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CORONARY is a large international randomised controlled trial comparing coronary artery bypass graft (CABG) surgery done with and without a bypass pump. Compared with on-pump, off-pump surgery may prevent acute kidney injury (AKI) in the short term and may better preserve kidney function 1 year following surgery. Secondary analyses may also clarify whether effects are similar in patients with and without pre-operative chronic kidney disease and whether AKI avoidance mediates preserved 1-year kidney function.
With respect to the study schedule, the last of 4752 patients from 79 sites in 19 countries were randomised in November 2011 to cardiac surgery performed with an on-pump or off-pump procedure. The authors will use regression models to compare the groups in the outcome of peri-operative AKI (per cent change in serum creatinine, ≥50% increase in serum creatinine) and 1-year kidney function (per cent change in estimated glomerular filtration rate (eGFR), ≥20% eGFR loss 1 year after surgery). The authors will use interaction terms in regression models to determine if there is a differential impact of the intervention in those with and without pre-existing chronic kidney disease. The authors will use regression-based tests to determine the proportion of the total effect of surgery type (off-pump vs on-pump CABG) on 1-year eGFR that is mediated by peri-operative AKI.
In the year 2009, the authors were competitively awarded a grant from the Canadian Institutes of Health Research to answer these kidney questions in CORONARY. Ethics approval was obtained for additional renal data collection in centres that agreed to study participation (>90% of participating centres). This collection began for patients enrolled after 1 January 2010. Remaining 1-year renal outcome data will be collected throughout 2012. Results will be reported in 2013.
Coronary artery bypass grafting (CABG) surgery is an effective way of improving symptoms and prolonging life in patients with severe coronary artery disease.1 Globally, 1.25 million cardiac surgeries are performed annually.2 A frequent and serious complication of CABG surgery is acute kidney injury (AKI), which is a sudden deterioration in kidney function. Peri-operative AKI is independently associated with increased short- and long-term mortality, a greater length of hospital stay and higher economic costs.3
CABG surgery is usually performed with the use of cardiopulmonary bypass (‘on-pump’). This requires cannulation of the heart and aorta, cross clamping of the ascending aorta and cardioplegic arrest. In this setting, AKI is multifactorial: the bypass circuit changes blood concentration and pulsatility and introduces a blood/circuit interface, which stimulates a generalised inflammatory response including increased catecholamine and free haemoglobin levels.4 Furthermore, aorta cannulation and cross clamping may lead to platelet or atheromatous embolic phenomena.
CABG surgery performed on a beating heart without the use of cardiopulmonary bypass (‘off-pump’) was developed to avoid post-operative complications from the pump. A systematic review published in 2010 evaluated AKI across 22 randomised trials comparing off-pump with on-pump surgery (total 4819 patients).5 Off-pump CABG resulted in a 40% RR reduction in AKI (variably defined in the primary trials). The results did not demonstrate statistical heterogeneity. However, as concluded by the review authors, ‘different definitions of AKI used in the individual trials and methodological concerns preclude definitive conclusions about the treatment effect’.
A limitation of most AKI prevention trials is a failure to consider any sustained effect of the intervention on longer term kidney function. It is possible that off-pump compared with on-pump surgery better preserves 1-year kidney function, but to our knowledge, this has not been previously evaluated. This effect may be mediated through the AKI itself (ie, avoiding AKI preserves kidney function at 1 year6 7) or may be mediated through other pathways (ie, off-pump bypass improves cardiac function which preserves 1-year kidney function). Understanding the degree to which avoiding AKI preserves longer term kidney function has broader implications for the acceptability of side effects and costs of interventions which prevent AKI. Finally, patients with pre-operative chronic kidney disease (CKD) are at high risk of AKI and are a subgroup where preservation of renal function is of key interest.8
We are conducting the CABG Off- or On-Pump Revascularisation Study (acronym CORONARY). The methods of this large, international randomised controlled trial are described elsewhere.9 In brief, after obtaining written informed consent, adult patients undergoing isolated CABG surgery (with a median sternotomy) are allocated to have the procedure done with or without a bypass pump. Allocation is done by a voice-activated telephone randomisation service. This trial is funded by the Canadian Institutes of Health Research. With respect to the study schedule, the last of 4752 patients from 79 sites in 19 countries were randomised in November 2011. The primary 30-day composite outcome is total mortality, stroke, non-fatal myocardial infarction or receipt of dialysis for severe AKI. The 30-day results will be analysed and reported in March 2012. One-year outcome data will continue to be collected throughout 2012 and will be reported in 2013.
