can be decreased in moderate renal disease as it alters duodenojejunal motility (Lefebvre et al, 2001).
Supplements
Supplements that can be used in cases of chronic renal diseases are designed to help reduce the phosphate levels in the diet. Phosphate binders that can be used include calcium carbonate, which can reduce the apparent digestibility of phosphorus more than two times than when fed a standard diet.
Chitosan (a derivative of chitin) has also been used in supplements for animals suffering from CRF. Chitosan acts as an absorbent in the intestines, and thus lowers the absorption of certain substances including phosphates and some uraemic toxins (Wagner et al, 2004).
A highly palatable formulation of Rheum officinale, medicinal rhubarb derived from the rhizome and roots of the rhubarb plant, which has been documented in tests with a variety of species to help protect healthy renal tissue, can also be used for cats with CRF (Hanzlicek, 2011). It helps to support normal renal function; the kidney can become damaged and fibrosis occur within the tissues by the cellular and enzymatic events occurring in renal injury through acute insult or persistent inflammation and the subsequent healing response. If untreated, this cascade of events can lead to fibrosis and related loss of glomerular filtration. One effective strategy in managing the disease is intervening at an early stage to minimise the negative consequences of fibrosis, therefore there is a role for use of these supplements in the management of patients with CRF.
Feeding a renal diet
Objectively, the role of renal veterinary diets is to help reduce azotaemia, hyperphosphataemia, and also to control secondary hyperparathyroidism ultimately improving both the clinical and biochemical status of the animal. However, the palatability of the diet has often been blamed for a reduced dietary intake. The effect of uraemia on the sense of taste and smell and the development of food aversions can all contribute towards inappetance. Changing any animal from a high salt, high protein diet to a commercial renal diet can be very difficult. Changing via a transitional intermediate diet over a more prolonged period of time can be beneficial in these animals.
When starting an animal on a renal diet, a gradual transitional period is required. Gradual changes through a range of diets from adult to senior to early renal can be beneficial for animals more sensitive to the changes in salt content of the diet, and can also prove of use in owners with preconceptions of changing an animal from a supermarket high salt brand to one of restricted salt and reduced protein levels. There is often a misconception that the palatability of renal diets is poor, palatability for these diets is good (Burkholder, 2000). As most animals are accustomed to a higher salt level in their diet, changing to a lower salt diet can cause difficulties, in the same way as when people that are used to adding salt to their diets stop doing this, their food tastes blander. This is nothing to do with the diet rather that the taste buds have to adjust to the reduction of salt in the food.
Supporting the owner during the transition period of diet change is important. As the majority of affected animal are older, fastidious cats, some of which have very precise food preferences, list of foods, which should not be added to any commercial clinical diet, can prove to be useful for the owner (Table 5).
| Cats | Dogs | |
|---|---|---|
| Grocery mooist diet | 2845 | 952 |
| Grocery dry diet | 1144 | 371 |
| Senior dry diet | 390 | 186 |
| Renal moist diet | – | 400 |
| Cardiac dry diet | – | 111 |
| Renal/cardiac moist diet | 175 | – |
| Renal/cardiac dry diet | 156 | – |
| 30 g cheese | 262 | |
| 1 slice of bread | 218 | |
| ½ tin tuna | 160 |
Conclusion
The feeding habits of animals suffering from renal failure will alter, especially if the animal is uraemic and suffering from anorexia and nausea. Small frequent meals can prove beneficial, as does feeding location and presentation. Food aversions can be a large problem with these animals, and if a clinical diet is flatly refused phosphate binders can prove to be useful.
The monitoring of animals with CRF is very important, and underutilised; both urine and blood sampling should take place. Regular monitoring to ensure dietary and medical management remain optimal for each individual animal is vital for long-term successful management. Compliance from the owner can also be improved with the extra support provided by the nurse-led clinic. The processes of the disease should be monitored, this will include laboratory evaluation of blood and urine, blood pressure monitoring, full clinical history, physical examination, bodyweight and body condition score. History should also be taken on the amount of food and water being consumed. Owners do find it difficult to differentiate between time spent at the food or water bowl and actual amount consumed. Careful questioning might have to be adopted.
The role of the veterinary nurse is important in aiding compliance, education of the owner, and discussion of care plans alongside guidance from the veterinary surgeon in charge of the case.