Preserved Acute Pulmonary Endothelial Homeostasis with Hydrogen Gas Inhalation After Neonatal Hypoxia-Ischemia Despite Increased Neutrophil Accumulation.

Background/Objectives: Hydrogen gas (H2) inhalation has shown neuroprotective effects in neonatal hypoxic-ischemic models. However, its impact on pulmonary endothelial activation and respiratory function after hypoxic-ischemic insult remains unclear. This study investigated whether H2 inhalation augments pulmonary endothelial activation or impairs pulmonary function during the acute phase after neonatal hypoxic-ischemic injury. Methods: Sixteen newborn Camborough® piglets within 24 h of birth were subjected to hypoxic-ischemic insult and randomized to an untreated group (HI, n = 8) or an H2-treated group (HI-H2, n = 8). The HI-H2 group received 2.1-2.7% H2 for 6 h. Pulmonary ICAM-1 and inducible nitric oxide synthase (iNOS) expression were assessed by immunofluorescence, lung neutrophils were quantified histologically, and pulmonary function was evaluated using arterial blood gases, oxygen index, and alveolar-arterial oxygen difference. The relationship between lung neutrophil counts and right ventricular cardiac output was also examined. Results: H2 inhalation did not increase pulmonary ICAM-1 expression or iNOS induction compared with untreated animals. Although lung neutrophil counts were significantly higher in the HI-H2 group, pulmonary gas exchange remained preserved, with no significant differences in arterial blood gases, oxygen index, or alveolar-arterial oxygen difference. Lung neutrophil counts were positively correlated with right ventricular cardiac output, suggesting that enhanced pulmonary perfusion may contribute to neutrophil redistribution rather than inflammatory recruitment. Conclusions: H2 inhalation preserved pulmonary endothelial homeostasis without impairing respiratory function during the acute phase after neonatal hypoxic-ischemic insult. The observed increase in lung neutrophils was not accompanied by evidence of endothelial activation or deterioration of gas exchange, supporting the acute pulmonary safety of H2 inhalation while suggesting that neutrophil accumulation may, at least in part, reflect hemodynamic redistribution rather than injurious inflammatory infiltration.