Introduction

Molecular hydrogen is the lightest of all gas molecules. It is an odorless, colorless, tasteless, nonmetallic, and nontoxic gas at room temperature [1]. Hydrogen is not dangerous when its concentration is under 4% [1]. Owing to its small size, a hydrogen molecule has the ability to diffuse through the cell membrane and enter the cytosol. This characteristic of hydrogen makes it superior when it comes to the transport efficacy of most hydrophilic compounds, which are retained at membranes and cannot reach the cytosol; the majority of hydrophobic ones cannot penetrate biomembranes without specific carriers [2, 3]. Many antioxidants, including vitamins, can enter the cytoplasm but not the mitochondria [2]. It has been shown that hydrogen can be rapidly distributed into the cytosol and organelles, and it can enter the mitochondria and nucleus with excellent efficacy and lack of adverse effects [3].

There is growing evidence to demonstrate that hydrogen could be an effective treatment in various diseases due to its ability to reduce oxidative stress by selectively eliminating toxic reactive-oxygen species (ROS) and reactive nitrogen species (RNS) [4]. Hydrogen was found to increase the level of antioxidants in vitro studies, animal models, and clinical studies [5,6,7]. Furthermore, anti-inflammation and anticell death have also been reported as hydrogen properties [8]. In addition, the previous studies in both animal models and clinical trials demonstrated the potential benefits of hydrogen application in various pathological conditions, including postcardiac arrest syndrome [9] and cardiovascular diseases [10,11,12,13].

In this review, we comprehensively summarize the reports regarding the potential role of the therapeutic application of molecular hydrogen in the cardiovascular aspect and describe the potential mechanisms responsible for the benefits of hydrogen. These findings from both preclinical and clinical studies will encourage further investigations to warrant the application of hydrogen as a novel treatment in a clinical setting in the near future.

Effects of hydrogen treatment on cardiomyocytes: reports from in vitro studies

Hypoxia and reoxygenation (H/R) induced oxidative stress and inflammatory reaction is one of the main factors contributing to myocardial cell injury [6, 14, 15]. It has been demonstrated that hydrogen exerts antioxidative stress, antiapoptotic, and anti-inflammatory effects [16]. Following 4 h of hypoxia and 24 h of reoxygenation, a hydrogen-rich medium was shown to increase the survival of H9c2 cells by decreasing inflammatory cytokine release and apoptosis [15]. The protective effect of hydrogen against cell death, inflammatory process, or oxidative stress was shown to be through various pathways, including the PI3K/Akt signaling pathway and the activation of the Nrf2/HO-1 signaling, leading to increased OH-1 levels which is considered a potent antioxidant, and a decrease in 8-OHdG which is regarded as an indicator of oxidative stress (Figs. 1 and 2) [6, 14].

Fig. 1
figure 1

Potential mechanisms of the action of molecular hydrogen on selective antioxidants and anti-inflammation. The possible mechanisms of molecular hydrogen proposed have been those which increased antioxidants, and decreased oxidative stress, and inflammation. CAT catalase, 4-HNE 4-hydroxyl-2-nonenal, 8-OHdG 8-hydroxydeoxyguanosine, CCL chemokine (C–C motif), DNA deoxyribonucleic acid, ER endoplasmic reticulum, ETC electron transport chain, GPx glutathione peroxidase, GPX1 glutathione peroxidase 1, GRP78 glucose-regulated protein 78, GSH glutathione peroxide, GST glutathione-S-epoxide transferase, HMGB high mobility group box 1, ICAM intercellular adhesion molecule, IFNγ interferon γ, IL interleukin, iNOS inducible nitric oxide synthase, JAK/STAT janus kinase/signal transducer and activation of transcription signal pathway, MCP-1 monocyte chemotactic protein-1, MDA malondialdehyde, MPO myeloperoxidase, NK cell natural killer cell, NOX Nox protein, Nrf2 nuclear factor erythroid 2-related factor2, ·OH hydroxyl radicals, RNS reactive nitrogen species, ROS phosphatidylinositol 3-kinase, SOD superoxide dismutase, TNF-α tumor-necrosis factor-α, TRAF2 tumor-necrosis factor-a (TNF-a) receptor-associated factor 2

