The effects of ischaemic conditioning on lung ischaemia–reperfusion injury

Ischaemia–reperfusion injury (IRI) encompasses the deleterious effects on cellular function and survival that result from the restoration of organ perfusion [1]. Counterintuitively, IRI further aggravates ischaemic organ damage as the degree of injury after reperfusion surpasses that caused by ischaemia per se [2]. It is mediated by sterile inflammation, enhanced oxidative stress and coagulation, endothelial dysfunction, and activation of cellular death pathways [1, 2]. Crucially, it is a systemic process with the potential to evoke distant organ injury and progress to multiple organ dysfunction syndrome [2]. Despite their unique tolerance to ischaemia and hypoxia, afforded by their dual (pulmonary and bronchial) circulation as well as direct oxygen diffusion from the airways [3], lungs are particularly susceptible to IRI (LIRI) [4]. Importantly, the cessation of ventilation leads to functional impairments similar to those induced by hypoperfusion [3], to which it is also interrelated by way of hypoxic pulmonary vasoconstriction (HPV) [5]. LIRI may be observed in a variety of clinical settings, including lung transplantation [4, 6], lung resections [7, 8], cardiopulmonary bypass (CPB) during cardiac surgery [9‐12], aortic cross-clamping for abdominal aortic aneurysm (AAA) repair [13], as well as tourniquet application for orthopaedic operations [14]. It culminates in the breakdown of lung endothelial and epithelial barriers, leading to pulmonary oedema with attendant gas exchange impairment [4] and increased pulmonary vascular resistance resulting in pulmonary hypertension [15].

Ischaemic conditioning (IC) signifies the original paradigm of treating IRI. It entails the application of short, non-lethal ischemia and reperfusion manoeuvres to an organ, tissue, or arterial territory, which activates mechanisms that reduce IRI [16]. The concept of IC has several temporal and anatomical variations. In specific, the protective ischaemic stimulus may be applied before, during, or following the index reperfusion episode (pre-, per-, and post-conditioning respectively) with similar beneficial effects [16]. Furthermore, this intervention has systemic protective properties which are exploited by remote ischemic conditioning (RIC): biochemical and neuronal mechanisms confer protection to organs distant to the conditioning stimulus [16]. There is accumulating experimental and clinical evidence that IC may ameliorate LIRI in various pathophysiological contexts. In specific, it reduces the underlying oxidative stress and sterile inflammation in animal [17] and human models [7]. This is translated into decreased histologic damage [18], reduced pulmonary oedema [19], and improved respiratory function [8, 19] and pulmonary vascular haemodynamics [10, 20]. Considering the detrimental effects of LIRI, ranging from acute lung injury (ALI) following lung resections [15] to primary graft dysfunction (PGD) after lung transplantation [4], the association of these entities with adverse outcomes [4, 21], as well as the paucity of protective or therapeutic clinical interventions [4], ischaemic conditioning holds promise as a safe and effective strategy to protect the lung. This article aims to provide a review of the existing experimental and clinical evidence regarding the effects of IC on LIRI.

