1 | INTRODUCTION 

Dramatic improvements in living conditions and health care have  significantly increased human life expectancy by up to 40% over the past 50 years (worldbank.org). With the aging of the popula tion, the incidence of pathologies associated with myocardial and cerebral ischemia is expected to increase, being largely favored by the fast-rising pandemic of diabetes mellitus and obesity (Go et al., 2014). Importantly, ischemia–reperfusion (I/R) injury of both heart and brain shares common pathomechanisms represented by oxi dative stress (Muntean et al., 2016; Sanderson, Reynolds, Kumar, Przyklenk, & Hüttemann, 2013), inflammation (Goldfine & Shoelson, 2017; Ong et al., 2018), microvascular dysfunction (Granger & Kvietys, 2017; Gursoy-Ozdemir, Yemisci, & Dalkara, 2012), and, ul timately, cell death. 

A great success has been achieved in reducing the ischemic in jury, with the advent of revascularization procedures and the suc cessful recanalization of the occluded arteries (Bhaskar, Stanwell, Cordato, Attial, & Levi, 2018) since past three decades. However, no treatment capable of mitigating the cell death occurring during  

the postischemic reperfusion is currently available in the daily prac tice (Heusch, 2017; Ibanez, Heusch, Ovize, & Van de Werf, 2015). A recent study shows that, although the mortality of heart attack decreased, the morbidity increased due to the development of heart failure (Hausenloy & Yellon, 2016). Reperfusion injury of the heart occurring most frequently in the setting of acute myocardial infarc tion and cardiac bypass surgery has been recently acknowledged  

as a “neglected therapeutic target” (Bulluck & Hausenloy, 2015; Hausenloy & Yellon, 2013). 

Pathophysiology of myocardial I/R injury comprises reperfusion induced arrhythmias, myocardial stunning, microvascular obstruc tion, and lethal reperfusion injury (Bulluck & Hausenloy, 2015). Over the past 30 years, the quest for novel therapies able to protect myo cardium against the deleterious effects of lethal reperfusion injury  

has lead to the identification of “ischemic conditioning” as the most powerful strategy of endogenous protection. The term refers to a  series of brief episodes of ischemia alternated with reperfusions  applied prior to or after a prolonged ischemia either locally (isch emic pre- and postconditioning) or at distance (remote ischemic  pre- and postconditioning) that resulted in infarct size reduction in  experimental setting and/or clinical outcome improvement in the clinical arena (reviewed by Heusch, 2015; Cohen & Downey, 2015; Hausenloy, 2013; Duicu, Angoulvant, & Muntean, 2013). A large body of research has aimed at characterizing the signal transduction  of conditioning maneuvers in order to identify cellular/molecular  targets that can be pharmacologically modulated (“pharmacologi cal conditioning”). However, neither ischemic nor pharmacological conditioning strategies were translated so far into an effective pro tective therapeutic protocol in daily practice mainly due to various  confounders such as comorbidities (e.g., diabetes and renal fail ure), several cotreatments, and aging (Bulluck & Hausenloy, 2015; Heusch, 2017). 

Hyperbaric oxygen (HBO) has emerged more than a decade ago as putative protective pharmacological therapy in the setting of I/R  injuries of brain and heart, in particular in the settings of ischemic stroke and acute myocardial infarction/revascularization procedures  with encouraged outcomes. (Camporesi & Bosco, 2014; Francis & Baynosa, 2017; Yogaratnam et al., 2006). 

In this study, we briefly review the pathophysiology of I/R injury and current treatment strategy. We further address the protective  effects and mechanisms of in the treatment of I/R injury. 