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Fig. 2
figure 2

Potential mechanisms associated with the action of molecular hydrogen in alleviating cell death. It has been proposed that molecular hydrogen effectively decreases apoptosis, ER stress, and pyroptosis. Molecular hydrogen has been shown to increase autophagy, mitophagy, and survival kinases, leading to the alleviation of cell death. Akt protein kinase b, ASC apoptosis-associated speck-like protein containing a card, ATF activating transcription factor, ATG autophagy-related protein, Bax apoptosis regulator Bax, Bcl-2 apoptosis regulator Bcl-2, CHOP the proapoptotic transcriptional factor c/ebp homologous protein, elF2 α eukaryotic initiation factor 2 alpha, ER endoplasmic reticulum, ERAD endoplasmic-reticulum-associated protein degradation, ERK extracellular signal-regulated kinase, FADD fas associated via death domain, GRP78 glucose-regulated protein 78, GSDMD gasdermin D, IRE1 Er stress sensor and cell fate executor, JNK c-jun N-terminal kinase, LC3-I microtubule-associated protein 1 light chain 3α, MAPK mitogen-activated protein kinase, MFN2 mitofusin-2, mTOR mammalian target of rapamycin, NFkB nuclear factor kappa-light-chain-enhancer of activated b cells, NLRP3 nod-like receptor (NLR) family pyrin domain containing protein 3, P38 38-kda protein, P53 53-kda protein, P62 62-kda protein, PERK protein kinase RNA-like endoplasmic reticulum kinase, PI3K phosphatidylinositol 3-kinase, PINK PTEN-induced kinase, TGF β transforming growth factor beta, XBP1 x-box binding protein 1, XBP1s active/spliced form of XBP1

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In cardiotrophin-I (CT-I)-induced hypertrophy neonatal rat cardiomyocytes, a hydrogen-rich medium effectively reduced cardiomyocyte hypertrophy via down-regulation of IL-6 and activation of the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) signaling pathway, leading to attenuation of adverse cardiac remodeling and the cell inflammatory response (Fig. 1) [17, 18]. These in vitro reports are comprehensively summarized in Table 1.

Table 1 Effects of hydrogen treatment on cardiomyocytes: reports from in vitro studies
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Effects of hydrogen treatment on the heart: reports from in vivo studies

Hydrogen treatment has been investigated in various in vivo models of cardiac pathology, including cardiac ischemia − reperfusion injury, myocardial infarction and chronic intermittent hypoxia, radiation, atherosclerosis, sepsis, cardiotoxicity from chemotherapy, and cardiac hypertrophy. The cardioprotective effects of hydrogen interventions are reported and summarized in Tables 2 and 3. The potential mechanisms of action of molecular hydrogen on selective antioxidants, anti-inflammation, and alleviating cell death are demonstrated in Figs. 1 and 2.

Table 2 Effects of hydrogen treatment on cardiac ischemia − reperfusion injury models: reports from in vivo studies
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Table 3 Effects of hydrogen treatment on the heart: Reports from in vivo studies ‘other models”
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Effects of hydrogen treatment on cardiac ischemia − reperfusion injury models

Cardiac ischemia − reperfusion injury (I/R) could negatively affect outcomes in various clinical settings, including post myocardial infarction, cardiac transplantation, or cardiopulmonary bypass. Oxidative stress induced by I/R was found to cause direct cellular injury and apoptosis, leading to impaired cardiac function [15]. The oxidative stress involved in the cell death pathway is a consequence of the presence of reactive oxygen species (ROS), including the hydroxyl radical (•OH), superoxide anion (O2−), hydrogen peroxide (H2O2), in addition to reactive nitrogen species (RNS), including nitric oxide (NO), and peroxynitrite (ONOO−). Both ROS and RNS are known to trigger the production of inflammatory cytokines and proteins, including IL-1 \(\beta \), IL-6, and TNF-\(\alpha ,\) HMGB1, and ICAM-1 [19]. Hydrogen has been demonstrated to potentially protect against I/R injury through the mechanisms of reducing oxidative stress, inflammation, and cell death in various in vivo experimental settings.

In rats with cardiac I/R, hydrogen-rich saline (HRS) injected intraperitoneally was shown to improve cardiac function, reduce infarct size, and alleviate cardiac injury [5, 15, 20]. Studies using either injection into the myocardial tissue around the infarct zone or inhalation in rats showed consistently beneficial results [21,22,23]. In swine with cardiac I/R, inhalation of 2–4% hydrogen treatment resulted in the reduction of both myocardial infarct size and the incidence of ventricular fibrillation (VF)/ventricular tachycardia (VT) and improved cardiac function [24].

In the cardiopulmonary bypass model (CPB), rats treated with hydrogen-rich water (HRW) via intravenous injection showed an improvement in cardiac function and a reduction in cardiac injury [6]. Within the last few decades, studies using the heart transplant rat model have demonstrated that hydrogen given either orally or by inhalation resulted in reduced infarct size and cardiac injury and enhanced the survival of cardiac grafts [11, 25]. Overall, evidence from these in vivo reports indicated that hydrogen treatment effectively reduced infarct size and myocardial injury, leading to improved cardiac function.

