Introduction

A high percentage of worldwide deaths is due to traumatic injuries caused by severe burns (Alencar de Castro et al. 2013; Lindahl et al. 2013). For patients with severe burns, there are several complications that increase morbidity and mortality, such as inflammation, compromised immune system, infections, respiratory disorders, cardiovascular and renal dysfunction. Another important factor that participates in increasing the mortality rate is the increased length of stay in hospital (Liberati et al. 2006).

Burns are responsible for modifying the functionality of the immune system, making patients vulnerable to infections. Systemic infections are according to the literature the most important causes of death in these type of patients, being directly involved in multiple organ dysfunction syndrome (MODS) and death (Rosanova et al. 2014).

The burned patient also presents pathophysiological disorders with strong implications in the survival rate, such as acute respiratory distress syndrome (ARDS), acute kidney injury (AKI), hypercatabolism, and bacterial translocation due to increased intestinal permeability (Arlati et al. 2007; Vf et al. 2012).

Moreover, inflammation influences the fluid coagulation balance by activating the coagulation cascade, which has as results the compromise of the microvascular system (Mühl et al. 2011; Farina et al. 2013; Rosanova et al. 2014). One of the most significant side effect in patients with severe burns is the systemic inflammatory response syndrome (SIRS), which occurs in a few hours postburn. This is responsible for increased capillary permeability, the release of proinflammatory factors, and for inducing the production of highly reactive molecular species, commonly known as free radicals (FR) (Lazarus et al. 2015). Excess production of FR leads to inflammation and infection potentiation, forming a vicious circle in terms of biochemical and metabolic reactions. By high concentrations of FR and by decreased antioxidant capacity, physiological redox balance is severely disrupted, thus setting up as oxidative stress (OS).

Numerous studies correlate OS in patients with severe burns with an increased rate of severe posttraumatic complications and with a high mortality rate (Rosenfeldt et al. 2013). For increasing the body’s antioxidant capacity, a number of studies recommend the administration of substances with antioxidant capacity. At the moment, a number of research groups are studying intensively the action of antioxidant therapy regarding the clinical outcomes of patients with severe burns (Oudemans-van Straaten et al. 2014).

In this current work, we want to present the pathophysiology of the OS in patients with severe burns, as well as the implications of the antioxidant therapy on the outcome of such patients.

Molecular Damage in Critically Ill Patients with Severe Burns

FR are molecular or ionic species with an increased reactivity, mainly due to their electronic configuration. In the human body, this reactive species are produced in a physiological pathway following biochemical processes, their destructive action being inhibited by the endogenous antioxidant systems (Lazarus et al. 2015). Along with the disturbance of physiological metabolisms due to trauma, the production of FR is accelerated. Moreover, the body’s antioxidant capacity progressively decreases because it is susceptible to OS aggression (Scheibmeir et al. 2005; Rao et al. 2011) (Fig. 1).

Fig. 1
figure 1

The diagram of biochemical mechanisms of free radicals. Pathophysiological implications of oxidative stress. The massive losses of fluids through the burned area have as effect a decrease of the blood flow. Following the peripheral ischemia, the cell metabolism goes from the aerobic phase to the anaerobic one, leading to modifications of the enzyme systems and to free radicals production. The overproduction of free radicals occurs through various biochemical pathways, such as xanthine oxidase pathway, NAD(P)H oxidase pathway, electron transport chain, lipid peroxidation, and protein peroxidation pathway. Lipid oxidation is responsible for destroying the mitochondrial cell membranes. Also, the intense redox activity is responsible for the destruction of the vascular endothelium. Cell apoptosis and tissue necrosis emphasizes SIRS increasing MODS and the death rate

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The most common FR described in the case of severe burns are represented by the hydroxyl radical, the superoxide radical, hydrogen peroxide, the alkylperoxyl radical, nitric oxide, nitroperoxide, and the lipid radical (Horton 2003; Chaturvedi and Beal 2013; Tsakiridis et al. 2014). The main sources of FR generation are represented by mitochondria, the peroxisomal oxidases, the xanthine oxidase, the nicotinamide adenine dinucleotide phosphate NAD(P)H oxidases, and by the cytochrome P450 enzymes (Halliwell 2007; Lazzarino et al. 2014).

