Why do on-pump patients live longer?

For the past 3 decades there has been a heated controversy in the world of coronary artery bypass graft (CABG) surgery. This has concerned whether to perform CABG with (on pump) or without (off pump) cardiopulmonary bypass. Each technique has advantages and shortcomings. For example on pump has often been associated with a small risk of stroke, whereas off pump reduces aortic manipulation [1]. Both approaches have been implicated in a systemic inflammatory response [1]. Despite >60 meta-analyses investigating this topic no conclusive answer has been reached and the debate continues [1]. Two of the problems encountered when seeking evidence are the sparsity of large randomised controlled trials (RCTs) and a focus on short-term clinical outcomes. A possible resolution to the latter was the long awaited publication of the five-year outcomes of two of the largest RCTs to date, ROOBY (Randomized on/off bypass) [2] and CORONARY (CABG off or on pump revascularization) [3] trials. This enabled a fresh metaanalysis investigating long-term clinical outcomes (mortality, myocardial infarction, stroke, angina and the need for revascularisation) incorporating 6 RCTs, and, most importantly, including ROOBY and CORONARY. The results showed a small but significant benefit of on pump in terms of mortality with all other comparisons showing no differences [4]. This raises a new question what is the reason/mechanism underlying on pump’s seemingly superior long-term survival rate?