The precise mechanisms involved during hydrogen treatment with regard to improving cardiac function and alleviating myocardial infarction and cardiac injury are still unclear, but the reduction in oxidative stress and inflammation could be key [5, 15, 20,21,22]. Hydrogen has been shown to effectively decrease oxidative stress indicators, including MDA, 8-OHdG, MPO, and ROS in rats with I/R, CBP, and in transplantation models [5, 6, 15, 20,21,22,23] as well as decreasing endoplasmic reticulum (ER) stress including TRAF2, and GRP78 in an I/R rat model [5, 6, 15, 20,21,22,23]. An increase in antioxidants, including SOD, was also demonstrated in rat models of I/R injury and CBP [5, 6, 15, 20,21,22,23]. Hydrogen treatment also led to a reduction in inflammation via increased autophagy, PINK/Parkin-mediated mitophagy [6, 11, 15, 20, 21, 26], and antiapoptosis [5, 6, 11, 15, 21, 23]. All of these mechanisms could lead to improved cardiac function in these models.

Effects of hydrogen treatment in myocardial infarction (MI) and chronic intermittent hypoxia (CIH) models

Myocardial infarction (MI), widely accepted as one of the major causes of death, can induce myocardial necrosis and interstitial fibrosis resulting in heart failure, and increasing the mortality rate [27]. In rats with MI, hydrogen treatment via ingestion, inhalation, or intraperitoneal injection was shown to improve cardiac function and attenuate myocardial pathological changes by reducing the infarct size and apoptosis [28,29,30]. In rats with CIH, hydrogen therapy has been shown to reduce cardiac dysfunction by reducing oxidative stress. In addition, hydrogen attenuated ER stress-induced apoptosis via PERK-eIF2 \(\alpha \)-ATF4, IRE 1-XBP1, and ATF6 pathways [31]. Moreover, a combination of HRS with exercise was shown to promote the repair of both the mitochondria and DNA in a rat MI model, which could be involved in the cardioprotective mechanism of hydrogen treatment [29].

Effects of hydrogen treatment in a radiation model

Radiation can cause myocardial damage as a consequence of radiation-induced myocardial fibrosis, leading to the chronic impairment of cardiac function [32]. In a radiated rat model, it has been demonstrated that an intake of oral hydrogen prior to radiation increased survival rate by increasing the level of antioxidants, reducing oxidative stress, and preventing DNA damage [33]. However, the effect of hydrogen treatment on cardiac function in these conditions is unknown.

Effects of hydrogen treatment in an atherosclerosis model

Atherosclerosis is a multifactorial process which is related to cardiovascular disease. It represents a state of inflammation and oxidative stress characterized by the accumulation of macrophages and oxidized products of lipoproteins in the affected blood vessels [34]. Interestingly, the consumption of HRS for 6 months effectively decreased oxidative stress and had the potential to decrease atherosclerotic lesions in the aorta [35].

Effects of hydrogen treatment on the heart in a sepsis model

Sepsis is systemic inflammation in response to an infection associated with the cardiovascular system. Cardiac myocytes are involved due to the oxygen consumption of the cell being compromised. Correspondingly, mitochondrial dysfunction occurs, leading to cellular energy depletion [36]. A recent study showed that hydrogen gas treatment reduced mitochondrial dysfunction by up-regulating the protein expression of mitofusin-2 (Mfn2), peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α), and protein heme-oxygenase-1 (HO-1) [37]. However, the effect of hydrogen treatment on cardiac function in these conditions is unknown.

Effects of hydrogen treatment in a chemotherapy-induced cardiotoxicity model

Doxorubicin is an anthracycline anticancer drug that can cause cardiotoxicity, a condition known as doxorubicin-induced cardiomyopathy, via oxidative stress, apoptosis, and intracellular calcium dysregulation [38]. The use of HRS via intraperitoneal injection in rats treated with doxorubicin has been shown to improve survival rate and reduce cardiac dysfunction by attenuating oxidative stress, inflammation, and apoptosis [12].

Effects of hydrogen treatment in a cardiac hypertrophy model

Cardiac hypertrophy, consisting of interstitial and perivascular fibrosis, can lead to heart failure, which results in increased mortality [39]. Hypertension is the major factor associated with left ventricular hypertrophy [40]. Studies into cardiac hypertrophy in rat models reported that hydrogen therapy using HRS via IP resulted in a reduction in heart and atrial weight [13, 17, 18, 41]. Hydrogen also decreased the incidence of atrial fibrillation (AF), atrial fibrosis, apoptosis, and inflammation through the downregulation of the JAK-STAT signaling [17, 18]. In another rat model with cardiac hypertrophy, the benefit of hydrogen therapy was shown via a reduction in oxidative stress, the inflammatory process, and angiotensin II, and the preservation of mitochondrial function in the left ventricle [13, 41]. These benefits could be due to the inhibition of the TGF-β/Smad signaling pathway, leading to reduced cardiac hypertrophy [41].