By the action of reactive oxygen species (ROS) on the polyunsaturated fatty acids, a series of important lipid reactive species are being produced, which are responsible for initiating the lipid peroxidation chain reactions. The aggression of lipid peroxidation on the physiological function of the human body is given especially by high spreading ability of the redox chain reactions, as well as by toxicity and by high concentrations of metabolites, which include malondialdehyde (MDA), 4-hydroxy-2-nonenal (4-HNE) si F2-isoprostanes (Leipnitz et al. 2011; Le Lay et al. 2014; Rahal et al. 2014).

Another target of FR are DNA species, which lead to significant structural and functional changes. Following this a number of alterations were outlined in the mechanisms of protein synthesis, which are being directly involved in certain pathologies (Ramos et al. 2012).

In trauma patients with severe burns, a significant amount of superoxide radical is produced by activating macrophages, eosinophils, T-lymphocytes, and B-lymphocytes (Vinha et al. 2013). The biochemical mechanism of superoxide radical release includes reactions produced by NAD(P)H oxidase and xanthine oxidase. Also during the mitochondrial reactions of electron transfer significant amounts of superoxide radical are generated. Subsequently, through the reaction catalyzed by superoxide dismutase (SOD), superoxide radical is transformed into hydrogen peroxide (Miller 2012).

Hydrogen peroxide is enzymatically reduced through catalase enzyme (CAT) in peroxisomes, and through glutathione peroxidase in the cytoplasm or in the mitochondria, forming water and oxygen molecules. By combining hydrogen peroxide with iron ions (II), the so-called Fenton reaction, large amounts of hydroxyl radical are obtained (Yazihan et al. 2008). Hydroxyl radical is a free species with the highest reactivity, being responsible for destroying cell membranes by distorting lipoproteins (Marín-Prida et al. 2012).

Acute endothelial dysfunctions are also present in patients with severe burns in a high percentage. NO is synthesized from l-arginine by nitric oxide synthases (NOS); this reaction is being catalyzed by oxygen, especially in the endothelial cells (Fox et al. 2013). Through the reaction with superoxide radical, nitroperoxide is obtained which is responsible for a number of pathophysiologies in trauma patients with severe burns (Rao et al. 2011; Zapatero-Solana et al. 2014; Duchesne et al. 2015). Endothelial dysfunctions, responsible for disrupting the physiology of the microvascular system, led in many cases to metabolic acidosis, cell apoptosis, sepsis, MODS, and death (Burkhardt et al. 2012). Another syndrome commonly seen in patients with severe burns is the ischemia–reperfusion syndrome, following this large amounts of proinflammatory mediators are released, which include complement fragments, cytokines, or lipid mediators. Han et al. studied a number of aspects of the inflammatory status in patients with severe burns, such as levels of endotoxin, soluble adhesion molecules, and cytokine levels. After the study they reported significantly modified profiles in such patients in terms of inflammatory aspects, due to the significant increase of endotoxins, cytokines, and of proinflamatory mediators (Han et al. 2004). Mühl et al. showed in a similar study, that in critical patients with severe burns, the antioxidant capacity dramatically decreases, being observed increases in proinflammatory marker expressions (Mühl et al. 2011). The endogenous antioxidant system is well represented under physiological conditions by enzymes with antioxidant capacity and by a number of vitamins. Endogenous antioxidant systems. Three types of enzymes were identified: SOD1 (CuZnSOD) in the nucleus and cytosol, SOD2 (MnSOD) identified in the mitochondrial matrix, respectively, SOD3 (EcSOD) located extracellular (Gerbaud et al. 2005; Pilon et al. 2011). During antioxidant mechanism SOD enzymes are involved in converting the superoxide ion into water and oxygen (Miller 2012). Catalase is another enzyme which is able to reduce the oxidative effect of hydrogen peroxide by decomposing it into oxygen and water molecules (Rahman and Adcock 2006; Comar et al. 2013). The antioxidant activity of glutathione (GSH) is represented by maintaining a normal redox balance (Lazzarino et al. 2014). Glutaredoxins (GRXs) are another endogenous antioxidant systems which are present in human body in two forms, Grx1 present in the cytosol and intermembranous space and Grx2 present in the mitochondrial matrix. Another endogenous antioxidant system responsible for the breakdown of hydrogen peroxide is represented by peroxiredoxins (PRXs) (Elkharaz et al. 2013).

