Present study demonstrates that pre-treatment with 60 minutes of hyperoxia followed by 33 (18–59) min normoxia before CPB does not afford protection against myocardial injury (cTn T release or myocardial function) associated with coronary artery bypass grafting. This is in accordance with our previous study where a different protocol of hyperoxic pretreatment was implemented . We established that surgery was associated with a well-defined release of systemic inflammatory mediators, and in patients ventilated with >95% oxygen before CPB the ratios of IL-10/IL-6 and IL-10/TNF-α were drifted towards anti-inflammatory. It could be a reflection of changes in the inflammatory response network/profile due to pre-treatment with hyperoxia.
By now we have tested two different times of hyperoxic exposure in clinical studies – 60 minutes followed by normoxia in the present and about 130 minutes immediately before CPB in a previous study . We did not find clinically relevant reduction of myocardial injury (release of cardiac troponin), although this manipulation resulted in some changes in the inflammatory profile. There can be several reasons for that. First, it could be speculated that in case of modern myocardial protection techniques only minimal myocardial injury is caused by cardioplegic cardiac arrest and there is no need for activation of intrinsic protective mechanisms. The situation is different in animal experiments where myocardial protective techniques are not used and injury is therefore well described. Although we included patients with at least 3-vessel CABG surgery, the aortic cross-clamping time of about 46 minutes could have been too short. Also, in power analysis we expected reduction of cTn T by 0.4 ng/mLin, which probably overestimates cardioprotective effect of whatever hypothetical intervention in these particular patients. To detect a difference of 0.1 ng/mL (the clinical relevance of which remains questionable), a study with several hundreds of patients needs to be conducted. Secondly, in difference from mice and rats, hyperoxic exposure of one or two hours in humans might be too weak stimulus to activate protective pathways. There is no direct evidence to support or oppose this idea. Some speculations might be drawn from the inter-species comparisons of myocardial injury in relation to duration of ischemia. For example, in rodent models of myocardial infarction, 30–40 minutes of either regional or global myocardial ischemia is sufficient to induce an infarct size of 50% of the area at risk. In the porcine heart, significantly longer duration (60–90 minutes) of myocardial ischemia is required to achieve equivalent levels of infarction. Human myocardial infarcts generally require 90 minutes to become established . Whether these data can be extrapolated to the exposure time of hyperoxia that is needed to activate endogenous cardioprotective mechanisms, is not clear. In addition, it has been shown recently that in comparison with human broncho-alveolar fluid proteome, oxidative stress response was selectively enriched only in mice . Regarding the response to pretreatment by hyperoxia, interspecies differences in evoking protective effect have been described even in the hearts of mice and rats5. Thirdly, there is a possibility that the phenomenon of hyperoxia induced myocardial protection does not exist in humans at all. In vitro studies, however, suggest that human myocardium can be preconditioned . Several in vivo surrogate models of ischaemic preconditioning have showed protective effect - preinfarction angina, repeated balloon inflations, intermittent aortic cross-clamping. Similar to these, hyperoxia has been shown to be a preconditioning evoking stimulus both in rodents and in human in vitro studies [2–10].
One factor that could also have an impact on the results is the preoperative usage of statins and angiotensin converting enzyme (ACE) inhibitors. Statins have shown to evoke cardioprotective effects both in animal models of ischemia-reperfusion injury and in clinical studies (reviewed by Ludman et al. ), and have the ability to reduce IL-6 and IL-8 after coronary surgery . ACE inhibitor treatment is associated with a reduction of IL-6 response to CABG  and confers added myocardial protection during surgical revascularisation .
Observed activation of inflammation in response to CPB is well in accordance with previous studies. TNF-α is a proinflammatory cytokine, which plays a central role in initiating and sustaining inflammation . It enhances oxidative stress in adult cardiac myocytes both by increasing reactive oxygen species generation as well as by decreasing antioxidants. Overexpression of TNF-α leads to cell injury due to excessive oxidative stress . IL-10 demonstrates potent anti-inflammatory properties through inhibiting the production of TNF-α and other proinflammatory cytokines . It has revealed to possess antioxidant like properties in situations where oxidative stress is increased .
IL-10 has been shown to inhibit the production of reactive oxygen species in isolated macrophages  and has been suggested to modulate TNF-α mediated, oxidative-stress- induced acute lung injury [14, 26, 27]. After elective cardiac surgery the ratio of IL-10/TNF-α messenger RNA has shown to be an independent predictor of outcome, and thus IL-10 may have a protective role after cardiac surgery . Hence, an appropriate balance between IL-10 and TNF-α may be of crucial importance. In the present study we observed a temporary increase of IL-10/IL-6 and IL-10/TNF-α ratios in the hyperoxia pre-treated patients, which suggests a change of the inflammatory profile towards anti-inflammatory. This can be a circumambage that hyperoxia may reduce inflammatory response immediately after CPB. Changes in immune system occur in 2 phases after cardiac surgery . The 1st phase represents the pro- and anti-inflammatory reaction returning to normal by the 3rd postoperative day. This is the cause of systemic inflammatory response with well known clinical manifestations as acute organ dysfunctions and failures. The 2nd phase emerges on postoperative day 5 and is characterised mainly by anti-inflammatory type of reaction. The synthesis of IFN-γ is significantly reduced after cardiac surgery and postoperative immunosuppression phenomenon in cardiac surgical patients has been well described .