The effects of mechanical ventilation on oxidative stress

Abstracts

OBJETIVO: A ventilação mecânica constitui um dos pilares terapêuticos da unidade de terapia intensiva, entretanto, deve-se avaliar os efeitos deletérios por ela ocasionados, logo objetivamos avaliar o estresse oxidativo de pacientes internados em unidade de terapia intensiva submetidos à ventilação mecânica invasiva. MÉTODOS: Estudo transversal onde foram incluídos 12 pacientes que estavam em ventilação mecânica invasiva. As coletas sanguíneas (3 mL) foram realizadas no primeiro e último dia em que o paciente estava submetido a ventilação mecânica invasiva e utilizou-se o plasma para avaliação das substâncias que reagem ao ácido tiobarbitúrico (TBARS) e os glóbulos vermelhos para dosagem de superóxido dismutase (SOD) e da catalase. RESULTADOS: Os pacientes apresentaram média de idade de 64,8±17,6 anos; volume corrente de 382±44,5 mL e APACHE II de 15±7. Quando comparado o TBARS inicial e ao final da ventilação houve diferença significativa (3,54±0,74 vs. 4,96±1,47; p=0,04). Em relação às enzimas antioxidantes não houve diferença. Observa-se correlação entre as variáveis PaO2/FiO2 e TBARS (r = 0,4); SOD e PaO2/FiO2 (r = 0,51) e SOD e APACHE II (r = 0,56). Quanto ao desfecho da internação, 6 pacientes foram a óbito. CONCLUSÃO: Pacientes submetidos à ventilação mecânica invasiva podem apresentar alteração do estado redox, marcado pelo aumento no TBARS e redução das enzimas antioxidantes.

Estresse oxidativo; Unidades de terapia intensiva; Respiração artificial; Substâncias reativas com ácido tiobarbitúrico

ORIGINAL ARTICLE

The effects of mechanical ventilation on oxidative stress

João Batista Raposo Mazullo Filho^I; Silvia Bona^II; Darlan Pase da Rosa^II; Fabiano Gomes da Silva^III; Luiz Alberto Forgiarini Junior^IV; Alexandre Simões Dias^V; Norma Possa Marroni^VI

^IPostgraduate Program (MSc level) in Genetics and Applied Toxicology, Universidade Luterana do Brasil - ULBRA - Canoas (RS), Brazil

^IIPostgraduate Program (PhD level) in Medicine: Medical College, Universidade Federal do Rio Grande do Sul - UFRS - Porto Alegre (RS), Brazil

^IIIGraduation Biomedicine Course, NOVAFAPI College, Teresina (PI), Brazil

^IVMethodist University Center - IPA - Porto Alegre (RS), Brazil

^VPhysical Therapy Course, Universidade Federal do Rio Grande do Sul - UFRS - Porto Alegre (RS), Brazil; Postgraduate Program in Movement Sciences and Pulmonology Sciences, Universidade Federal do Rio Grande do Sul - UFRGS - Porto Alegre (RS), Brazil

^VIPostgraduate Program in Genetics and Applied Toxicology, Universidade Luterana do Brasil - ULBRA - Canoas (RS), Brazil, and Postgraduate Medical Program: Medical College and Physiological Sciences, Universidade Federal do Rio Grande do Sul - UFRS - Porto Alegre (RS), Brazil

Corresponding author

ABSTRACT

OBJECTIVE: Mechanical ventilation is a mainstay of therapy in intensive care units; however, its deleterious effects need to be assessed. Therefore, we aimed to assess oxidative stress in patients admitted to an intensive care unit undergoing invasive mechanical ventilation.

METHODS: This cross-sectional study included 12 invasive mechanical ventilation patients. Blood samples (3 mL) were collected on the first and last days on invasive mechanical ventilation. Thiobarbituric acid-reacting substances (TBARS) were assessed in plasma, and superoxide dismutase (SOD) and catalase (CAT) were assessed in erythrocytes.

RESULTS: The mean age was 64.8 ± 17.6 years, the tidal volume (VT) 382 ± 44.5 mL, and the APACHE II score 15 ± 7. When initial and final TBARS were compared, a significant difference was identified (3.54 ± 0.74 vs. 4.96 ± 1.47, p = 0.04). Antioxidant enzymes showed no significant differences. Correlations between PaO₂/FiO₂ and TBARS (r = 0.4), SOD and PaO₂/FiO₂ (r = 0.51) and APACHE II and SOD (r = 0.56) were identified. Six patients died.