2 | PATHOPHYSIOLOGY OF ISCHEMIA/ REPERFUSION INJURY 

Pathophysiology of myocardial I/R injury recognizes four types of  specific lesions, namely reperfusion-induced arrhythmias, myocar dial stunning, microvascular obstruction, and the most severe lethal reperfusion injury. The intimate mechanisms responsible for the  occurrence of these lesions are the direct results of I/R-triggered  

changes in several cells that are briefly summarized in Table 1. Ischemia/reperfusion injury of the brain can be either focal as  occurs in ischemic stroke which arises in a specific territory due to  atherothrombotic or thromboembolic vascular occlusion (the most  common clinical presentation) or global—in the setting of cardiac  arrest followed by resuscitation and the neonatal hypoxic–ischemic encephalopathy (Sanderson et al., 2013). The mechanisms underly ing cerebral injury at the postischemic reperfusion are similar to the  ones triggering the above-mentioned specific myocardial lesions, with a major contribution of mitochondria-dependent oxidative stress (Sanderson et al., 2013). The brain exhibits a unique sensi tivity to ischemia due to its highest metabolic activity, dependence on constant glucose delivery, and structural and functional partic ularities that render neurons more vulnerable to oxidative damage, namely increased polyunsaturated fatty acids in the cellular mem branes and lower levels of antioxidant enzymes and mitochondrial cytochrome c oxidase as compared to the heart (Kalogeris, Baines, Krenz, & Korthuis, 2017). 

3 | THE SAGA OF CONDITIONING  STRATEGIES 

In the setting of acute I/R injury, the most powerful cardioprotec tive strategy, apart from revascularization, is the so-called ischemic preconditioning (IPC). The term was coined by the group of Robert  Jennings which firstly reported that four episodes of nonlethal is chemia applied prior to the onset of a prolonged lethal episode (index ischemia) dramatically reduced (by 75%) the size of experimental myo cardial infarction in dogs (Murry, Jennings, & Reimer, 1986). 

 | THE SIGNAL TRANSDUCTION OF IPC 

4.1 | The triggers 

The IPC triggers are stimuli that act during the brief ischemic epi sode, activate the signal transduction pathways in a receptor/non receptor manner, and transmit the protective signal to the effector(s) through mediators (Downey, Krieg, & Cohen, 2008). Some trigger  

molecules (adenosine, bradykinin, opioids, natriuretic peptides, and other cytokines) released during the conditioning IPC episodes ac tivate the signaling cascades through specific membrane receptors.  Other triggers, such as reactive oxygen species (ROS) and nitric oxide (NO), initiate the signaling cascades in a receptor-independent manner (Heusch, 2008). Inside the cell, cytosolic signal transducers interact at different levels and at different time points (before lethal  ischemia or at reperfusion) to convey information to the end effec tors: mitochondria, the main organelles that ultimately control cell 

death in the setting of I/R injury (Cohen & Downey, 2015). Adenosine, bradykinin, and opioids are triggers that act on G protein-coupled receptors which, in turn, activate protein ki nase C (PKC). Although the pathways are slightly different, they all converge on the PKC, the blockade of which results in the lack of any possible protection attributable to those triggers (Cohen & Downey, 2015). At variance, both exogenous (Nakano, Liu, Heusch, Downey, & Cohen, 2000) and endogenous (Cohen, Yang, & Downey, 2006; Krieg et al., 2009) NO can trigger myocardial protection in a receptor-independent manner; in this case, protection occurs ei ther dependent or independent of the activation of protein kinase G (PKG) signaling pathway (Sun et al., 2013). The next step identified within the IPC signal transduction consisted in the activation (open ing) of the ATP-sensitive K+ channel (KATP) at the inner mitochondrial  membrane (Garlid et al., 1997; Gross & Auchampach, 1992; Liu, Sato, O’Rourke, & Marban, 1998). The opening of mitochondrial KATP is  related to electrochemical changes in the mitochondrial matrix that are responsible for an increased ROS production reported to occur  mainly (but not exclusively) at the postischemic reperfusion. ROS can 

directly activate the PKC isoforms whose contribution to protection is species-dependent, with PKCε being responsible for protection  in the rodent heart, PKCα in large mammals, whereas controver sial data are available about PKC (Cohen & Downey, 2015; Heusch, 2015). Activated PKC phosphorylates several downstream targets, among which connexin 43 (Cx43) plays a critical role in transferring the protective signal to mitochondria (recently reviewed by Boengler & Schulz, 2017). 

