Renal replacement strategies: dual benefits of hydrogen gas inhalation and hydrogen-enriched dialysate

Chronic kidney disease remains a significant global health challenge, with a substantial proportion of patients progressing to end-stage renal disease, necessitating renal replacement therapies (RRT) such as hemodialysis (HD).1 Despite advancements in dialysis techniques, patients on long-term HD experience heightened oxidative stress and systemic inflammation, contributing to an elevated risk of cardiovascular disease and mortality.　Malnutrition-inflammation-atherosclerosis syndrome, first described in 2000, is a pivotal contributor to the significantly high mortality rates observed in patients on HD.2

This syndrome is underpinned by chronic inflammation driven by oxidative stress, the accumulation of uremic toxins, and the inherent stress of HD itself.3 Together these factors form a vicious cycle of malnutrition, systemic inflammation, and accelerated atherosclerosis, significantly impairing patient survival and patient-reported outcomes.

Recent advances in therapeutic strategies have highlighted the potential of molecular hydrogen (H₂) as an innovative intervention. Unlike conventional antioxidants, H₂ selectively scavenges harmful reactive oxygen species, such as hydroxyl radicals, while preserving essential physiological processes. These properties make H₂ an attractive candidate for mitigating oxidative stress, a central mechanism in malnutrition-inflammation-atherosclerosis syndrome. In this context, H₂-enriched dialysate has emerged as a promising approach, with initial studies demonstrating its ability to reduce oxidative stress during dialysis. For example, an 8-week trial of electrolyzed-water hemodialysis (EW-HD) with a dialysate containing 154 ppb of dissolved H₂ reported improvements in patient fatigue, although complete symptom resolution was not achieved.4

Beyond dialysate enrichment, H₂ gas inhalation has shown substantial therapeutic potential, particularly in emergency and critical care settings, by attenuating oxidative stress and systemic inflammation.5 These effects are particularly relevant for dialysis patients, who experience a chronic inflammatory state due to their underlying disease and the dialysis process. Emerging clinical evidence suggests that combining H₂ gas inhalation and RRT using H₂-enriched dialysate may enhance the management of oxidative stress and inflammation in patients with HD.

Despite these promising developments, the optimal concentration of H₂ in dialysate—a critical parameter for maximizing therapeutic efficacy—remains defined. This study aims to explore the combined effects of H₂ gas inhalation and H₂-enriched dialysate during HD, focusing on their potential to reduce oxidative stress and inflammation. By investigating this novel approach, we aim to contribute to the growing body of evidence supporting the integration of H₂ into RRT for improved patient outcomes.

Methods: This article evaluated the combined effects of H₂ gas inhalation and RRT using H₂-enriched dialysate across multiple RRT modalities. The primary focus was on oxidative stress, inflammation, and clinical outcomes in patients on HD.

RRT modalities utilized included conventional HD, online hemodiafiltration (OL-HDF; pre-dilution), off-line hemodiafiltration (off-line HDF; post-dilution), EW-HD, and electrolyzed-water OL-HDF (EW-OL-HDF).

EW-HD employed a novel system utilizing electrolyzed water containing H₂ as the dialysate, prepared through reverse osmosis (Trim Medical Institute, Osaka, Japan; EW-SP11-HD: the personal use model). The system delivered dialysate with an average H₂ concentration of 300 ppb.

Off-line HDF used H₂-enriched substitution fluid prepared with an H₂-absorbing alloy canister (6.06 L SUBPACK-Bi, Nipro Corp., Osaka, Japan).6 H₂ gas pressurization (0.05 MPa) was performed using a DAYS hydrogen gas filling system (Doctors Man Co., Ltd., Yokohama, Japan). The substitution fluid was shaken for 30 seconds to enhance H₂ dissolution, followed by pressure normalization before infusion. The process ensured the H₂-enriched substitutional fluid was prepared immediately before each infusion session.

Patients inhaled 99.99% pure H₂ gas generated by the H2JI1 H₂ inhaler (Doctors Man Co., Ltd.) at a flow rate of 250 mL/min.7 The inhalation involved a 2.5% H₂ gas mixture delivered via nasal cannula three times weekly during HD sessions (4–5 hours/session).