CONCLUSION: Patients undergoing invasive mechanical ventilation can develop redox state changes, showing increased TBARS and reduced antioxidant enzymes.

Keywords: Oxidative stress; Intensive care units; Respiration, artificial; Thiobarbituric acid reactive substances

INTRODUCTION

Invasive mechanical ventilation (IMV) consists of a positive pressure device connected to a tracheal tube. IMV is used for the treatment of patients with acute respiratory failure or in acute worsening of chronic respiratory failure. The purposes of IMV are to maintain gas exchange, to reduce respiratory muscle workload (increased in acute conditions with increased metabolic requirements), to reduce oxygen consumption, thereby reducing respiratory distress, and to allow for the use of specific therapies.⁽¹⁾

Maintaining gas exchange with IMV has consequences to organ systems. Both higher positive pressures and inspired oxygen fractions greater than 21% are believed to cause oxidative stress levels so high that antioxidant methods are ineffective.⁽²⁾

The inspired oxygen fraction (FiO₂) is a mechanical ventilation parameter used to optimize tissue oxygenation. However, an inappropriately adjusted FiO₂ can lead to the harmful effects of hypoxia or hyperoxia. Some tissue hypoxia consequences are cell changes and increased anaerobic metabolism. Toxic oxygen effects are not well established in humans; however, high or prolonged oxygen doses can cause both pulmonary and systemic injuries. In hyperoxia, most of the damage is caused by oxidative stress, which can cause organ molecule degeneration and consequent cell failure and death. Many intensive care unit (ICU) patients are on prolonged mechanical ventilation and therefore on prolonged oxygen use. These patients should receive FiO₂ levels sufficient to fit their metabolic requirements but not high enough to change their respiratory patterns and vital signs.^(3-5)

The severity and prognosis of ICU patients must be assessed.⁽⁶⁾ The Acute Physiology and Chronic Health Evaluation II (APACHE II) is a useful and affordable prognostic score that was validated by Kruse and that is widely used in ICUs. This score is used for a range of diagnoses and is based on physiological parameters and clinical information that are available in most hospitals.⁽⁷⁾

Growing evidence shows that oxidative stress has a relevant role in several clinical conditions, such as neoplasms, diabetes, atherosclerosis, neurovegetative diseases, chronic inflammatory diseases and ischemia/reperfusion injuries, among other common conditions in critically ill patients.⁽⁸⁾

This article reports on a study aimed to assess and correlate oxidative stress in the plasma and erythrocytes of ICU patients undergoing IMV.

METHODS

This study was approved by the ethics committee of Faculdade NOVAFAPI - Teresina - Piauí, Brazil, and is considered to be in compliance with the Brazilian National Health Council's Law 196/96 on research involving human subjects.

This was an observational, quantitative study assessing oxidative stress in patients admitted to the ICU of Hospital São Marcos, in Teresina, Piaui, Brazil.

Male and female patients between 18 and 80 years of age, admitted between September and October 2011 and signing a written informed consent form (ICF), were included during their first day on IMV. No exclusion criteria were applied.

Data and sample collections

After inclusion and completion of the subjects' identification sheets, initial and final IMV respiratory monitoring data were collected. The data were collected daily, therefore allowing for the assessment until the day of mechanical ventilation discontinuation or death. The case report form included identification (name, age, weight, height, gender, APACHE II score, date of admission, underlying disease and comorbidities), date of each blood collection and ventilator parameters (ventilation mode, peak pressure [Ppeak], tidal volume [TV], inspiratory time [Ti], positive end-expiratory pressure [PEEP] and FiO₂), arterial blood gases (pH, PaO₂, PaCO₂, HCO₃), hemodynamic status (heart rate [HR] and mean blood pressure [MBP]) and peripheral oxygen saturation (SpO₂).

On the first and last days of IMV, 3-mL arterial blood samples were collected from the radial artery to analyze blood gases, and venous blood samples were collected for oxidative stress assessment. A GEM Premier 3000 blood gas meter was used for all blood gas measurements.