One of the most important discoveries with respect to the  preconditioning-related cardioprotection is that minute ROS gen eration during the brief reperfusions is mandatory for IPC-related  protection, as ROS scavenging blocked protection; moreover, pro tection was lost when a hypoxic solution was used for reperfusion during the preconditioning phase (Dost, Cohen, & Downey, 2008). Importantly, the identification of the ROS sources and the threshold at which ROS loses potentially protective effect and become dam aging to cellular function and integrity is still unclear in the field of  cardioprotection (Di Lisa et al., 2011). 

4.2 | The mediators 

The above-described triggers act as stimuli to activate a couple of  cytosolic enzymatic cascades that act as “mediators” during the index ischemia and/or at reperfusion in order to transmit the car dioprotective signal onto the final “effector(s)” that ultimately are responsible for the attenuation of the irreversible injury during the  postischemic reperfusion. 

By far, the most investigated signaling cascade activated during the early reperfusion following the index ischemia is represented by so-called reperfusion injury salvage kinases (RISK) pathway (Hausenloy & Yellon, 2004). The RISK pathway comprises phos phoinositide 3-kinase (PI3K), protein kinase B (Akt), and extracel lular signal-regulated kinase (ERK), which are proven effective in protecting the myocardium in rat (Hausenloy, Tsang, Mocanu, & Yellon, 2005) and rabbit (Yang et al., 2004). They act on endothelial nitric oxide synthase (eNOS) directly and on glycogen synthase ki nase 3 beta (GSK3β) through one ribosomal protein kinase, P70S6K (Kleinbongard & Heusch, 2015). 

Another signaling pathway is the survivor activating factor en hancement (SAFE) pathway (Lacerda, Somers, Opie, & Lecour, 2009; Lecour, 2009). At reperfusion, possibly due to the inflammatory re sponse, the tumor necrosis factor (TNF) activates the Janus kinase (JAK) (a tyrosine kinase associated with the membrane receptor; it has a major role in translating signals from the cytosol to the nucleus)  

and signal transducers and activators of transcription (STATs) (when phosphorylated by the activated JAK, these dimerize and translo cate to the nucleus, resulting in gene transcription; they may also be phosphorylated directly by receptor tyrosine kinases such as epider mal growth factor receptor, or by nonreceptor tyrosine kinases such as Src), playing an important role on the expression of the stress responsive genes (Willis, Homeister, & Stone, 2014). The effects on I/R happen far too quickly to be explained only by the gene transcrip tion. It seems that STAT also phosphorylates GSK3β, inactivating it

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(Lacerda et al., 2009). Isoform STAT3, shown to be present in mito chondria, may also act on cyclophilin D, the target for mitochondrial permeability transition pore (mPTP) inhibitor cyclosporin A, thus inhibiting pore opening. Other downstream targets of STAT include proteins involved in cell survival and proliferation (Bcl-2, Bcl-xl, Mcl 1, and p21) and growth factors (vascular endothelial growth factor) (Brantley & Benveniste, 2008). It also inactivates the proapoptotic factor Bad. TNF-α’s effect is concentration-dependent, and high doses may increase the infarct size (Lecour, 2009). The SAFE path way may also be activated by triggers other than TNF-α via STAT: opioids, insulin, and sphingosine-1 (Willis et al., 2014). 

4.3 | The effectors 

The end effector of preconditioning through these signaling path ways, which interact with each other at different levels and dif ferent time points, is the mPTP, a protein structure—the structure of which is still controversial—located in the inner mitochondrial  

membrane. Inhibition of this high-conductance pore is considered  to be the final step in the protective signal transduction (Griffiths & Halestrap, 1993, 1995; Hausenloy, Maddock, Baxter, & Yellon, 2002; Hausenloy, Ong, & Yellon, 2009). When open, this pore dissipates the transmembrane electrochemical gradient used for ATP genera tion, resulting in ATP depletion, enhanced ROS production, the fail ure of energy-driven membrane ion pumps, solute entry, organelle swelling, and, finally, mitochondrial rupture. The acidosis during the ischemic phase inhibits the formation of the pore. But during the reperfusion phase, the formation of the pore is stimulated due to al kalization of the pH, increasing mitochondrial Ca2+, and ROS (Cohen & Downey, 2015). 

