Study population

We performed a retrospective single-centre study. All patients undergoing primary coronary angioplasty for STEMI between January 2010 and December 2011 in our University Hospital (Tours, France) were considered for inclusion.

STEMI was defined according to the European Society of Cardiology criteria, as: any chest pain lasting > 10 minutes associated with a significant increase in cardiac biomarkers and/or associated with new or presumed new ST-segment elevation of 0.2 mV in at least two contiguous leads or with the onset of a left bundle branch block . Patients’ aged ≥ 18 years who had had primary coronary angioplasty (successful or not) were eligible. Patients were excluded if they had been exposed to contrast injection within 7 days before coronary angioplasty or were on chronic renal replacement therapy.

Study protocol

Baseline serum creatinine concentration was measured from a blood sample obtained immediately after hospital admission or at the beginning of primary coronary angioplasty. The result was usually unavailable at the time of primary coronary angioplasty. Measurements were usually repeated at 24, 48 and 72 hours during the patient’s stay in the intensive coronary care unit. All blood samples were processed by the same laboratory in our hospital (AU2700Plus™ chemistry analyser, Beckman Coulter, Brea, CA, USA; and AU640™ immunochemistry analyser, Olympus, Center Valley, PA, USA). Creatinine clearance was calculated using the modified modification of diet in renal disease (MDRD) equation.

According to local custom, most patients received intravenous infusion of a 5% glucose solution from initial management by emergency crew to arrival in the catheterization laboratory. No preventive measures for CIN were recommended. After coronary angioplasty, infusion of a 5% glucose solution followed at the usual rate of 1 mL/kg/h, reduced to 0.5 mL/kg/h in patients with an impaired left ventricular ejection fraction (< 40%), overt heart failure or cardiogenic shock.

Coronary revascularization

Following our local procedure, primary coronary angioplasty was performed using either radial or femoral access (at the discretion of the operator), with size 6F guide catheters. Patients received a prehospital injection of acetylsalicylic acid 250 mg in addition to clopidogrel 600 mg or prasugrel 60 mg when eligible, and an intravenous bolus of unfractionated heparin (4000–6000 IU, adapted to weight). All patients received a monomer non-ionic low-osmolar contrast agent (iohexol, OMNIPAQUE™, GE Healthcare Pharmaceuticals, Chalfont St. Giles, UK). After identification, the culprit lesions were treated by direct coronary stenting or by balloon predilatation followed by stenting, at the discretion of the operator. The volume of contrast agent, the angioplasty technique and the use of pharmaceutical or mechanical haemodynamic support were in accordance with the European Society of Cardiology guidelines .

Cardiogenic shock was defined as acute circulatory failure of cardiac origin with a systolic arterial pressure < 80 mmHg, refractory to intravenous infusion of 500 mL of saline over 30 minutes and resulting in at least two additional organ failures.

According to our emergency protocol, renal support (haemodialysis or haemofiltration) was proposed in cases of anuria lasting > 24 hours, independent of the patient’s haemodynamic status.

Endpoints

The main endpoint was the occurrence of CIN, defined as a relative (≥ 25%) or absolute (≥ 0.5 mg/dL; 44 μmol/L) increase in serum creatinine from baseline within 3 days after primary coronary angioplasty. Kidney disease and chronic renal failure were defined according to the recommendations of the European Society of Nephrology, as a glomerular filtration rate (GFR) < 60 mL/min/1.73 m ² , estimated with the modified MDRD formula .

Concomitantly, the risk of CIN was assessed by application of Mehran’s score , based on the following eight variables: age (> or < 75 years); hypotension; congestive heart failure; need for haemodynamic support with intra-aortic balloon pump (IABP); baseline serum creatinine; diabetes; anaemia; and volume of contrast agent. Mehran’s score categorized our population into four different groups according to their estimated risk of CIN: a low-risk group (score < 5); a medium-risk group (5 < score < 10); a high-risk group (10 < score < 15); and a very high-risk group (score > 15).

Statistical analysis

Continuous data were expressed as mean values ± standard deviations. Categorical data were reported as percentages and absolute values. Comparisons between groups were made using the Chi ² test for categorical variables and Student’s t -test for continuous variables. A continuous regression model was used to identify variables independently associated with the occurrence of specified adverse events during follow-up. Potential confounders were included in the statistical model to fit. The results are expressed as relative risks with 95% confidence intervals (CIs). A P value < 0.05 was considered statistically significant. Statistical analyses were performed with JMP 9.1 software (JMP ^® , SAS ^® and all others, SAS Institute Inc., Cary, NC, USA). We tested the sensitivity, specificity, positive predictive value and negative predictive value of Mehran’s score for each patient risk group. We used several indicators to assess the informative value of the score, including positive likelihood, negative likelihood, Youden’s index and ROC curve. Youden’s index, also known as Youden’s J statistic, is a helpful validated tool to assess the performance of a diagnostic test ; it is easily calculated according to the formula J = sensitivity + specificity − 1.