Impact of Perioperative Acute Kidney Injury as a Severity Index for Thirty-Day Readmission after Cardiac Surgery

BACKGROUND:

Of patients undergoing cardiac surgery in the
United States, 15% to 20% are re-hospitalized within 30 days. Current
models to predict readmission have not evaluated the association between
severity of postoperative acute kidney injury (AKI) and 30-day
readmissions.

METHODS:

We collected data from 2,209
consecutive patients who underwent either coronary artery bypass or
valve surgery at 7 member hospitals of the Northern New England
Cardiovascular Disease Study Group Cardiac Surgery Registry between July
2008 and December 2010. Administrative data at each hospital were
searched to identify all patients readmitted to the index hospital
within 30 days of discharge. We defined AKI stages by the AKI Network
definition of 0.3 or 50% increase (stage 1), twofold increase (stage 2),
and a threefold or 0.5 increase if the baseline serum creatinine was at
least 4.0 (mg/dL) or new dialysis (stage 3). We evaluate the
association between stages of AKI and 30-day readmission using
multivariate logistic regression.

RESULTS:

There were 260
patients readmitted within 30 days (12.1%). The median time to
readmission was 9 (interquartile range, 4 to 16) days. Patients not
developing AKI after cardiac surgery had a 30-day readmission rate of
9.3% compared with patients developing AKI stage 1 (16.1%), AKI stage 2
(21.8%), and AKI stage 3 (28.6%, p < 0.001). Adjusted odds ratios for AKI stage 1 (1.81; 1.35, 2.44), stage 2 (2.39; 1.38, 4.14), and stage 3 (3.47; 1.85 to 6.50). Models to predict readmission were significantly improved with the addition of AKI stage (c-statistic 0.65, p = 0.001) and net reclassification rate of 14.6% (95% confidence interval: 5.05% to 24.14%, p = 0.003).

CONCLUSIONS:

In addition to more
traditional patient characteristics, the severity of postoperative AKI
should be used when assessing a patient’s risk for readmission.