All cardioprotective signaling pathways inhibit the mPTP from opening. Both the RISK and SAFE cascades appear to have a final kinase, GSK3β, which seems to act differently to the other kinases, GSK3β being essential in pore formation. Conditioning signals lead  

to the inhibition, not activation, of this kinase, thus blocking mPTP formation and opening (Gross, Hsu, & Gross, 2004; Juhaszova et al., 2004; Tong, Imahashi, Steenbergen, & Murphy, 2002). Pharmacological activation of P70S6K leads to phosphorylation and inhibition of GSK3β, which further inhibits mPTP formation and opening, mimicking ischemic conditioning (Förster et al., 2006). 

5 | HYPERBARIC OXYGEN THERAPY 

Hyperbaric oxygen (HBO) refers to the administration of 100% oxy gen at two to three times the atmospheric pressure at sea level. HBO is a therapeutic strategy aimed at raising the arterial oxygen tension and the oxygen supply via an increase in oxygen dissolved in plasma that, ultimately, drives cellular respiration and sustains ATP synthe sis in ischemic/hypoxic tissues. Over the time, HBO has been proven to be beneficial in acute conditions associated with general hypoxia/ anoxia, such as carbon monoxide poisoning, circulatory arrest, and local ischemia/hypoxia, that is, cerebral and myocardial ischemia. 

The systematic investigation of HBO as therapeutic measures during or after an ischemic insult of the brain and heart can be  traced back to pioneering studies of George Smith (Smith, 1964; Smith & Lawson, 1963; Smith, Lawson, Renfrew, Ledingham, & Sharp, 1961). Indeed, this author firstly reported the preserva tion of cortical electrical activity in an experimental model of cerebral ischemia in the presence of compressed oxygen (Smith, 1964; Smith & Lawson, 1963; Smith et al., 1961). As for the heart, he reported in the in vivo model of regional I/R injury in dogs a  significant decrease in mortality by preventing the occurrence of  ventricular fibrillation in animals that breathed oxygen at two at mospheres absolute as compared to the groups that breathed room  air or oxygen at one atmosphere absolute (Smith, 1964). A pleth ora of experimental studies further confirmed the HBO-related neuroprotective effects and improved survival in animal models  of middle cerebral artery occlusion, especially when applied at 2.0 absolute atmospheres (ATA) immediately after occlusion and for more than 6 hr (Xu et al., 2016). By facilitating oxygen delivery, HBO ameliorated cerebral circulation, decreased cerebral edema, blocked inflammatory cascades, and ultimately reduced infarct size via the mitigation of cell death and the restoration of mito chondrial oxidative phosphorylation (Sanchez, 2013). 

In the coming years, several proof-of-concept clinical studies have been carried out to confirm the beneficial effect of HBO in the setting of brain ischemia associated with stroke with both positive  results (several applications of HBO at 1.5 to 2 atmospheres absolute (ATA)—Neubauer & End, 1980) and neutral results (Nighoghossian, Trouillas, Adeleine, & Salord, 1995; Rusyniak et al., 2003). However, the current opinion is that the reduced number of randomized, double-blind controlled trials does not provide enough evidence based decisions for the design of appropriate clinical protocols (Zhai, Sun, Yu, & Chen, 2016). Indeed, in the most recent meta-analysis, seven of the 11 randomized trials showed no significant difference  

observed in the mortality rates at 6 months in the HBO-treated pa tients as compared with the nontreated ones. However, these au thors did not exclude the potential clinical benefit of the therapy as they found an improvement in a couple of disability and neurolog ical function scale scores with HBO therapy (Bennett et al., 2014). Clearly, future randomized clinical trial will shed light on the bene fits of HBO application together with thrombolysis within the same therapeutic window of 3 to 6 hr in acute stroke as well as in the post stroke stage in stable patients via the modulation of neuroplasticity.