Enhanced oxidative stress appears to play a prominent role in the pathophysiology of LIRI [22]. Ischaemia, in the presence or not of apnoea as determined by ventilatory manoeuvres, creates a hypoxic environment with inhibited mechanotransduction in the arterioles and capillaries [23]. This triggers the production of reactive oxygen species (ROS) by endothelial cells, macrophages, and other immune cells [24]. The lung antioxidative mechanisms are overwhelmed upon reperfusion, resulting in an imbalance between ROS production and clearance [22]. Eventually, direct oxidative damage ensues with carbonylation of proteins and peroxynitration of proteins, lipids, and DNA [25]. Furthermore, ROS instigate a robust innate immune response by activating alveolar macrophages, which in turn release proinflammatory cytokines, including interleukin (IL)-8, -12, 18, and tumour necrosis factor (TNF)-a [26]. Additionally, neutrophils are recruited [26], and in concert with macrophages further enhance ROS generation; thus, a self-perpetuating cycle of oxidative stress enhancement is created [27, 28]. These activated leucocytes transmigrate into the extravascular space and cause increased microvascular permeability, thrombosis, oedema, and parenchymal cell death, by way of proteases, elastases, and ROS production [2]. Adherence of neutrophils to the endothelium further promotes the formation of gaps between the endothelial cells [6]. Moreover, these inflammatory cascades trigger platelet aggregation and coagulation, leading to formation of microthrombi and microvascular constriction [29]. The attendant activation of vasoactive agents, including thromboxane A2 and platelet activating factor, further promotes oedema formation [29, 30]. LIRI is also characterised by apoptotic phenomena, in part initiated by inflammatory cytokines (including IL-1β, -2, -8, and TNF-a) and enhanced by the release of proapoptotic factors due to mitochondrial rupture [31]. More specifically, the hypoxia-induced adenosine triphosphate (ATP) deficiency induces dysfunction of ATP-dependent ion pumps that results in mitochondrial calcium overload [29]. This leads to increased permeability of the mitochondrial transition pores and swelling, eventually leading to rupture [31]. Type II epithelial cells play a key role as both victim and as culprit of LIRI, as their reperfusion-induced dysfunction leads to impaired production and composition of pulmonary surfactant [32]. Crucially, the above described prooxidative and inflammatory signalling may exert systemic effects: on the one hand, distant IRI is known to cause pulmonary injury [33]; on the other hand, LIRI induces remote organ inflammatory and oxidative damage [34], while unilateral lung reperfusion may lead to similar deleterious processes in the contralateral, non-ischaemic lung [35] (Fig. 1).

The end result of LIRI is disruption of the alveolar-capillary barrier, causing non-cardiogenic pulmonary oedema and ventilation-perfusion (V/Q) [5]. Total and extravascular lung water is increased, causing gas exchange and lung mechanics impairment, with decreased arterial oxygen tension (PaO2) [36], increased airway pressures and increased alveolar-arterial oxygen gradient [(A-a) DO2] [37]. In addition, the attendant defective surfactant production and composition leads to reduced static (Cs) and dynamic lung compliance (Cd), while further increasing (A-a) DO2 [38]. Moreover, pulmonary vascular resistance (PVR) is increased up to three-fold following reperfusion, mainly due to pulmonary precapillary vasoconstriction [39]. Thus, LIRI is further compounded by pulmonary hypertension, which might also additionally hydrostatically promote the formation of pulmonary oedema [40].

In the absence of specific diagnostic criteria, LIRI is a diagnosis of exclusion, manifesting clinically as ALI or acute respiratory distress syndrome (ARDS) [41]. It has a detrimental impact in various clinical scenarios, particularly following lung transplantation: LIRI leads to PGD, which is the major cause of both short- and long-term morbidity and mortality in this setting [15, 42]. Importantly, the incidence of severe (grade 3) PGD within 72 h of transplantation is approximately 30% [42], while PGD is also associated with late graft rejection, which is the primary mortality aetiology beyond 1 year of the procedure [43, 44]. ALI/ARDS is frequent following lung resections, with a reported incidence of 0.88%, 2.96%, and 7.9% following sublobar, lobar/bilobar resection, and pneumonectomy respectively [21]. The associated mortality is considerable, ranging from 22% in sublobar resections to 50% following pneumonectomy [21]. LIRI is also a major source of morbidity and mortality after cardiac surgery, as it contributes to the development of ARDS, with an incidence of up to 20% and a mortality reaching 80% [45]. Interestingly, hepatosplanchnic IRI in aortic surgery has been found to cause lung injury, with increased pulmonary leak index (PLI) in up to 74% of the patients, especially in cases involving clamping of major aortic branches and in direct correlation with the aortic clamping time [46]. ALI also complicates 30–50% of major trauma cases, with an associated mortality of 10% depending on the severity of lung dysfunction [47].

LIRI has a detrimental effect on respiratory function and pulmonary haemodynamics, with dismal consequences for patient prognosis in a variety of clinical settings. The abundance of experimental evidence revealing the beneficial effects of IC, both on the underlying inflammatory and oxidative cascades, as well as the resultant functional derangements is being gradually complemented by encouraging clinical data. Given its promising preliminary results, safety, and ease of application, IC appears to be an intervention worth of further investigation and a useful addition to our deficient armamentarium against LIRI.