Pharmacokinetics of a single inhalation of hydrogen gas in pigs

The benefits of inhaling hydrogen gas (H₂) have been widely reported but its pharmacokinetics have not yet been sufficiently analyzed. We developed a new experimental system in pigs to closely evaluate the process by which H₂ is absorbed in the lungs, enters the bloodstream, and is distributed, metabolized, and excreted. We inserted and secured catheters into the carotid artery (CA), portal vein (PV), and supra-hepatic inferior vena cava (IVC) to allow repeated blood sampling and performed bilateral thoracotomy to collapse the lungs. Then, using a hydrogen-absorbing alloy canister, we filled the lungs to the maximum inspiratory level with 100% H₂. The pig was maintained for 30 seconds without resuming breathing, as if they were holding their breath. We collected blood from the three intravascular catheters after 0, 3, 10, 30, and 60 minutes and measured H₂ concentration by gas chromatography. H₂ concentration in the CA peaked immediately after breath holding; 3 min later, it dropped to 1/40 of the peak value. Peak H₂ concentrations in the PV and IVC were 40% and 14% of that in the CA, respectively. However, H₂ concentration decay in the PV and IVC (half-life: 310 s and 350 s, respectively) was slower than in the CA (half-life: 92 s). At 10 min, H₂ concentration was significantly higher in venous blood than in arterial blood. At 60 min, H₂ was detected in the portal blood at a concentration of 6.9–53 nL/mL higher than at steady state, and in the SVC 14–29 nL/mL higher than at steady state. In contrast, H₂ concentration in the CA decreased to steady state levels. This is the first report showing that inhaled H₂ is transported to the whole body by advection diffusion and metabolized dynamically.