Oxidation-reduction potential (ORP) was measured in heparinized plasma using the RedoxSYS Analyzer (Aytu BioScience Inc., Englewood, CO, USA).8 Dialysate ORP was assessed with an RM-30P ORP meter (TOA DKK, Tokyo, Japan). Blood oxygen partial pressure was analyzed using a blood gas analyzer (OPTI CCA-TS2, SYSMEX Corp., Kobe, Japan). H₂ concentration in blood was quantified by gas chromatography (TRIlyzer mBA-3000, TAIYO Instruments Inc., Osaka, Japan). Dialysate H₂ concentration was measured with a dissolved hydrogen meter (DH-35A, TOA DKK, Tokyo, Japan).

Dialysate solutions included Kindaly (Fuso Pharmaceutical Industries, Osaka, Japan; acetate content) and Carbostar (AY Pharmaceuticals Co., Ltd., Tokyo, Japan; acetate-free).

Quality of life and health status were evaluated using the Euro-Qol Visual Analog Scale (EQ-VAS), with scores ranging from 0 (worst imaginable health) to 100 (best imaginable health).9

All data were analyzed using appropriate statistical methods. ORP, oxidative stress markers, and patient-reported outcomes were assessed pre- and post-intervention. Continuous variables were compared using paired t-tests, while categorical variables were analyzed with chi-square tests. A P-value < 0.05 was considered statistically significant.

Results: This comprehensive approach explored the combined therapeutic potential of H₂ gas inhalation and H₂-enriched dialysate in mitigating oxidative stress and inflammation while improving patient outcomes during RRT.

Blood H₂ concentrations during various RRT modalities are shown in Table 1. Notably, in EW-OL-HDF with the dialysate H₂ levels of 300 ppb, the arterial blood H₂ concentration at the inlet of the hemodiafilter was minimal (approximately 0.5 ppb), despite the elevated venous blood H₂ concentration at the outlet. The venous concentrations closely mirrored the dialysate H₂ levels (e.g., 110 ppb), indicating efficient H₂ transfer during treatment (Figure 1A).

Table 1.

Typical concentrations of blood hydrogen in various renal replacement therapy modalities

RRT	Normal Vol.	Conv. HD(F)		EW-OL-HDF		Off-line HDF
H2 gas inhalation	Yes	No	Yes	No	Yes	Yes
A-side DH (ppb)	20	0	20	0.5	20	20
V-side DH (ppb)	10	0	1–2	300	300	60–70

A-side: Artery side (inlet of the dialyzer or hemodiafilter); Conv.: conventional; DH: dissolved hydrogen; EW-OL-HDF: electrolyzed-water online hemodiafiltration; HD(F): hemodialysis or hemodiafiltration; HDF: hemodiafiltration; RRT: renal replacement therapy; Vol.: volunteers with normal renal function; V-side: venous side (outlet of the dialyzer or hemodiafilter).

Figure 1.

Patient-reported outcomes, dialysate and plasma oxidation-reduction potential, and blood partial pressure of oxygen in various renal replacement therapy modalities.

(A) Blood hydrogen concentrations during electrolyzed-water hemodialysis with hydrogen gas inhalation in three patients. (B) Euro-Qol Visual Analog Scale scores for two cases across renal replacement therapy modalities. (C) Dialysate and plasma oxidation-reduction potential levels. Data are expressed as mean ± SD (n = 11, 5, 5, 4, 9, 6, 13). Kindaly (Fuso Pharmaceutical Industries) and Carbostar (AY Pharmaceuticals Co., Ltd.) dialysis solutions were used. (D) Arterial blood partial pressure of oxygen levels 1 hour post-dialysis initiation. (E) Changes of ORP levels in dialysate and draining dialysate (after passing through a dialyzer or a hemodiafilter). Carbostar (AY Pharmaceuticals Co., Ltd.) dialysis solutions were used. Created with Microsoft PowerPoint. A-side: Artery side (inlet of the dialyzer); Conv.: conventional; DH: dissolved hydrogen; EQ-VAS: Euro-Qol Visual Analog Scale; EW-HD: electrolyzed-water hemodialysis; EW-OL-HDF: electrolyzed-water online hemodiafiltration; HD(F): hemodialysis or hemodiafiltration; HD: hemodialysis; HDF: hemodiafiltration; inhal.: inhalation; OL-HDF: online hemodiafiltration; ORP: oxidation-reduction potential; PaO_2: partial pressure of oxygen in arterial blood; V-side: venous side (outlet of the dialyzer).

The EQ-VAS scores for two representative cases over 2 to 12 weeks of RRT are presented in Figure 1B. Among the RRT modalities, EW-OL-HDF combined with H₂ gas inhalation exhibited the most significant improvement in EQ-VAS scores, with participants reporting scores approaching 100, indicative of excellent perceived health. Off-line HDF with H₂-enriched substitution fluid (60–70 ppb dissolved H₂) achieved a modest EQ-VAS score of approximately 80, while conventional HD yielded lower scores.

