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.