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Diatomic hydrogen (H2) is the lightest and most abundant molecule in the universe. Hydrogen's reactivity with oxygen is well recognized as a thermodynamically favorable redox reaction that forms water. H2's small size results in unsurpassed diffusivity (even diffusing through steel), enabling it to reach therapeutic targets effectively even in low-flow states. In vivo, inhaled H2 undergoes solubilized transport to the tissues and undergoes redox reactions with reactive oxygen species (ROS) within the mitochondrion, resulting in a substantial arteriovenous difference in dissolved H2 content (∼10 ng/mL).
Shown in Figure 1, H2 is an ideal therapy for ischemia and reperfusion, where the underlying pathophysiology is mitochondrial oxygen deprivation leading to the subsequent formation of ROS. Unreduced ROS initiate a complex cascade that leads to oxidation of DNA, lipid membranes, and other structures culminating in apoptosis.
In this issue of the Journal, Kimura and colleagues
describe a series of well-controlled experiments demonstrating that H2 attenuates the effects of experimental spinal ischemia in a dose-dependent fashion with improved motor function, reduced histopathologic injury, and reduced cerebrospinal fluid glutamate and hydroxyperoxide concentrations. These findings are consistent with a preponderance of evidence that H2's primary effect is related to reactivity with and chemical reduction of oxygen species. In addition to glutamate, mitigation of this central pathology has been shown to alter various signal transduction pathways and gene expression, including calcium-dependent signaling pathways such as nuclear factor of activated T cells,
H2 appears to have a favorable safety profile in a diverse range of clinical settings. Perhaps most important is the environmental safety of handling hydrogen gas mixtures, since mixtures containing an excess of 4% H2 exceed the lower flammability limit and pose a substantial hazard in a clinical setting. This can be overcome through the use of commercially produced, certified, nonflammable hydrogen mixtures that contain less than 4% H2. Specifically, we have shown that 2.4% H2 mixtures with air, oxygen, and oxygen−carbon dioxide mixtures can be attached to the gas inlets of standard ventilators, anesthesia machines, and membrane oxygenators (for cardiopulmonary bypass sweep gases) even in the surgical setting.
When so arranged, administered H2 concentration remains constant independent of inspired oxygen fraction or cardiopulmonary bypass or ECMO flow rates, and the environmental hazards are eliminated.
Work to determine whether H2 will prove efficacious in the clinical setting is underway. Our group has recently embarked on a phase I Investigational New Drug trial examining the clinical safety and feasibility of H2 inhalation in healthy adults (NCT04046211). This is the first step in determining whether H2 becomes an important therapeutic in the treatment of clinical conditions in which ischemia and reperfusion play a central role.
Hydrogen acts as a therapeutic antioxidant by selectively reducing cytotoxic oxygen radicals.
Disclosures: The authors reported no conflicts of interest.
The Journal policy requires editors and reviewers to disclose conflicts of interest and to decline handling or reviewing manuscripts for which they may have a conflict of interest. The editors and reviewers of this article have no conflicts of interest.
This experimental study aimed to assess the efficacy of hydrogen gas inhalation against spinal cord ischemia–reperfusion injury and reveal its mechanism by measuring glutamate concentration in the ventral horn using an in vivo microdialysis method.