Neuroprotective gases--fantasy or reality for clinical use?

The neuroprotective properties for certain medical gases have been observed for decades, leading to extensive research that has been widely reported and continues to garner interest. Common gases including oxygen, hydrogen, carbon dioxide and nitric oxide, volatile anesthetics such as isoflurane, sevoflurane, halothane, enflurane and desflurane, non-volatile anesthetics such as xenon and nitrous oxide, inert gases such as helium and argon, and even gases classically considered to be toxic (e.g., hydrogen sulfide and carbon monoxide) have all been supported by the evidence alluding to their use as potential neuroprotective agents. A wide range of neural injury types such as ischemic/hemorrhagic, stroke, subarachnoid hemorrhage, traumatic brain injury, perinatal hypoxic-ischemic brain injuries, neurodegenerative disease as well as spinal cord ischemia have been used as platforms for studying the neuroprotective effects of these gases, yet until now, none of the gases has been widely introduced into clinical use specifically for protection against neural injury. Insufficient clinical data together with contradictory paradigms and results further hinders the clinical trials. However, pre-clinical models suggest that despite the various classes of gases and the broad range of injuries to which medical gases confer, protection, several underlying mechanisms for their neuroprotective properties are similar. In this review, we summarize the literature concerning the neuroprotective effect of each gas and its underlying mechanisms, extract common targets reported for the neuroprotective effects of different gases, highlight the conflicting observations from clinical trials and further discuss the possible hindrances impeding clinical applications in order to propose future research perspectives and therapeutic exploitations.