Oxidative stress was measured from 3-mL venous blood samples collected and stored in heparin and EDTA bottles. The blood was centrifuged to separate the plasma and erythrocytes. Plasma was stored and later analyzed for lipid peroxidation using the thiobarbituric acid-reacting substances (TBARS) technique. Erythrocytes were washed and centrifuged three times with saline 0.9%, discharging the supernatant. Following the last washing, erythrocyte precipitates were stored in acetic acid and magnesium sulfate solution for anti-oxidant enzyme analysis.

For lipid peroxidation and anti-oxidant enzyme assessment, the protein concentration in pulmonary tissue homogenate was calculated using the Bradford method, using bovine albumin as the standard. The samples were measured spectrophotometrically and the values expressed as mg/mL. Plasma lipid peroxidation was determined with the TBARS method. This technique consists of heating the material with thiobarbituric acid until the formation of a pink material, which is subsequently measured at 355 nm using a spectrophotometer. The results are expressed as nmol/mL TBARS.⁽⁹⁾ Superoxide dismutase (SOD) anti-oxidant enzyme activity was measured on a plate reader, assessing epinephrine degradation and adenochrome formation; the data are expressed as SOD units per mg of protein (U-SOD/mg prot.)⁽¹⁰⁾ Catalase (CAT) activity was measured with hydrogen peroxide in a plate reader at 240 nm, and the results are expressed as nmol/mg protein.⁽¹¹⁾

Statistical analysis

A data bank was prepared using Microsoft Excel software. The data were analyzed using the SPSS statistical package, version 17.0, and the results are expressed as the mean ± standard deviation. A significance level of 5% was adopted, with p < 0.05 considered statistically significant. Before/after comparisons were made using a pairwise t test, and correlations were tested with Pearson's correlation test. The sample's normality was assessed with the Kolmogorov-Smirnov test.

RESULTS

Table 1 displays the patients' overall characteristics. Most of the patients were female — 75% of the sample. The mean age was 64.8 ± 17.6 years. The mean height was 155 ± 0.05 centimeters. The predicted weight was 47.7 ± 5.5 kilograms, and the predicted tidal volume (TV) was 382 ± 44.5 mL. The median APACHE II score was 15 ± 7. The mean IMV time was 5.5 ± 3.5 days. The following comorbidities were recorded: three patients (25%) with systemic hypertension (SH), two patients (17%) with diabetes mellitus (DM) and two patients with acute renal failure (ARF) (17%). Six patients (50%) died during their hospitalizations.

Analyzing initial and final oxidative stress data, the TBARS levels were significantly higher after IMV (p < 0.05). No statistically significant differences were found for SOD or CAT; however, enzyme activity was identified as decreasing after IMV (Table 2).

No arterial blood gas parameters were shown to change significantly (Table 3). Similarly, as shown in Table 4, the initial and final ventilation parameters were not different.

A statistically significant correlation was observed between FiO₂ and TBARS before (r = 0.4) and after (r = 0.4) IMV, suggesting that the higher FiO₂ was, the more oxidative stress there was (Figure 1). Analyzing the correlation between the final PaO₂/FiO₂ and final SOD, r = 0.51 (p < 0.05) was found, suggesting that patients with higher PaO₂/FiO₂ ratios have more active SOD following IMV (Figure 2). Analysis of the correlation between the IMV time and final CAT showed r = -0.30 (p < 0.5), i.e., patients with shorter mechanical ventilation time had larger CAT values after IMV (Figure 3).

Figure 4 shows the correlations between the APACHE II score and initial SOD (r = 0.56; p < 0.05) and between the APACHE II and final SOD (r = 0.59; p < 0.05).

DISCUSSION

This study assessed the correlation between oxidative stress and IMV in ICU patients; our findings suggest that IMV might influence the redox state.

