Ninety patients were enrolled in the study between May 2011 and December 2013 (clinicaltrials.gov identifier: NCT01408420). All patients were operated at Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark. The study was approved by The Regional Committee on Biomedical Research Ethics. Written informed consent was received from all patients prior to their inclusion in the study.
In- and exclusion criteria
To ensure that the cohort had a high preoperative risk of developing AKI, the inclusion criteria were age > 70 years and complex cardiac surgery procedures (heart valve and bypass surgery in combination). Exclusion criteria were serum creatinine (sCr) > 200 μmol/L, previous heart surgery, endocarditis and acute operation defined as coronary angiography < 24 h of surgery.
A parallel-group study was conducted where patients were randomised into either a control group (CG) or a high pressure group (HPG) using closed opaque envelopes that were sequentially numbered. The allocation ratio was 1:1. Randomisation was performed the day before surgery after informed consent. The patients and outcome assessors who analysed GFR, blood- and urine samples were blinded until follow-up. The generation of the random allocation sequence, the randomisation and enrolment of patients were done by the first author of this paper.
Patients were premedicated with triazolam 0.125–0.250 mg orally 1 hour prior to surgery. Anaesthesia was induced with fentanyl (10 μg/kg), propofol (1–2 mg/kg) and cisatracurium (0.1 mg/kg). Maintenance of anaesthesia was achieved using sevoflurane (0.5–3%) and continuous infusion of remifentanil (15 to 30 μg/kg/hour). Intravenous administration of 1500 mg of cefuroxime and a single-shot of 240 mg of gentamicin were given after induction of anaesthesia. No specific guidelines were used in regards to the administration of the single-shot gentamicin. AP was recorded through a cannula placed in the radial artery.
After heparinisation (350 IU/kg, ACT > 480 s), normothermic CPB (36.5–37.0 °C bladder temperature) was initiated as follows: in the ascending aorta an angled arterial cannula was placed (DLP 24 FR, Medtronic, Minneapolis, Minnesota) and a two-stage venous cannula (36/46 FR, Medtronic) was inserted through the right atrial appendage. A membrane oxygenator (Capiox RX25, Terumo, Tokyo, Japan) and a roller pump (Stockert S5, Sorin Group, Milano, Italy) were used for perfusion with non-pulsatile flow. The arterial line included a 40-μm filter (AL06, Pall, Port Washington, New York). Pump flow was calculated by multiplying 2.4 L/minute by the body surface area in square metres.
Intraoperative CPB data were retrieved from electronic perfusion charts. The AP during CPB was sampled electronically every minute by the heart-lung machine. The average AP during CPB was calculated using the values from clamping to de-clamping of the ascending aorta.
The primary end-point of this study was mean change in glomerular filtration rate (GFR) at follow-up compared to baseline.
The secondary end-point were change in urinary Neutrophil Gelatinase-Associated Lipocalin (uNGAL) and urinary creatinine ratios at different postoperative time-points compared to baseline, a method that has been shown to be highly sensitive in predicting AKI .
It was not possible to calculate sample size since no data on 51Cr-ethylenendiaminetetra acetic acid plasma clearance technique (Cr-EDTA) GFR measurements existed on cardiac surgery patients at the initiation of the study.
Baseline u-NGAL and urine creatinine were measured using urine samples taken just after induction of anaesthesia and insertion of a urinary catheter. Postoperatively the samples were taken at arrival to the intensive care unit (ICU) and 6-, 18-, 48- and 120 h postoperative. The samples were centrifuged for 5 min at 1000 RPM and the supernatant was pipetted into cryotubes. The cryotubes were placed in a freezer at − 80 degrees Celsius for a maximum of 9 months before analysis, a period that has previously been found to be safe for storing uNGAL . The urine samples were analysed for uNGAL using the NGAL Test reagent kit (BioPorto Diagnostics, Gentofte, Denmark) on a Cobas c501 analyser (Roche, Basel, Switzerland) using fully automated particle-enhanced turbidimetric immunoassay. On the same analyser urine creatinine levels were measured to adjust for postoperative hydration status using the Creatinine Plus version 2 reagent kit (CREP 2, Roche, Basel, Switzerland).
GFR was measured by the 51Cr-ethylenendiaminetetra acetic acid (EDTA) plasma clearance technique the day before surgery and 4 months postoperative . Four plasma samples were collected during 60 min within the period of three to 5 hours after tracer injection.
Baseline sCr values were defined as the preoperative value closest to the day of surgery. Postoperative sCr samples were taken in the morning of the first and second postoperative day. The patients were characterised as either AKI or no-AKI based on the RIFLE criteria. AKI was present when an increase in sCr values of > 50% or absolute increase of > 27 μmol/l occurred within the first 48 h postoperative, compared to baseline. An estimated GFR (eGFR) was calculated based on sCr using the Cockroft-Gault formula. In accordance with the RIFLE criteria an eGFR decrease of > 25% was also used to define AKI.
In the CG the patients underwent standard anaesthesia and CPB with the exception of a maximum of 110% flow on the heart-lung machine.
In the HPG an infusion containing isotonic sodium chloride and norepinephrine was mixed according to patient weight so that 1 ml/hour equalled 0.01 μg/kg/min of norepinephrine. This mixture was used when the AP was < 60 mmHg during CPB. If the target pressure could not be reached the infusion rate was not increased above 30 ml/min. Maximum flow on the heart-lung machine was 110%.
The study was conducted on an intention-to-treat basis.
Continuous data are presented as means ± standard deviations (SD) or median (interquartile range). Continuous variables were compared by Student’s unpaired t-test; categorical variables were compared by Pearson’s chi-squared test. Mann-Whitney U-test was used to compare sCr levels between the groups and u-NGAL/creatinine ratios between the groups at each time-point after correcting for the preoperative measurement. Paired samples t-test was used to compare preoperative GFR, eGFR and sCr with values at follow-up.
Differences were considered to be statistically significant when the p value was < 0.05.
Statistical analysis was performed using the statistical software package SPSS, version 188.8.131.52; SPSS Inc.; Chicago IL.