Conservative medical management of chronic renal failure in the cat (Proceedings)
MANAGEMENT OF HYPERTENSION
The prevalence of hypertension in cats with CRF is variable and ranges from approximately 30 to 75% of affected patients. The prevalence of hypertension may be higher in animals with glomerular disease. Cats especially are prone to "white coat artifact" making it difficult to determine if a given cat is truly hypertensive. In clinical practice, systolic blood pressure usually is measured by Doppler technique. Sufficient time for acclimation should be allowed, and several sequential measurements should be made to assess the animal's blood pressure. Averaging sequential readings improves reliability. Cats with systolic blood pressure readings consistently above 170 mm Hg or those with abnormally high blood pressure readings that also have fundic lesions consistent with hypertensive retinopathy (e.g., retinal edema, intra-retinal serous exudation, retinal hemorrhages, arterial tortuosity, retinal detachment) are considered candidates for anti-hypertensive therapy. Angiotensin-converting enzyme (ACE) inhibitors (e.g. enalapril, benazepril) may have protective effects in patients with chronic renal disease due to their ability to block adverse effects of angiotensin II. Potential beneficial effects include reduction in proteinuria, limitation of glomerular sclerosis and slowing of progression of renal failure as well as improvement in systemic blood pressure. Enalapril (0.5 mg/kg PO q12h) typically is recommended in dogs with glomerular disease and hypertension, but enalapril has not been very effective for treatment of hypertensive cats. The calcium channel blocker, amlodipine has been used successfully to manage hypertension in cats at a dosage 0.625 to 1.25 mg per cat given orally once per day. Follow-up evaluations should be scheduled for one week after beginning treatment with amlodipine. Adverse effects (including hypotension) are very uncommon with the use of amlodipine in cats. In one study, amlodipine controlled hypertension in nearly 60% of CRF cats treated over a period of 3 months or more. Benazepril (0.5-1.0 mg/kg/day) may decrease proteinuria with minimal adverse effect on serum creatinine concentration in CRF cats. In one study of 192 cats, median survival in cats treated with benazepril (637 days) was not significantly longer than in those treated with placebo (520 days). However, when the small number of cats with urine protein/creatinine ratios ≥ 1.0 was considered, the difference in survival was more marked: 484 days for cats in the benazepril group (4 cats) versus 124 days for those in the placebo group (9 cats). Another study of CRF cats showed similar effects of benazepril on proteinuria with no difference in survival time between groups but some evidence of decreased rate of progression in the benazepril-treated group.
HORMONE REPLACEMENT: CALCITRIOLCalcitriol may enhance gastrointestinal absorption of calcium and reduce parathyroid hormone (PTH) synthesis and secretion in cats with CRF. Calcitriol should not be administered until hyperphosphatemia has been controlled. If the Ca × P solubility product exceeds 60-70, calcitriol should be avoided because of the risk of soft-tissue mineralization. An extremely low dosage of calcitriol (2 to 3 ng/kg/day) has been used in cats with stable CRF to reverse renal secondary hyperparathyroidism. Plasma PTH concentrations decrease dramatically during calcitriol administration. Calcitriol is manufactured in capsule (250 or 500 ng) and liquid (1000 ng/ml) forms. Reformulation by a compounding pharmacy is necessary to provide accurate dosing. During treatment of CRF patients with calcitriol, simultaneous monitoring of serum ionized calcium and PTH concentrations is the ideal way to document successful and safe control of renal secondary hyperparathyroidism.
HORMONE REPLACEMENT: ERYTHROPOIETIN
Recombinant human erythropoietin (EPO) has been used to correct nonregenerative anemia in CRF cats. Treated animals demonstrate resolution of anemia, weight gain, improved appetite, improved haircoat, increased alertness, and increased activity. Therapy may be started in symptomatic cats with PCV values < 20%. The starting dosage is 100 U/kg administered subcutaneously 3 times per week. Elemental iron supplementation should be provided at a dosage of 2 mg/kg/day, but some treated cats may experience gastrointestinal upsets. When the lower end of the target PCV range (30-40%) is reached, frequency of administration of EPO is reduced to twice a week. Depending upon the severity of anemia, it may require 3-4 weeks for the PCV to enter the target range. There is a high risk of anti-EPO antibody formation in cats receiving recombinant human EPO. Formation of antibodies against EPO may result in severe anemia and prolonged transfusion dependence. Although initially effective in correcting the anemia of CRF, use of recombinant human EPO is associated with antibody formation in up to 50% of treated animals after 1 to 3 months of treatment. The resulting anemia can be more severe than that present before treatment because the induced antibodies can cross-react with the animal's native EPO. Feline recombinant EPO has been produced and shown to be effective, but unfortunately unexplained red cell aplasia developed in 26% of treated cats that had not previously been exposed to recombinant human EPO. Other adverse effects that have been reported after administration of recombinant human EPO to cats include vomiting, seizures, hypertension, pruritus and excoriations at the injection site, and, rarely, life-threatening anaphylaxis.
SUBCUTANEOUS FLUIDS AT HOME BY THE OWNER
Subcutaneous administration of crystalloid fluids (lactated Ringer's solution) to CRF cats by their owners at home assures optimal hydration and subjectively seems to improve the quality of life for many CRF cats. Some owners give a fixed volume each day (usually 120 ml) whereas others judge whether or not to administer fluids on a given day based on their observation of the cat's behavior (e.g. appetite, activity level, playfulness). The clinician should consider placement of a gastrostomy tube in CRF cats with poor appetites because this approach allows convenient delivery of calories, fluids, and medications and the tubes are well tolerated by most cats.
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