Advanced, Clinical - Renal/Urine/Electrolytes
Oliguria is defined as having a urine output of <0.5mL/kg/hour. While urine output does not always correlate with glomerular filtration rate (GFR) and rarely results in acute kidney injury, it does serve a purpose in identifying the level of kidney injury defined by the RIFLE and AKIN (Acute Kidney Injury Network) criteria. One should consider all the possible reasons for oliguria in the perioperative period before assuming there is injury to the kidneys. Without a Foley catheter, oliguria or anuria may be an obstructive phenomenon. Even with a catheter, it is possible to have blood clots, kinking of the catheter, or other reasons to have obstruction of flow. Intra-operatively, it is not uncommon for urine output to decrease secondary to hypovolemia, hypotension, or an acute increase in ADH release as part of the surgical stress response. Interestingly, the use of intra-operative desmopressin for hematologic indications does not correlate well with a decrease in urine output. Surgical ligation of the ureters may also result in oliguria without kidney damage. As stated in the AKIN criteria, oliguria for a certain length of time in conjunction with an increase in serum creatinine (sCr) is correlated with the stage of kidney injury. Specifically, oliguria for 6 hours with an increase in sCr of at least 0.3mg/dL and up to 2 times baseline is stage 1 AKI, stage two is defined as increase in sCr of 2-3 times baseline with oliguria for 12hours, and stage 3 is an increase in baseline sCr of greater than 3 times the baseline, or greater than 4mg/dL, with urine output of <0.3mg/kg/hr for 12 hours.