Effects of hydrogen treatment on the heart: reports from ex vivo studies

Heart transplant is one of the causes of I/R injury. A period of cold ischemia due to tissue matching and transportation is inevitable after retrieval of the heart. The organ preservation solutions have been found to only partially alleviate ischemia injury during storage [42]. In isolated hearts mounted on the Langendorff apparatus for aerobic perfusion, it has been shown that preservation in H2-rich with Histidine − Tryptophan − Ketoglutarate (HTK) significantly improved cardiac function in a hydrogen concentration-dependent manner as well as attenuated the microscopic pathology of the myocardium [43]. The protective mechanism of hydrogen was via inhibition of cold ischemia-induced up-regulation of oxidative stress, inflammation mediators, and apoptosis (Figs. 1 and 2) [43]. In a study using syngeneic heart grafts from elderly donors or allografts from adult donors and exposing them to prolonged cold preservation, the cardiac grafts immersed in the cold-water bath with hydrogen showed ameliorated myocardial injury [26]. The grafts exhibited inflammatory responses, including neutrophil infiltration, and increases in pro-inflammatory cytokines and chemokines, whereas hydrogen induced lower levels of mitochondrial damage and higher adenosine triphosphate content [26]. In a recent study using an isolated heart model with I/R injury, perfusion with HRW resulted in a decrease in apoptosis by up-regulating the JAK-STAT and PI3K-AKT signaling pathways (Fig. 2) [44]. All of these ex vivo reports are comprehensively summarized in Table 4.

Table 4 Effects of hydrogen treatment on the heart: reports from ex vivo studies
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Effects of hydrogen treatment on the heart: Evidence from clinical studies

Because molecular hydrogen has various potential therapeutic effects, it has been investigated in various pathophysiological conditions in clinical settings. It has been suggested that hydrogen has an effective therapeutic approach in the heart for improving outcomes associated with I/R injury. A randomized single-center prospective, open-label, blinded study to investigate the feasibility and effects of hydrogen on the infarct size and adverse left ventricular (LV) remodeling in patients with ST-elevated MI (STEMI) was conducted after primary percutaneous coronary intervention (PCI) [10]. This first clinical trial showed that hydrogen inhalation during PCI is genuinely feasible, promotes LV reverse remodeling 6 months after STEMI, and improves cardiac function [10]. Another recent clinical trial enrolled five comatose postcardiac arrest patients [45]. The study demonstrated that oxidative stress was reduced while the cytokine levels were unchanged in cardiogenic patients. However, the oxidative stress was unchanged in septic patients, but the cytokine levels were diminished. Nevertheless, the effect of inhaled hydrogen on oxidative stress and cytokines remained inconclusive due to potential methodological weaknesses [45].

Various in vivo and in vitro studies demonstrate hydrogen’s ability to reduce inflammation and antiapoptotic properties. A randomized, double-blind, controlled trial showed hydrogen increases antioxidant capacity, thereby reducing inflammatory responses and apoptosis in healthy adults [46].

Because metabolic syndrome remains a serious concern, those patients are at increased risk of developing cardiovascular disease. Hydrogen decreases serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and apo-B levels. Moreover, hydrogen therapy was shown to improve high-density lipoprotein (HDL) function and reduced oxidative stress in patients with metabolic syndrome [47, 48]. All of these clinical studies are comprehensively summarized in Table 5. The potential mechanism of action of molecular hydrogen on selective antioxidants, anti-inflammation, and alleviating cell death are demonstrated in Figs. 1 and 2.

Table 5 Effects of Hydrogen treatment on the heart: Reports from clinical studies
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Conclusion and future perspectives

Molecular hydrogen has versatile therapeutic effects due to its small size. It can penetrate the cell membrane and affect metabolism in the body. Molecular hydrogen can be administered via several methods including inhalation, drinking of hydrogen-rich water, injection with hydrogen-rich-saline, and bathing of an organ in a preservation solution. Cumulative evidence from in vivo, in vitro, ex vivo, and clinical studies demonstrated the possible mechanisms underlying the potential benefits of molecular hydrogen, including those increasing antioxidants and decreasing oxidative stress, cell death, metabolism, and inflammation.

For future research, searching for the mechanism of molecular hydrogen to reduce ventricular dilation, decrease wall stress, and reverse adverse cardiac remodeling should be thoroughly investigated. In addition, future clinical studies investigating oxidative stress and inflammatory pathways may provide information to improve the current treatment of various inflammatory diseases, including Kawasaki disease, COVID-19 infection, a multisystem inflammatory syndrome in children or adult (MIS-C or A). Although various in vitro and in vivo models have demonstrated the beneficial effects of molecular hydrogen treatment on the heart, clinical investigations are still limited. Future large-scale randomized control trials are needed to determine the crucial clinical impact of using hydrogen as a therapy, and to verify the efficacy and safety of clinical interventions with molecular hydrogen to warrant its use and to improve medical treatment in this field.