Predictors for Acute Kidney Injury Following Cardiac Surgery

Patients undergoing cardiac surgery may experience acute kidney injury (AKI), a common and serious complication. Nearly one in five patients undergoing cardiac surgery experience mild AKI, which is associated with a 19-fold increase in short-term mortality. Severe AKI, defined as requiring renal replacement therapy, occurs in 2% of patients; in those patients, mortality rates approach 20%. In addition to increased risk of morbidity and mortality, patients who develop AKI have longer hospital admissions, increasing the burden of healthcare costs. The risk of developing chronic kidney disease (CKD) is also associated with AKI.

Researchers in Australia, led by Keren Grynberg, MD, recently conducted a single center, prospective observational cohort study designed to: (1) define the incidence of AKI following cardiac surgery; (2) identify pre-morbid and operative risk factors for developing AKI; and (3) accurately predict patients at risk for AKI using routine biochemical tests. The researchers sought to test the hypothesis than an early serum creatinine, performed within 2 hours of cessation of surgery, would be predictive of AKI, which would serve as an early identifier of at-risk patients. Results of the study were reported online in BMC Nephrology [doi:10.1186/s12882-017-0504-y].

The study included patients who underwent elective on-pump cardiac surgery at Cabrini Hospital, Malvern, Victoria, Australia, between August 2011 and December 2012. Inclusion criteria were ≥18 years of age and the ability to provide informed consent. Exclusion criteria were end-stage renal disease (ESRD) treated with renal replacement therapy (RRT) or transplantation and lack of routine pre-operative urinalysis.

Acute Kidney Injury Network criteria were used to define and stage AKI: Stage 1: rise in serum creatinine of ≥26.4 mmol/L or 1.5 times baseline serum creatinine; Stage 2: increase of ≥2- to 3-fold from baseline serum creatinine; Stage 3: increase of >3-fold from baseline serum creatinine, a serum creatinine of ≥354 mmol/L with an acute increase of at least 44 mmol/L or initiation of RRT, all within 48 hours. The researchers then calculated the percentage change in serum creatinine and further divided the cohort into three categories: (1) increase of ≥15% from baseline; (2) decrease of ≥15%; (3) within 15% of baseline (control group).

There were 200 consecutive patients who underwent elective cardiac bypass surgery at the center. Of those, four were excluded (one had ESRD treated with RRT and three lacked pre-operative urinary analysis data).

Of the remaining 196 eligible patients, 73.5% (n=144) were male and median age was 70.8 years (range, 30-91 years). Mean pre-operative creatinine was 86 mmol/L with a mean estimated glomerular filtration rate (eGFR) of 75.63 mL/min/1.73m2. Thirty-seven patients (19%) had diabetes mellitus.

Forty of the 196 patients (20.4%) developed AKI. Thirty-five of those patients had stage 1 AKI (87.5%), three had AKI stage 2 (7.5%), and two had stage 3 AKI (5%). Two patients needed RRT and two died; both of those occurrences were in patients with stage 3 AKI. Patients with AKI were older than those without AKI (median age, 75.2 vs 69.7 years; P=.023), had more diabetes (30% vs 16%, P=.04), and had poorer preoperative kidney function (eGFR 69 vs 82 mL/min/1.73 m2; P=.002). Patients with AKI more commonly had CKD stages 3 and 4 (38% vs 16%; P=.013) and required longer stay in the intensive care unit (mean duration of 2.8 vs 1.6 days for patients without AKI; P<.001). Total length of hospital stay was longer for patients with AKI (mean duration of stay 15 vs 11.1 days; P=.002). In the group with AKI, eGFR at discharge was significantly lower compared with the group without AKI (46 vs 84 mL/min/1.73 m2); mean serum creatinine levels were 115 and 79 mmol/L, respectively; P<.001).

Results of univariate logistic regression analysis demonstrated a significant association between hypertension (odds ratio [OR], 2.64; 95% confidence interval [CI], 1.10-6.36; P=.02), diabetes mellitus (OR, 2.25; 95% 1.01-5.0; P=.04), and proteinuria (OR, 2.48; 95% CI, 1.1-5.56; P=.02) and increased likelihood of AKI. Higher pre-operative eGFR was protective (OR, 0.74 per 10/mL/min/1.73 m2, 95% CI, 0.61-0.89; P=.002).

Compared with a change of <15% in the early post-operative serum creatinine, a rise of >15% over baseline was associated with a nonsignificant increased risk of AKI (OR, 3.44; 95% 0.72-16.55; P=.122). There was protective value for the development of AKI in a fall of >15% from baseline in the early post-operative serum creatinine (OR, 0.34; 95% CI, 0.16-0.73; P=.006).

A multivariate logistic model with preoperative and surgical factors (age, sex, eGFR, proteinuria, hypertension, diabetes, and type of cardiac surgery) demonstrated moderate discrimination for AKI. The addition of immediate post-operative serum creatinine improved model discrimination for AKI and was independently associated with AKI (OR, 7.17; 95% CI, 1.27-40.32; P=.025).

Study limitations cited by the authors included recruiting patients from a single center, assessing AKI at 48 hours only, and the low rate of stage 3 AKI.

In summary, the researchers said, “We found that a rise of >15% in serum creatinine over baseline, taken within 2 hours of arrival to the intensive care unit, equated to an odds ratio of seven for the development of AKI. This is important as serum creatinine is an inexpensive, readily available test and measurements of early serum creatinine could easily be incorporated into clinical practice. even in resource poor settings. We suggest that patients who demonstrate an increase in early serum creatinine should be referred to a nephrologist in a timely manner. This would enable early interventions and potentially improve outcomes.”

Takeaway Points

  1. Acute kidney injury (AKI) following cardiac surgery is common and is associated with significant morbidity and mortality.
  2. In a single-venter, prospective study, researchers in Australia aimed to define the incidence of AKI post-cardiac surgery and to identify risk factors for developing AKI.
  3. Factors associated with increased risk for post-operative AKI were hypertension, diabetes mellitus, proteinuria, and a lower baseline estimated glomerular filtration rate.