The occurrence of oxidative stress (OS) has been linked to the pathophysiology of multiple human diseases and physiological processes such as aging. Among these diseases, one of the most important is Chronic Obstructive Pulmonary Disease (COPD), which is currently a global public health problem and the fourth leading cause of death and is expected to reach third place in 2020 (GOLD). In Chile, it is the third cause of respiratory death among causes of death in men. This disease is characterized by respiratory symptoms and a persistent airflow limitation, which is caused by respiratory or alveolar airway abnormalities, usually the result of significant exposure to particles or harmful gases. It is inflammatory, non-reversible and progressive (GOLD). Its main cause is cigarette smoke, although the inhalation of other types of particles, such as environmental pollution, becomes very important, especially in Chile. Within the pathogenesis of COPD, the development of oxidative stress, pulmonary, muscular and systemic, plays a key role in the progression of the disease.

OS can be defined as an imbalance between an increase in the production of oxidizing agents and/or a decrease in the endogenous antioxidant capacity, which is produced by both the inhaled reactive oxygen species (ROS) of cigarette smoke and those produced endogenously as a product of the systemic inflammatory reaction that characterizes this pathology.

As COPD progresses, many patients in their moderate or advanced stage become oxygen-dependent to supplement the gasometric alterations they present and to prevent exacerbations; however, it has been demonstrated in recent years that short and long- term oxygen concentrations produce an increase in the production of oxidizing agents, which may aggravate the disease even more.

Currently, the measurement of some OS biomarkers, such as total antioxidant capacity, or some parameters of oxidative injury such as plasma lipid peroxidation, have been associated with clinical variables of progression and/or exacerbation of this pathology. Available studies have focused on long-term exposure under hypoxic conditions in healthy subjects. In these individuals, oxygen-poor environmental conditions (such as those found in expeditions at height) are associated with an increase in ROS and OS production in the systemic circulation, at rest and in exercise. The literature on environmental hypoxia suggests that chronic hypoxemia in COPD patients contributes to OS in the systemic circulation under conditions of rest and post-exercise [1].

Recent studies have indicated that COPD is usually associated with significant extrapulmonary abnormalities, called “COPD systemic effects”. These systemic effects could be clinically relevant and could contribute to a better management and disease understanding. Muscle dysfunction, considered one of the major COPD systemic effects, contributes significantly to exercise limitation in these patients [2]. The identification of this disorder plus nutritional support rich in antioxidants added to exercise would improve the quality of life and prognosis of these patients [3]. On the other hand, OS biomarkers have been mainly measured in lungs, little has been described their systemic effect, and taking into consideration that PR produces several benefits without affecting pulmonary function, it remains to be studied whether any of these benefits are related to the modulation of the systemic OS.

In 2006, Rahman proposed that a rational approach to COPD treatment should also include supplementation with antioxidants, not only to neutralize the increased OS and subsequent inflammatory response, but also to identify the source of the oxidants and delay their generation. This could be done by two strategies: 1) augmenting endogenous antioxidant defenses, and 2) boosting non-enzymatic defenses through diet, drugs or exercise [4].

Current studies have focused on finding therapeutic tools to counteract the undesirable effects of the damage produced by ROS, such as pharmacological or non-pharmacological measures or a combination of both, in order to find the most appropriate treatment for the great diversity of patients with this disease.

Pulmonary rehabilitation is a non-pharmacological therapy: its widespread application in chronic COPD patients should be followed by demonstrable improvements in function (health-related quality of life, functional and maximum exercise capacity) attributable to these programs [5]. Rehabilitation serves as an important component of the management of COPD and is beneficial in improving health-related quality of life and exercise capacity. However, studies should focus on identifying which components of pulmonary rehabilitation are essential, its ideal length and location, the degree of supervision and intensity of training required, and how long treatment effects persist. Recent clinical studies have shown that some markers of DNA damage were higher in the group of non-responders to a 3-week PR program and in those patients receiving oxygen therapy [6]. Regarding oxidative stress markers, PR reduced erythrocyte lipid peroxidation at sessions 10 and 20. In contrast, with 8 weeks of PR in stage IV COPD, the group of patients did not improve exercise capacity [7, 8]. Moreover, the mechanistic and molecular effects associated with oxidative stress occurrence are not well characterized.

The aim of this work was to determine markers of oxidative stress levels in plasma and erythrocytes in patients with COPD through the application of a PR protocol.

The results presented here show the effect of pulmonary rehabilitation on OS markers in COPD patients. A pulmonary rehabilitation protocol is associated with a lower systemic OS as well as with an enhancement of antioxidant defense systems. Therefore, it is reasonable to state that the stimulation of long-term resistance training would induce an antioxidant response of such magnitude that it could counteract the damage caused by ROS. In this regard, it has been shown that the repetitive exercise stimulus generates a permanent increase in antioxidant activity as well as an improvement in capillary and mitochondrial density, evidenced in active muscle recruitment [13]. In this regard, Radak et al., report that this phenomenon is not paradoxical, but that it is only the result of the adaptation induced by the exercise, involving the activation of the antioxidant system, the blockade of ROS and the repair of biomolecules [14].