In addition to the primary outcome, CORONARY is uniquely positioned to answer important kidney-specific questions. In the year 2009, we were competitively awarded another grant from the Canadian Institutes of Health Research to collect additional renal information within CORONARY to address these questions. Ethics approval was obtained for additional renal data collection in centres that agreed to study participation (>90% of participating centres). This collection began for patients enrolled after 1 January 2010. Remaining 1-year renal outcome data will be collected throughout 2012. Results will be reported in 2013.
The kidney questions detailed in the grant are presented below and are followed by pre-specified analytic plans.
The pre-operative serum creatinine value (within 7 days prior to randomisation) has been recorded since the start of the trial, along with the peak value during the hospital stay. We define pre-operative CKD as an estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m2, using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation (which requires knowledge of whether a patient is of black race; <0.1% of CORONARY patients).10 In centres that agreed to additional renal data collection, we started recording all serum creatinine measurements during the hospital stay for patients enrolled after 1 January 2010 (not just the peak value) and a serum creatinine 1 year after surgery.
To refine the analytic plan in December 2011, we reviewed all CORONARY data without knowledge of patient allocation. Table 1 presents AKI according to modern staging systems.11 12 Table 2 presents changes in kidney function 1 year after surgery. The analytic plan was finalised without any knowledge of CORONARY outcomes by allocation group.
For CORONARY kidney substudy analyses, all randomised patients will be included except as follows: (1) patients receiving chronic haemodialysis prior to randomisation (as these patients cannot develop AKI—to date 1.3% of randomised patients), (2) patients with a baseline eGFR <15 ml/min/1.73 m2 prior to randomisation (ie, patients with end-stage renal disease (ESRD)13; to date an additional 0.2% of patients), (3) patients missing a pre-randomisation serum creatinine value as we cannot reliably define AKI without knowledge of the baseline value (<1% of patients) and (4) patients who never undergo CABG surgery as they will not have the opportunity to have any post-operative serum creatinine measurements (<1% of randomised patients).
Patients are randomised to on-pump or off-pump CABG, with randomisation stratified by centre. The intention-to-treat principle will guide all analyses, irrespective of whether there is a deviation from the randomly allocated therapy. Currently, the cross-over rate is 4% from on-pump to off-pump and 10% from off-pump to on-pump surgery.
We will use a linear regression model to compare the groups in the outcome of per cent change in serum creatinine ((peak post-operative serum creatinine—pre-randomisation serum creatinine)/pre-randomisation serum creatinine), stratified by centre and adjusting for the following covariates: age (per year), sex, left ventricular function categories (≥50%, 35%–49%, 20%–34%, <20%), diabetes, pre-randomisation ACE inhibitor or angiotensin receptor blocker use, pre-randomisation statin use, pre-randomisation diuretic use, urgent versus elective surgery and pre-randomisation eGFR category (>60 ml/min/1.73 m2, ≤60 ml/min/1.73 m2). We will include a missing data indicator value for each covariate (at present, there is <1% missing for each variable).14 In patients who underwent surgery but have a missing post-operative peak serum creatinine value (<4% patients), we will carry the pre-randomisation serum creatinine forward as the post-operative value (which should provide a more conservative estimate of the intervention effect than the alternative of removing such patients). We will test model assumptions (detailed in online appendix) and interpret a p value ≤0.05 as statistically significant. We will report the result as the average difference in per cent change in serum creatinine between the surgical groups with 95% CI. Visually the unadjusted results will be graphed as box-plots. A sample of ~4700 patients will have over 80% power to detect a 5% or greater difference in the mean per cent change in serum creatinine between the two groups (α 0.05, independent samples t test; adequate power to detect a small effect in relation to expected SD of 60).
We will use a logistic regression model to compare the groups in the outcome of ≥50% increase in serum creatinine, stratified by centre and adjusting for previously defined covariates11 15 16 We will test model assumptions (detailed in online appendix) and will estimate the adjusted RR of AKI with 95% CI (bootstrap method detailed in online appendix). We will have over 90% power to detect a 20% or more RR reduction should it exist (two-tailed α 0.05, χ2 test). Of note, many factors including haemodilution add ‘noise’ to post-operative serum creatinine measurements. When the value is markedly elevated, we are confident that the reason is due to AKI. It is conceivable that an intervention effect will be observed for categorised AKI but not per cent change in creatinine. If the p value is greater for per cent change than categorised AKI, we will interpret a p value ≤0.025 for categorised AKI as statistically significant.17 Because categorised AKI is easy to interpret, if it meets criteria for significance, we will primarily focus on this outcome.
