Special issue on carbon monoxide and exhaled biomarkers in human disease

Abstract

Human exhaled breath is a complex mixture of low molecular weight gases, containing a multitude of organic and inorganic molecules. Recent intense research interest has focused on the promise that fluctuations in the relative concentrations of the molecular components of exhaled breath can not only be monitored in clinical settings, but can also potentially serve as diagnostic indices of disease, including lung and airways diseases, and other systemic diseases [1–6]. To examine this hypothesis, a number of approaches have been applied, including the direct measurement of expired gases, the analysis of volatile organic compounds (VOC) on exhaled breath, as well as the analysis of pH and volatile/non-volatile compounds in the exhaled breath condensate (EBC). In general, the primary advantage of applying exhaled breath analyses to human clinical diagnostics and therapeutic monitoring is that these techniques are relatively non-invasive [1–6].This special issue on exhaled biomarkers in human disease begins with an overview of the clinical analysis of EBC with emphasis on asthma diagnostics, in a summary by Kazani and Israel [7]. As described in this article, a large number of compounds can be detected in EBC using spectrometric methods. These include lipid metabolites, cytokines, hydrogen peroxide, nitric oxide (NO) metabolites, and oxidative stress markers. The article discusses the relative utility of these species as biomarkers of asthma.In addition to EBC, a major portion of exhaled breath research has focused on low molecular weight gas analysis, such as the measurement of exhaled NO, and carbon monoxide (CO). Among these, the analysis of NO in human airway disease has been the most widely studied [8]. The reader is referred to other recent monographs for discussion on the predictive value of NO in breath analysis with respect to lung and airways diseases [8–10]. In addition to NO, a number of studies have focused on CO, which is the subject of this special issue. Although these two simple diatomic gases diverge in biochemical properties (NO being reactive as a free radical while CO is relatively inert), both represent the products of endogenous enzymatic processes [11]. Both NO and CO have been proposed as biomarkers of inflammatory tissue injury [8–13], though CO may also selectively reflect increased oxidative stress in the lung [14].Exhaled CO (E-CO) may originate from the inspiration of ambient CO, as found in smoke and air pollution, but also from endogenous metabolic sources that include heme metabolism catalyzed by the heme oxygenase (HO, E.C. 1:14:99:3) enzymes [11, 15]. HO, the principle source of endogenous CO, occurs as both constitutive (HO-2) and inducible (HO-1) isozymes [16]. The expression of HO-1 is inducible by a wide spectrum of cellular stress conditions, including those that elicit inflammatory or pro-oxidant states [17, 18]. Therefore, endogenous CO generation attributable to the HO system can generally increase in response to systemic stress. E-CO can arise in the airways, as the product of inducible HO-1 activity in the airway epithelium, alveolar macrophages and other lung cell types, as a consequence of inflammation. CO in the airways may also arise from the alveolae in equilibrium with carboxyhemoglobin (Hb–CO) in the pulmonary circulation [11]. Elevations in Hb–CO in turn may reflect increases in inhaled ambient CO, increased HO activity in peripheral tissues, or increased systemic hemoglobin turnover. E-CO increases dramatically in active smokers, and can be used to monitor smoking cessation. The dramatic increase in E-CO arising in smokers without airways disease is a confounding variable in attempts to use E-CO to generally detect disease [19].A number of early studies relying on measurements of E-CO have been limited by relatively insensitive methods for detection of this gas. For example measurements of E-CO relying on electrochemical detection methods typically report values in the parts per million (ppm) range, whereas equivalent studies focused on NO report values in the parts per billion (ppb) range. Recent studies have attempted to implement more sensitive methodologies for the detection of CO, such as infrared laser spectroscopic detection systems [20]. The article in this special issue by Sowa et al [21] features new experimental methodology for the detection of CO on exhaled breath. In this study the authors measure E-CO by monitoring 13CO using the technique of cavity leak out spectroscopy (CALOS). Using this technique, the authors describe acute exercise-dependent as well as long term natural variations in E-CO output in healthy humans.A number of recent and past studies have reported associations between E-CO levels and various disease states. For example, modulations in E-CO levels have been observed in critically ill or post-surgical patients and those with pulmonary diseases associated with inflammation, including chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, and bacterial infections [22–28]. The potential application of E-CO as a predictive tool for assessing pulmonary inflammation and disease is summarized in this issue in the articles by Gajdócsy and Horváth [29] and Morimatsu et al [30]. The current evidence for the role of E-CO as a marker of various acute and chronic diseases of the airways, including asthma and COPD, and of lung transplantation, is reviewed by Gajdócsy and Horváth. In a related review, Morimatsu et al discuss the observation and implications of E-CO measurements in critically ill patients with systemic inflammation and/or sepsis. Finally, in an article by Sethi et al [31], the authors examine differential effects of methacholine and allergen challenges on the exhaled levels of CO and NO in a cohort of adult subjects with atopic asthma. The authors conclude that bronchospasm negatively modulates E-CO and exhaled NO values, whereas the inflammatory stimulus of allergen exposure increases exhaled NO.In conclusion, many studies in recent years have collectively reported the occurrence of changes in E-CO levels as a function of disease state, though some studies have reported negative findings [32]. Despite improvements in the standardization and sensitivity of methods to detect E-CO, the predictive value of E-CO measurements as a diagnostic tool in human diseases remains uncertain. Nevertheless, we are optimistic that the collection of articles in this issue will stimulate further interest in this field. Continued efforts in this area might eventually yield tests and procedures of clinical value, and undoubtedly will shed further light on fundamental aspects of human metabolism.