Anuria and Oliguria

Anuria is defined as the absence of urine production, and oliguria is defined as scant or subnormal urine production. Ihese two disease conditions are often difficult to distinguish at the onset of evaluation.

Volume of urine produced by healthy animals varies tre­mendously according to breed, age, physiologic status, exercise, diet, and environmental factors. In 1961 Altman determined normal urine output for the large animal species to be as follows: horses, 3 to 18 mL/kg/day; cattle, 17 to 45 mL/kg/day; and sheep and goats, 10 to 40 mL/kg/day.³² Neonates urinate more frequently than adults and often directly after nursing. Anuria and oliguria frequently become apparent to the clinician when an azotemic animal is diagnosed with a primary disease in which renal function is commonly threatened (e.g., severe diarrhea, congestive heart failure) and urine output after initia­tion of fluid therapy is subnormal or absent.

If urine collection and measurement cannot be performed, the adult animal with potential anuria or oliguria can be closely watched or placed in a stall with no or minimal bedding or fine, fresh bedding (e.g., sawdust) to facilitate detection of urine on the floor. When indicated, fluid therapy should be initiated and expectations for urine output predicted based on the rate and route of fluid administration. Animals with painful thoracic or abdominal disease may refrain from urination, and the clinician must remember to evaluate bladder filling by transrectal or transabdominal ultrasonography, and not just the volume of urine voided. Recumbent, obtunded foals, particularly males, may not reliably void urine; monitoring bladder fill by ultrasound or directly monitoring urine output by placement of an indwelling urinary catheter is recommended to avoid bladder rupture. The same applies for some recumbent neurologic horses and cattle.

When evidence for anuria mounts, the clinician must determine whether the urinary tract is patent and rule out the presence of rupture of the bladder, urethra, or ureters. Repeat physical examination and ultrasonography of the upper and lower urinary tract and abdominal cavity is warranted in such instances. Obstructive diseases of the LUT are typically accompanied by signs of colic and stranguria. However, these signs will not be present if the patient has a preexisting rent in the urinary tract or if the animal is severely obtunded from uremia.

Oliguria is a physiologic adaptation to dehydration and can be expected in animals with prerenal azotemia. Assessment of UspG is critical in determining the presence of renal failure in such cases. If urine cannot be collected from the dehydrated animal, fluid therapy should be initiated and the azotemia monitored. Oliguria is a relatively common feature of acute renal failure, particularly if the disease process involves obstruc­tion of the nephron lumen with crystals, cellular debris, or proteinaceous casts. However, volume of urine produced by a patient in renal failure can vary greatly according to hydration status, diet, and the inciting cause. Urine production can be normal, increased, or decreased in renal failure. Renal failure is covered in detail in Chapter 34.