(1412) Predicting Kidney Injury in Cardiac Surgery Patients Based on Urine Output, an Observational Study

Introduction: Acute Kidney Injury (AKI) following cardiac surgery is associated with increased morbidity and mortality. AKI is staged using RIFLE, AKIN, KDIGO, or SOFA definitions, all including consensus criteria for oliguria. While many feel that oliguria is the earliest and most sensitive sign of worsening renal function, there is criticism that it is poorly defined and overly sensitive. This retrospective study compares AKI as defined by several metrics using urine output data from the first 48 hours post-surgery with kidney injury defined by creatinine upon hospital discharge.

Methods: A retrospective cohort study was conducted including 453 consecutive patients who underwent cardiac surgery at the University of Vermont Medical Center from January 1, 2015, through December 31, 2016. Excluded were intraoperative deaths and patients undergoing off-pump surgery, circulatory arrest, or preoperative dialysis. Data collected included urine output during surgery and hourly urine output during the first 48 hours after surgery. AKI was assessed using RIFLE, AKIN, KDIGO, and SOFA consecutive and averaged hourly urine output criteria during the first 48 postoperative hours. This gave a total of eight versions of urine output criteria for kidney injury. Each of these was then compared to kidney injury as defined by 1.5x preop creatinine or 0.3+preop creatinine at hospital discharge resulting in a total of 16 comparisons.

Results: The incidence of AKI varied based on the criterion used. From 3.3% (n=15) (≥1.5x preop creatinine) to 5.5%(n=25) (≥0.3 + preop creatinine) of patients had kidney injury at discharge. The percentages of postoperative AKI by urine output ranged from the most sensitive “Risk” criterion (6 consecutive hours, the average of which was < 0.5 ml/kg/hr) at 91.8% (n=416) to the least sensitive “Failure” (24 consecutive hours, each of which had < 0.3 ml/kg/hr) and SOFA Renal Score 4 criteria ( < 200 ml for 24 hours) at 0.88%(n=4). There was poor agreement between postoperative AKI as measured by any criterion for urine output and the presence of kidney injury at hospital discharge (Kappa: 0% to 24%).

Conclusions: Reduced urine output within the first 48 hours post-surgery is common and does not predict decreased renal function upon discharge in cardiac surgery patients.