review article Polyuria is excessive volume of urine for an adult (i.e. >3 liters/day). It’s a fairly common symptom, which is often noticed when one has to get upto use the bathroom at night. The causes include diabetes insipidus, diabetes mellitus, drinking a large amount of fluids, especially fluids that contain caffeine or alcohol, kidney failure, medications, especially diuretics, psychogenic polydipsia, sickle cell anemia and tests such as CT scan, that involve injecting contrast media. Role of Urine Output in ICU Settings Real-time and accurate monitoring of urine output could improve the clinical management of patients in the intensive care unit (ICU) and enable clinicians to early recognition of kidney injury.4 The decrease of urine output may be associated to a decrease of GFR due to decrease of renal blood flow or renal perfusion pressure, neurohormonal factors and functional changes may influence diuresis and natriuresis in critically ill patients. Systemic inflammation can induce natriuresis and diuresis changes due to functional changes unrelated to hypoperfusion, histological or tubular damage.2 Increased vascular response of the renal microcirculation to vasoconstrictors has been proposed to elicit intense renal vasoconstriction in sepsis-induced acute kidney injury (AKI).5 Endotoxemia also can increase urine output and water clearance despite decrease in GFR due to tubular aquaporin-2 dysfunction.6 The total interruption of renal blood flow for a prolonged period of time followed by reperfusion is always associated with major tubular and microvascular damage. In this condition, cellular lesions result from a combination of cellular hypoxia-reperfusion injury and oxidative stress-associated damage, it is expected that preventing a decrease of renal blood flow may prevent or limit the occurrence of AKI in ICU patients.7 Urine Output in Perioperative Period Inhalation and intravenous anesthetics cause cardiac depression or vasodilation, therefore decrease arterial blood pressure. The sympathetic blockade associated with regional anesthesia similarly causes hypotension. Decrease in blood pressure below the limits of autoregulation reduces RBF, GFR, urinary flow and sodium excretion. Also, sympathetic activation commonly occurs in the perioperative period as a result of light anesthesia, intense surgical stimulation, tissue Asian Journal of Critical Care Vol. 9, No. 2, January-March 2013
trauma or anesthetic-induced circulatory depression, which increases renal vascular resistance and activates various hormonal systems. Both effects tend to reduce RBF, GFR and urinary output.8 The endocrine response to surgery and anesthesia (catecholamines, renin, angiotensin II, aldosterone, antidiuretic hormone [ADH], adrenocorticotropic hormone and cortisol) is probably at least partly responsible for the transient postoperative fluid retention that is seen in many patients.8 Certain surgical procedures can significantly alter renal physiology. The pneumoperitoneum produced during laparoscopy produces an abdominal compartment syndrome-like state. The increase in intra-abdominal pressure typically produces oliguria (or anuria). Other surgical procedures that can significantly compromise renal function include cardiopulmonary bypass, crossclamping of the aorta and dissection near the renal arteries.8 Fluid overload, hypovolemia and postoperative renal failure are major causes of postoperative morbidity and mortality. Diuretics are also used intraoperatively, particularly during neurosurgical, cardiac, major vascular, ophthalmic and urological procedures. Preoperative diuretic therapy is also common in patients with hypertension and with cardiac, hepatic and renal disease.8 Urine Output in AKI per se Acute renal failure or AKI is defined by an acute decline of GFR. Occurrence of AKI is a marker of severity of the underlying acute illness but also appears as an independent factor associated with mortality in unselected critically ill patients,9 in sepsis,10 pneumonia11 or cardiac surgery.12 The Acute Dialysis Quality Initiative Group has published a consensus definition/classification system for AKI termed the Risk, Injury, Failure, Loss and End-stage kidney disease (RIFLE) criteria (Table 1). The Acute Kidney Injury Network (AKIN) group has recently proposed modifications to this system (Table 2). It is currently unknown whether there are advantages between these criteria. Compared to the RIFLE criteria, the AKIN criteria do not materially improve the sensitivity, robustness and predictive ability of the definition and classification of AKI in the first 24-hour after admission to ICU.13 7