Hydrogen inhalation is associated with a transient rightward shift in prefrontal oxyhemoglobin asymmetry and...

Molecular hydrogen (H₂) has selective antioxidant and anti-inflammatory properties, yet its immediate effects on human cerebral oxygenation and autonomic function remain unclear. In this study, we evaluated acute central and autonomic responses to a single 30-minute session of hydrogen inhalation in healthy adults, using 99.9% hydrogen delivered via nasal cannula at a fixed flow rate of 300 mL/min. Cerebral oxygenation was assessed using time-domain near-infrared spectroscopy (TD-NIRS) to quantify the concentrations of oxyhemoglobin (oxy-Hb) and deoxyhemoglobin (deoxy-Hb) in the bilateral prefrontal cortex (PFC), and to calculate interhemispheric asymmetry indices before hydrogen inhalation, immediately after the end of inhalation, and at 30 and 90 min thereafter. Autonomic activity was assessed via continuous electrocardiography (ECG) to derive heart rate, R-R interval, and frequency-domain heart rate variability metrics (low-frequency (LF), high-frequency (HF) and LF/HF ratio). Hydrogen inhalation elicited robust, transient increases in the right-PFC asymmetry of the oxy-Hb concentration. Concurrently, the LF/HF ratio increased during inhalation, indicating sympathetic activation, followed by decreases in the heart rate after inhalation, consistent with parasympathetic recovery. These parallel cerebral and autonomic responses suggest a coordinated neurovascular-autonomic coupling in response to hydrogen inhalation. Our findings show that acute hydrogen inhalation transiently modulates PFC oxygenation lateralization and autonomic tone, suggesting potential relevance to cognitive and cardiovascular regulation.