Analyzing oxidative stress after IMV, significantly increased TBARS levels were identified. Although the medical literature lacks reports from clinical trials assessing oxidative stress and IMV, our data agree with the findings of other authors who assessed biological models.⁽¹³⁾ Experimental animal and human oxidative stress studies have shown that increased metabolic activity favors oxidative molecular injuries.⁽¹⁴⁾ Oxidative stress, shown by increased TBARS, could be associated with endothelial dysfunction and prolonged IMV due to an increased production of oxygen-derived free radicals and also reactive oxygen species, such as hydrogen peroxide, causing both pulmonary and systemic injuries.⁽¹⁵⁾ Oxidative stress markers are remarkable tools for the assessment of possible effects on and implications for several metabolic processes, and they might be markers of pathologic and inflammatory processes. These substances can be measured in the blood, thus making them suitable for bedside care.^(13,16) Respiratory muscle contractions, either active or passive due to IMV, promote transient ischemia, while relaxation leads to reperfusion. This process characterizes the oxygen paradox in which a lack of oxygen followed by reperfusion leads to increased reactive oxygen species and can cause an unbalanced condition, characterizing oxidative stress.⁽¹⁷⁾ Additionally, significant reductions of CAT and SOD anti-oxidant enzymes were observed after IMV. These serum enzyme decreases could be explained by the body's reduced self-defense mechanisms, as the subjects were prolonged IMV patients with associated comorbidities.⁽¹⁸⁾

The initial and final arterial blood gas and ventilation parameters demonstrated no statistically significant differences. However, the patients had increased oxidative stress and reduced levels of anti-oxidant enzymes.^(19,20)

A significant correlation was observed between the initial FiO₂ and initial TBARS (r = 0.4), as well as a correlation between the initial FiO₂ and final TBARS (r = 0.4; p < 0.05). This correlation shows that a higher FiO₂ leads to higher oxidative stress, as a high FiO₂, with the consequence of more circulating oxygen, causes oxidative injury. However, PaO₂ conditions that are too low can also cause oxidative stress.⁽¹⁷⁾

Additionally, a correlation between the final PaO₂/FiO₂ ratio and final SOD was identified (r = 0.51; p < 0.05). Patients with higher PaO₂/FiO₂ ratios showed higher final SOD values, therefore showing that more oxygen from circulating FiO₂ might stimulate the production of SOD, increasing anti-oxidant defenses, as SOD acts by transforming superoxide radical anions (produced from oxygen) into hydrogen peroxide.⁽²¹⁾

Analysis of the correlation between the IMV time and final CAT showed r = 0.3 (p < 0.05), demonstrating that patients with prolonged ventilation had higher CAT values by the end of IMV. This finding suggests that patients with less IMV time have more effective anti-oxidant systems and thus higher CAT concentrations. CAT's anti-oxidant action takes place when free radicals reach the bloodstream via erythrocyte catalase. Catalase prevents meta-hemoglobin accumulation and also degrades hydrogen peroxide — a toxic metabolism byproduct — into water and molecular oxygen.⁽²²⁾ Excessive H₂O₂ causes hemoglobin oxidation, consequently leading to a reduced oxygen concentration; this process can lead to infections, ulcers or even necrosis.⁽²³⁾

The predictive APACHE II score had a correlation with SOD anti-oxidant enzyme levels at the initial (r = 0.56; p < 0.05) and final assessments (r = 0.59; p < 0.05), suggesting that more severely ill patients had lower SOD values, both on the first and last days of IMV. This finding shows that, in general, ICU oxidative parameters are associated with severity and outcome.⁽²⁴⁾

This study had some limitations: a) the small sample size; b) the lack of control patients, which would have allowed for the assessment of whether oxidative stress changes are secondary to underlying disease progression; and c) the use of TBARS as a single oxidative stress marker. Most analytes are generated during heating, which is part of the TBARS technique. Not adding butylated hydroxytoluene (BHT) can increase endogenous production of analytes, thus changing TBARS assessments.

Additional longitudinal studies and larger sample sizes are required to assess the predictive value of the selected variables, as well as the possibility of stratifying oxidative stress, anti-oxidant enzymes and patients' outcomes.

CONCLUSION

This study's results suggest that patients with invasive mechanical ventilation have increased oxidative stress by the end of IMV, with increased TBARS and reduced superoxide dismutase and catalase serum levels.

REFERENCES

Submitted on January 23, 2012

Accepted on March 14, 2012

Conflicts of interest: None.

Study conducted at Hospital São Marcos - Teresina (PI), Brazil and Hospital de Clínicas de Porto Alegre – HCPA – Porto alegre (RS), Brazil.

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Publication Dates

• Publication in this collection
  24 Apr 2012
• Date of issue
  Mar 2012

History

• Received
  23 Jan 2012
• Accepted
  14 Mar 2012