Chronic Renal Insufficiency

History. You recommend routine dentistry for a 15-year- old male miniature schnauzer that appears to be in good health. Before administering general anesthetic to this aged animal, you obtain a CBC, serum chemistry profile, and urinalysis to detect any Subclinical organ dysfunction.

Clinical Examination. The CBC and serum chemistry profile are normal. The urinalysis is normal with a specific gravity of 1.010 (osmolality, 352 mθsm∕kg H₂O). You ask the owner to submit a sample of the dog’s urine from the first void of the day. The specific gravity of this sample is 1.012 (osmolality, 401 mθsm∕kg H₂O).

Comment. Chronic renal insufficiency is common in geriatric patients and is probably responsible for the two urine specific gravity values in the “fixed range” of 1.008 to 1.012. These values correspond to osmolalities similar to or slightly higher than normal plasma osmolality. Additional evaluation would have to be performed to verify that this animal could neither dilute nor concentrate his urine significantly; how­ever, in an animal of advanced age and in the absence of other clinical or laboratory abnormalities, further evaluation is probably not indicated.

In chronic renal insufficiency the loss of functional nephrons is first manifested by the inability to alter significantly the urine concentration in response to a water load or water depriva­tion. The residual nephrons are able initially to sustain adequate filtration rates to prevent azotemia (elevated serum creatinine and urea nitrogen levels), but the compensatory increase in flow rates in individual nephrons probably exceeds the capa­city of the thick ascending limb and distal convoluted tubule to dilute the tubule fluid significantly. The residual nephrons are also unable to generate a steep medullary concentration gradient, and thus the tubule fluid cannot be concentrated greatly above the level of the plasma osmolality. If there is progressive nephron loss, the glomerular filtration rate will continue to decline, and renal failure will ensue.

Treatment. It is important to be aware that your patient has chronic renal insufficiency and is unable to respond effi­ciently to changes in fluid and salt intake. Water should be with­held only briefly, and caution should be exercised in support­ing the animal with intravenous fluids during anesthesia while avoiding fluid overload.

A highly bioavailable, low-protein diet that is also low in sodium and phosphorus may hinder the progression of chronic renal disease and delay the onset of renal failure, at least in some species.