Dialysate and plasma ORP levels during various RRT modalities are depicted in Figure 1C. Conventional HD demonstrated significantly elevated dialysate ORP levels (> 300 mV), associated with correspondingly higher plasma ORP levels compared with control subjects. In contrast, EW-HD and EW-OL-HDF treatments significantly reduced dialysate ORP, with a corresponding decline in plasma ORP levels.

The partial pressure of oxygen in arterial blood (PaO₂)measured 1 hour after initiating dialysis is illustrated in Figure 1D. Conventional HD was associated with elevated PaO₂ levels, exceeding 120 mmHg, likely due to the high oxygen content in the dialysate. In comparison, EW-HD exhibited significantly lower PaO₂ levels, suggesting reduced oxidative stress during treatment.

Changes occur in dialysate and draining dialysate (after passing through a dialyzer or hemodiafilter). ORP is shown in Figure 1E. EW-OL-HDF demonstrated a marked elevation in dialysate ORP, meaning the oxidation of dialysate, i.e., the reduction of blood, compared with conventional HD, highlighting its potential to mitigate oxidative stress by creating a more reductive dialysis environment.

Discussion: This article highlights the potential of combining hydrogen H₂ gas inhalation with RRTs employing H₂-enriched dialysate to improve patient outcomes. While the combination demonstrated promising results, particularly in reducing oxidative stress and enhancing patient-reported outcomes, the optimal H₂ concentration in the dialysate may vary depending on the specific RRT modality.

The addition of H₂ gas inhalation to EW-OL-HDF and EW-HD proved to be a critical factor in achieving better patient-reported outcomes. This combination therapy may enhance overall health by leveraging complementary antioxidant mechanisms, suggesting synergistic effects between H₂ inhalation and H₂-enriched dialysate. Previous studies have established that the inhalation of 2.4% H₂ gas is safe and well-tolerated for healthy adults; no clinically significant adverse effects have been reported.10 H₂ gas is typically administered via a high-flow nasal cannula or mechanical ventilator at concentrations ranging from 1.3% to 3%. However, during the present study, a low-flow nasal cannula was used. Additionally, H₂ gas inhalation has been associated with improved recovery from ischemia-reperfusion injury, which is commonly encountered with kidney transplants and acute kidney injuries. Similarly, this therapy is expected to reduce the risk of myocardial stunning and cerebral hypoperfusion during dialysis.11

EW-HD has shown benefits such as reduced oxidative stress, improved blood pressure control, and better long-term prognosis in patients with HD.12 The observed blood pressure-lowering effect, particularly in managing systolic hypertension post-dialysis (exceeding 140 mmHg),12 aligns with previous findings that H₂-enriched dialysate can improve vascular outcomes. No marked improvement in patient-reported outcomes with EW-HD or EW-OL-HDF alone underscores the importance of combination therapies.13 Despite these promising results, detailed data on adverse effects and the optimal H₂ concentration remain insufficient. Although dialysate concentrations between 100 and 400 ppb are often recommended,14 this article observed better outcomes at 300 ppb compared with 150 ppb, emphasizing further research to establish an evidence-based range.

Dialysate oxygen content emerged as another influential factor in oxidative stress modulation. High oxygen concentrations in the dialysate may exacerbate oxidative stress; however, factors like the deaeration capabilities of console and dialyzer membrane properties also contribute significantly. This finding underscores the importance of a multifaceted approach to oxidative stress management in HD patients.

Critical gaps remain regarding the precise H₂ concentrations required to suppress oxidative stress pathways, such as neutrophil extracellular traps release during extracorporeal circulation. 15 Studies focusing on peripheral blood neutrophils could provide valuable insights into these mechanisms. Additionally, real-time monitoring of oxidative stress markers and personalized therapeutic strategies based on individual patient characteristics could help refine the optimal H₂ dosage and delivery methods.

Conclusion: In summary, the combination of H₂ gas inhalation and H₂-enriched dialysate offers a novel and promising approach to managing oxidative stress and improving the quality of life in patients on HD. However, further research is essential to optimize treatment protocols, ensure safety, and develop comprehensive clinical guidelines for the widespread adoption of this therapy.

We want to thank Mr. Sou Hashimoto and Ms. Yasuyo Aoyama (Doctors Man Co., Ltd., Japan) for their technical assistance with this research.