The Impact of Fluid Balance on the Detection, Classification and Outcome of Acute Kidney Injury After Cardiac Surgery

Abstract

Objective: To assess whether or not a positive fluid balance masks acute kidney injury (AKI) and is associated with adverse outcomes after cardiac surgery.

Design: Retrospective analysis.

Setting: Tertiary university-affiliated metropolitan hospital: single center.

Participants: Three thousand two hundred seven consecutive cardiac surgical patients admitted to the ICU from July 1, 2004 to June 30, 2012.

Interventions: None.

Measurements and main results: The authors used AKI Network criteria to classify AKI. They then adjusted creatinine levels for weight-corrected fluid balance and categorized patients into 3 groups: group A (No AKI); group B (AKI only after adjustment); group C (AKI before and after adjustment). No patients had "AKI" before but "No AKI" after adjustment. Among 2,171 patients with weight and baseline creatinine available, after adjusting for fluid balance, the proportion of patients classified with AKI increased from 25.3% to 37.2% (p<0.001). In patients with AKI only after adjustment (group B), ICU mortality approximated that of group C (1.9% v 3.1%, p = 0.35) but was almost 3 times greater than group A (1.9% v 0.7%, p = 0.04). For group B, use of renal replacement therapy also was greater than group A (4.3% v 1.5%, p = 0.004) but less than group C (4.3% v 14.4%, p<0.001). The same trend was found for ICU length of stay (p≤0.001) and other adverse outcomes.

Conclusions: Patients with AKI diagnosed after correction for the effect of a positive fluid balance on serum creatinine concentration have more adverse outcomes than patients without AKI by conventional criteria, but fewer than patients with AKI by conventional criteria.

Keywords: acute kidney injury; cardiac surgery; cardiothoracic surgery; classification; fluid accumulation; fluid balance.