Biomarkers for the Assessment of Oxidative Stress in Burned Patient

A number of specific biomarkers have been described for the assessment and for monitoring the oxidative effects. Some of the most useful markers for the evaluation and optimization of intensive care in patients with severe burns are represented by inflammatory markers, such as interleukin 1 (IL-1), interleukin 6 (IL-6), interleukin 2 (IL-2) interleukin 8 (IL-8), interleukin 17 (IL-17), tumor necrosis factor alpha (TNF-alpha) (Homsi et al. 2009; Trancă et al. 2014). IL-1 has been particularly studied due to its high specificity, being associated with poor outcomes (Abdul-Muneer et al. 2015). Regarding Il-6, numerous studies report the existence of a direct connection between elevated levels of IL-6 and the level of systemic inflammation (Dal-Pizzol et al. 2010; Abdul-Muneer et al. 2015; Kumar et al. 2014). 4-HNE (Ansari et al. 2008) is a specific biomarker for redox reactions in lipid peroxidation. For assessing injuries brought by FR on DNA and RNA 8-hydroxy-2′-deoxyguanosine is used (Abdul-Muneer et al. 2013), which expresses DNA oxidative damage. Regarding the lower immune response in patients with severe burns, a special place is represented by the CD163 receptor (Xie 2013). CD163 expression is particularly highlighted in the surface of macrophages and monocytes (Piatkowski et al. 2011). Piatkowski et al. investigated the expression of CD163 at 120 h postburn. After the study they concluded that dosing, evaluating, and monitoring the expression of CD163 can be a good predictor for inflammation, sepsis, or MODS for this type of patients (Piatkowski et al. 2011). Table 1 presents the most significant biomarkers for assessing and monitoring the OS in burned patient.

Table 1 Specific biomarkers for molecular changes in the case of severe burns
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Over the last period, microRNAs circulating species have been intensively studied, in order to use them as a biomarkers for various diseases. microRNAs species are short noncoding RNAs consisting of 19–24 nucleotides (Kodahl et al. 2014). microRNA formation begins in the nucleus with protein-coding transcription by RNA polymerase II, which represents the primary microRNA (pri-microRNA) (Courts and Madea 2010). Nucleocytoplasmic transporter (Exportin 5) is responsible for the transport into the cytoplasm of the pre-microRNA. Once in the cytoplasm, the pre-miRNA undergoes an additional processing by the Dicer complex, generating double-stranded mature microRNA (19–24 nucleotides in length) and microRNA* (passenger strand) (Kodahl et al. 2014). microRNAs can reach the systemic circulation either with cell death, as apoptotic bodies, or as microvesicles and exosomes. Due to the increased stability and specificity, microRNA circulating species are currently preferred for the assessment and monitoring of various pathologies (Starega-Roslan et al. 2011). In trauma patients with severe burns a series of studies were performed, regarding the expression of microRNAs. Tacke et al. studied the expression of microRNAs in trauma patients with sepsis, reporting an increased expression of microRNA-133a in patients with sepsis. Moreover, they concluded that increased expression of microRNA-133a can be a predictor of poor prognosis for sepsis patient (Tacke et al. 2014). Zhao et al. report changes in microRNA 23a-3p expression during ischemia-reperfusion syndrome (Zhao et al. 2014).

Other similar studies, report changes in microRNA-182, microRNA-199a-5p, microRNA-211, microRNA-203, microRNA-222, microRNA-29b, microRNA-150, microRNA-342-5p, and microRNA-122 expression in trauma patients, in patients with severe burns, in severe systemic inflammation, sepsis, and MODS (Vasilescu et al. 2009; Ding et al. 2012; Moore et al. 2012; Xie 2013; Zhao et al. 2014).