We will perform six analyses defining AKI in other ways. We will interpret a p value ≤0.05 as significant provided there is concordance with the primary results.
Similar to the previously described AKI analysis, we will use a linear regression model to compare the groups in the outcome of per cent change in eGFR 1 year after surgery ((post-operative eGFR measured 1 year after surgery—pre-randomisation eGFR)/pre-randomisation eGFR), stratified by centre and adjusting for covariates described for AKI. In patients with missing 1-year values (~5% for reasons of death, target <10% for reasons of missing data), we will substitute the pre-randomisation eGFR value. We will impute an eGFR value of 5 ml/min/1.73 m2 for any patient who developed ESRD anytime in follow-up (<0.1%) or who died after receiving acute dialysis (~0.6% patients). With at least 1800 patients, we will have over 80% power to detect a difference of 4% or more between the two surgical groups (α 0.05, independent samples t test, adequate power to detect a small to moderate effect in relation to expected SD of 25).
Similar to the previously described AKI analysis, we will use a logistic regression model to compare the groups on the outcome of ≥20% eGFR decrease 1 year after surgery (ie, loss of over a fifth of kidney function). Anyone who develops ESRD in follow-up, or receives acute dialysis and dies before a serum creatinine measurement can be obtained, will be counted as having met this definition. We will have between 75% and 89% power to detect a 25%–30% RR reduction should it exist (two-tailed α 0.05, χ2 test). As with AKI, it is conceivable that an intervention effect will be observed in the categorised but not continuous outcome. In this scenario, we will interpret statistical significance as described for AKI.
We will perform two analyses examining 1-year kidney function in other ways. We will interpret a p value ≤0.05 as significant provided there is concordance with the primary results.
We will use interaction terms in the previously described linear and logistic regression models to determine if there is a differential impact of the intervention in those with and without CKD. We will interpret a p value ≤0.05 as statistically significant. Despite the size of CORONARY, there will only be adequate statistical power for very large subgroup effects.
A mediating variable is one that explains all or part of the association between a predictor and a response.19 In CORONARY, mediation will occur if on-pump versus off-pump CABG surgery influences the development of AKI, which influences 1-year kidney function.20 Regression-based tests for mediation have been well developed and are widely used.21 22 Figure 1 presents our analytic framework. We will test for a significant mediation effect (methods detailed in online appendix). We will report the proportion of the total effect of surgery type (off-pump vs on-pump CABG) on per cent change in 1-year eGFR that is mediated by peri-operative per cent change in serum creatinine. As well, we will consider mediation with AKI defined as a categorical variable (methods detailed in online appendix).
The sample in CORONARY almost exceeds the combined number of patients enrolled across 70 randomised controlled trials, which tested strategies to prevent or treat AKI in cardiac surgery.23 It will be the largest AKI prevention trial conducted to date. International recruitment across 19 countries will provide generalisable estimates of the treatment effect. In this report, we have judiciously pre-specified the main questions and analytic protocol that will be used to test relevant kidney hypotheses in CORONARY. We have done so to minimise the chance of spurious post-hoc assertions of effect, so that the kidney results from this large international trial are robust and believable.
To cite: Garg AX, Devereaux PJ, Yusuf S, et al. Coronary Artery Bypass Grafting Surgery Off- or On-pump Revascularisation Study (CORONARY): kidney substudy analytic protocol of an international randomised controlled trial. BMJ Open 2012;2:e001080. doi:10.1136/bmjopen-2012-001080
Contributors: All the authors made substantial contributions to the conception and design, acquisition of data, and analysis and interpretation of data; drafting the article or revising it critically for important intellectual content and final approval of the version to be published.
Funding: CORONARY and the CORONARY kidney substudy are funded by the Canadian Institutes of Health Research. The authors are solely responsible for the design and conduct of this trial, all analyses, the drafting and editing of the report, its final contents and the decision to submit for publication.
Competing interests: None.
Ethics approval: Ethics approval was provided by all participating trial centres.
Provenance and peer review: Not commissioned; internally peer reviewed.
Data sharing statement: No additional data are available.