Regarding the total antioxidant capacity of the plasma, the results show an increase in FRAP in session 20 (at the end of the protocol) (Fig. 1A). When performing the analysis by degree of severity of COPD by session, both II / III and IV increase, but the latter in smaller magnitude. This could be in part due to the lower antioxidant capacity expressed in more severe COPD. The values of the GSH/GSSG ratio (Fig. 1B) also show this trend; however, its increase is evident only until session 10, a finding consistent with long-term effects on antioxidant mechanisms that would involve a genomic response. This was investigated by Viña et al., who reported long-term increases in oxidized glutathione (GSSG) [15]. In turn, the acute exercise would register an increase in GSSG and plasma MDA values associated with a decrease in GSH [16].

Based on OS biomarkers, plasma lipid peroxidation shows lower values of 8-isoprostanes (Fig. 2A) and MDA (Fig. 2B) in both sessions 10 and 20 compared to the baseline values. Leelarungrayub in 2017 also found lower MDA values in trained subjects after pulmonary rehabilitation and a respiratory training protocol, consistent with the data found in the present study [17]. Although the levels of MDA are higher in COPD patients with exacerbation, if they are compared with stable COPD or with healthy subjects respectively, by decreasing lipid peroxidation products as the PR sessions progress, it could be explained by the decrease in the oxidative load with exercise, reaching stable COPD levels [18].

These findings are consistent with the results shown by Kelsey et al., who also found that in COPD patients after six months of moderate-intensity aerobic exercise, there was a decrease in lipid peroxidation as well as an increase in the total content of reduced glutathione (GSH) [16]. The results of this study show that the GSH/GSSG ratio is higher in session 10 and then decreases to baseline levels in session 20. These values, together with the decrease in lipid peroxidation, could explain that until session 10 there is reduced OS resulting from the combination of the induction of antioxidant enzymes and the GSH pathway. Subsequently, a long-term effect, possibly genomic, predominantly determined by these intracellular enzymes would predominate. GSH, on the other hand, would have a higher consumption; therefore, only the action of these enzymes and other inducible systems would play a fundamental role in the reduction of plasma lipid peroxidation.

In a study conducted with aerobic and resistance exercise, maintained for six months, it was described that the activity of antioxidant enzymes increases steadily compared to baseline levels (pre-training of COPD patient subjects), explaining as a long-term effect, compared to acute exercise, where the enzymatic activity increases after exercise, and quickly falls to baseline levels [19]. Regarding the results of this study (Fig. 3), the activity of the antioxidant enzymes CAT, SOD and GSH-Px is greater after two months of training. As an antioxidant intracellular mechanism, the activity of the GSH-PX is expected to increase, and is thus consistent with that proposed by Paschalis et al. [20].

The strengthening of the antioxidant capacity due to physical training may be because exercise stimulates the expression of genes involved in the regulation of the antioxidant system, mainly Nrf2 (nuclear Factor-E2-related factor 2), a redox-sensitive transcription factor that binds to specific antioxidant response elements (AREs) in the promoter of the gene that encodes for these antioxidant enzymes and in other genes for defense proteins, regulating their expression in many body tissues [21]. It has also been shown that there are significant differences in the antioxidant enzyme content in erythrocytes and in the baseline levels of antioxidant activity in blood between healthy trained and untrained subjects. This suggests that the training status of the subjects influences not only the antioxidant capacity, but also the degree of oxidative molecular damage [22]. Extrapolated to COPD patients, PR would directly affect reducing oxidative damage in these patients since long-term training improves antioxidant defenses and decreases the basal oxidative grade typical of this pathology. Although the baseline levels of oxidants are higher when compared to healthy subjects due to the inflammation maintained in the pathology, among other factors described above. The lipid peroxidation products, such as MDA and 8-isoprostanes, decrease significantly throughout rehabilitation, but in comparison to the increase in antioxidant capacity and antioxidant enzymes as of training session 10, the concept of maintaining antioxidant characteristics as a long-time effect against oxidative injury is reinforced [23].

Another important point, and where more information is described in the face of improvements in PR lies in the topic of quality of life of COPD patients. The main measurement tool is Saint George’s Life Questionnaire, and in the analysis in this study, it is no exception, since a significant improvement was found in all the groups studied, although it was most significant in COPD stage IV, with an average improvement this score. This is consistent with what was described by Nemoto in 2014, where he described that improvements in quality of life do not lead to an increase in OS [7]. It is described that in a PR program, it is expected to obtain a difference equal to or greater than four points expressed by a significant change in the quality of life [24]. Although a score over 50 is associated with higher mortality, in COPD III patients, only 25% obtained a score greater than this value at the beginning of PR, whereas, for COPD IV, 85% of them obtained scores at that cutoff point at the beginning of the protocol. No COPD II exceeded 50 points. This demonstrates that PR significantly improves the quality-of-life aspect measured by the SGRQ questionnaire. And if we add good psychological and nutritional support to this, the most important deterioration aspects of these patients would be seen as a clear improvement with respect to their basal state (Fig. 4).

As a projection, it would be a useful and innovative mixed therapy, comprised of a pharmacological intervention with antioxidants and an adjunct therapy with PR protocol to ensure lower long-term oxidative damage or potentiation of antioxidant defenses against nociceptive pro-oxidant injury in these patients.