Modulation of the Oxidative Response: Antioxidant Therapy Models

The current protocols are focused mainly on the fluid management in such patients, unfortunately fighting against SIRS is not fully approached (Belîi et al. 2014; Mierzewska-schmidt 2015). Studies on experimental animals show a significant decrease of SIRS, sepsis, and MODS incidence associated with high doses of intravenous administration of substances with increased antioxidant capacity (Hu et al. 2015; Preiser et al. 2015) (Table 2). Among these, the most studied as having a strong antioxidant potential are N-acetylcysteine (Csontos et al. 2012), vitamin C (Biesalski and McGregor 2007), vitamin E (Oudemans-van Straaten et al. 2014), vitamin A (Aschauer et al. 2014), selenium (Sakr et al. 2014), and resveratrol (Lagouge et al. 2006). Atabak Najafi et al. studied the influence of N-acetylcysteine administration in patients with ventilator support. As a result of this prospective study, which included 44 patients with multiple trauma, they showed a reduction of systemic inflammation and of complications caused by this (Najafi et al. 2014). In a similar study, Al-Jawad et al. (2011) confirm the benefic effects of N-acetylcysteine on the inflammatory status, reporting a decrease in the mortality in such patients. Csontos et al. studied the effects induced by the antioxidant therapy with N-acetylcysteine in patients with severe burns, reporting a considerable decrease in plasma levels for proinflammatory cytokines, such as IL-6, IL-8, IL-10. Moreover, patients who received antioxidant therapy showed low serum levels of malondialdehyde and a lower necessary for catecholamine (Csontos et al. 2012). Other studies also revealed low plasma levels for MDA and myeloperoxidase activity, where N-acetylcysteine was given in trauma patients with severe burns (Heyland et al. 2005; Hall et al. 2012). Studies performed in humans regarding the administration of high doses of vitamin C, given intravenously, reported an increase in the survival rate for this type of patients (Lira and Pinsky 2014; Tompkins and Hospital 2015). Moreover, various studies associate the administration of intravenous vitamin C with a decrease in oxidative reactions given by neutrophils, systemic inflammation, sepsis, mechanical ventilation, as well as in the length of stay in intensive care unit (ICU) (Dubick et al. 2005; Oudemans-van Straaten et al. 2014).

Table 2 Substances with strong antioxidant capacity
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Tanaka et al. studied the effects of vitamin C administration in trauma patients with severe burns. They administered high doses of vitamin C (66 mg/kg/h) in the first 24 h posttrauma, in patients who had burns on more than 30 % of the body surface area. After the study, they concluded that the antioxidant therapy implemented in the first 24 h posttrauma reduced the need for fluids during fluid management in this type of patients, it also reduced the wound edema and the incidence of respiratory disorders (Tanaka et al. 2000). Juretic et al. studied the effects of vitamins A, C, and E administration on nuclear transcription factor-kappa B (NF-kB), inflammatory status, and cardiac function in patients with severe burns. The antioxidant therapy was achieved by coadministration of 38 mg/kg vitamin C (i.v.), 27 U/kg vitamin E (i.v.), and 41 U/kg vitamin A (i.v.). The animal group who received antioxidant therapy, showed significant improvements in heart function. Also, the study reported a reduction of the inflammatory response. Regarding NF-kB, a decrease of proinflammatory cytokines biogenesis was observed, due to the inhibition of NF-kB nuclear migration (Horton 2003).

Conclusions

In this review we resumed the biochemical and physiological implications of FR in trauma patients with severe burns. Available clinical and preclinical studies report a series of pathologies induced by OS in critically ill patient, such as microvascular system dysfunction, severe systemic inflammation, vulnerability to infection, metabolic disorders until in the end when multiple organ dysfunction or death occur.

In order to minimize the destructive effects induced by OS, a series of studies were performed regarding the administration of antioxidant substances, such as vitamins C, E, A, selenium, N-acetylcysteine, and resveratrol. These protective effects against FR refer to protection against inflammation, organ injury, and dysfunction, as well as to a more rapid recovery of patients with severe burns. Finally, we can conclude that the administration of vitamin C, associated or not with other antioxidants, brings beneficial effects in critically ill patient, by reducing particularly the systemic inflammation and the microvascular system dysfunctions. However, there still are a series of questions that need to be answered, including the optimal dose, the time of antioxidant substances administration, and the antioxidant combinations.