Hyperoxia: a review of the risks and benefits in adult cardiac surgery.

Perioperative hyperoxia has been claimed to have a number of therapeutic advantages. However, in the setting of cardiac surgery and cardiopulmonary bypass (CPB), enthusiasm for its use has been tempered by concerns regarding the effect of high partial pressures of oxygen on cardiac, vascular, and respiratory function and the potential for exacerbation of ischemia-reperfusion injury. There is encouraging evidence from animal studies that hyperoxia is effective in myocardial preconditioning, at least in nondiseased hearts. There is also evidence that hyperoxia reduces gas microemboli production and longevity during CPB, although it is unclear whether this translates into a clinical benefit in terms of a reduction in postoperative neurological morbidity. Hyperoxia leads to changes in cardiovascular function. However, the effects of these changes remain unclear. At a tissue level, there is evidence that hyperoxia does not lead to improvement in partial pressure of oxygen. Indeed, the opposite may be the case with reductions in capillary density leading to areas of reduced tissue oxygenation. The risks of hyperoxia in association with CPB include lung injury, increased systemic reactive oxygen species generation, and exacerbation of reactive oxygen species-mediated myocardial injury at the time of reperfusion. Again, it is difficult to know whether the changes demonstrated are temporary or if they translate into a worsening of clinical outcomes. In conclusion, perhaps the key to the use of hyperoxia is in the timing. In the period pre-CPB, hyperoxia may precondition the myocardium and, paradoxically, confer a degree of protection against reactive oxygen species-induced injury at the time of reperfusion. Hyperoxia during CPB is probably harmful and should be avoided unless the risk from gas microemboli is thought to be significant, in which case the risks and benefits to the individual patient must be weighed.