INTRODUCTION

Oliguria is defined as abnormally small production of urine. Normal urine output for the dog and the cat is 1 to 2 ml/kg/hr.¹ Oliguria has been defined as urine output ranging from less than 0.27 ml/kg/hr to less than 1 to 2 ml/kg/hr.¹ Oliguria can be classified as prerenal, renal, or postrenal and may represent either an appropriate physiologic response or a pathophysiologic process. It is essential to differentiate physiologic from pathologic oliguria when making clinical decisions and assessing the severity of disease in a critically ill patient.

PHYSIOLOGIC OLIGURIA

Physiologic oliguria is an appropriate decrease in urine production in response to physiologic stimuli (Box 7-1). When considering the causes of physiologic oliguria, the concept of effective circulating volume (ECV) is paramount. The ECV refers to the portion of extracellular fluid that is in the arterial circulation and is effectively perfusing tissues. A useful physiologic reflection of ECV is the pressure perfusing the arterial baroreceptors, because changes in pressure (or stretch), rather than volume or flow, is generally perceived at these sites.² Reductions in ECV, due to either true loss of volume (i.e., hypovolemia) or perceived low volume (i.e., decreased cardiac output), result in the same physiologic responses.³

The ECV is sensed in a variety of locations in the body, including baroreceptors in the aortic and carotid bodies, as well as the afferent arterioles of the kidney.² For example, when the ECV is decreased in a hypovolemic animal, the subsequent decrease in baroreceptor stimulation results in a number of physiologic responses. The first and most rapid response is mediated by the sympathetic nervous system and is manifested by an increase in heart rate, contractility, and systemic vascular resistance. In addition to these immediate changes, there is also an increase in the secretion of renin, angiotensin II, arginine vasopressin, and aldosterone. Increased reabsorption of sodium and water, as well as additional vasoconstriction, results. There is also an increase in thirst, which serves to further augment ECV if the patient drinks water.

Oliguria that occurs in response to a decrease in ECV is an appropriate physiologic response and is not a result of acute renal failure. It is important to remember that although this response to a decrease in ECV is appropriate, it may prove maladaptive in certain clinical situations, such as congestive heart failure. In most instances of physiologic oliguria, a urinalysis will reveal a maximally concentrated urine (specific gravity >1.040) and a fractional excretion of sodium of less than 1%.^2,4-6 However, these changes may not be present if the animal has been receiving diuretics (e.g., furosemide or mannitol), has inappropriate neurohormonal abilities (e.g., hypoadrenocorticism), or suffers from a disease causing hyperosmolality (e.g., diabetes mellitus).