IEEE SENSORS JOURNAL, VOL. 10, NO. 1, JANUARY 2010
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What is Normal Breath? Challenge and Opportunity Steven F. Solga and Terence H. Risby (Invited Commentary)
HE current resurgence in the interest of breath analysis is a reflection of advances in analytical chemistry instrumentation. For despite terrific promise of breath analysis and the fact that there have been literally thousands of publications published during the last one hundred years, only a handful of breath tests are used clinically and a few others used for research purposes. The subsequent discussion presents some of the steps that researchers in the field of breath analysis must take to ensure that breath analysis will be accepted by clinicians. Breath analysis is not normally used clinically for a number of reasons. Most breath analyses have been performed using typical analytical chemistry instrumentation that was not designed for breath analysis. This has hindered both research and, ultimately, enthusiasm for clinical applications. The development of monitors that are custom designed for breath analysis that are portable and easy to use would be a significant advance. There are also multiple potential confounders unique to breath analysis, including contamination from ambient air, interference from other molecules, a subject’s cardiopulmonary status, tobacco use, and so on. Without careful attention to these factors, breath analysis research can fall victim to the cliché “garbage in/garbage out.” Ideally, standard breath collection or breath sampling protocols should be generated for the collection and analysis of single breath samples, for the collection and analysis of endtidal breath samples, and for the analysis of breath samples collected during constant tidal breathing. These guidelines must be developed under the auspices of a consortium of international societies. Once agreement on guidelines has been achieved standard breath sampling and collection protocols must be used to report data for publication in the peer-reviewed literature. Also, guidelines should be generated for methods of breath collection that involve breath holding. The development of these guidelines will allow breath samples collected and analyzed with the same or different analytical chemistry instrumentation in different laboratories to be compared and contrasted. In this same vein, a standard commercial gas mixture should be developed that includes all the major species that are found in exhaled
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Manuscript received November 11, 2008; accepted January 20, 2009. Current version published December 09, 2009. The associate editor coordinating the review of this paper and approving it for publication was Prof. Cristina Davis. The authors are with the Johns Hopkins Medical Institutions, Baltimore, MD 21205 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/JSEN.2009.2035201
human breath. Analysis of this gas mixture should always be included whenever a novel or improved method of breath analysis is submitted for publication. At this time, it is reasonable to propose that the method for the collection of a single breath could be the same as the standard method for on-line breath nitric oxide analysis [1]–[3]. If the method does not involve the use of a real-time monitor for the biomarker, then an end-tidal concentration of the carbon dioxide should also be monitored. In general, when breath is collected under any sampling protocol, mouth pressure, and carbon dioxide should be monitored continuously. The profile of carbon dioxide will define the quality of the breath sample and the variation of mouth pressure with breathing cycle will demonstrate that the subject is maintaining a tight seal at the mouth and is exhaling through the mouthpiece. In addition, volume flow should be monitored continuously during tidal breathing. The composition of the inspiratory air contributes significantly to breath analysis. Many potentially clinically relevant molecules are present in the ambient environment. Currently, there is no consensus for a standard method to allow the background levels to be subtracted. At least part of the reason for this deficiency is the fact that there are no data that define how long it takes for a subject to reach steady state with his or her ambient environment. It has been suggested the lung can be washed out in approximately 4 min if a subject breathes pure air [4]. However, the washout of the entire body may take days or weeks depending upon the identity of the molecule. Similarly, the body may take a significant time to reach steady state with the composition of inspiratory air. At this time, there are few toxicokinetic models that can predict the disposition of molecules present in inspiratory air [5], [6]. Until robust models are generated, analytical data should be treated with caution when a sample of inspiratory air is greater than 25% of the concentrations in breath. This limitation is proposed since the study subject may not be in steady state with his or her environment and the resulting analysis will have a significant error. Guidelines should include a definition for the way the results of breath analysis are expressed. These guidelines will allow intrasubject and intersubject breath analyses to be compared and contrasted. Breath analysis for single breath samples could be expressed in terms of concentration units that are dimensionless (i.e., parts-per-million, etc.) or in terms of moles per unit volume (pmol/l). Alternately, single breath analyses could be normalized to a physiological based parameter such as carbon
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dioxide production (i.e., pmol/ml of CO ) or oxygen consumption (i.e., pmol/ml of O ). Normalization to carbon dioxide or oxygen allows breath analysis data for subjects with widely different body masses to be compared. This latter method of data expression should definitely be used for reporting analysis of breath collected after breath holding [7]. Collection of breath during tidal breathing presents additional problems, since when human subjects are asked to breathe normally they tend to hyperventilate. Hyperventilation will change the distributions of molecules across the alveolar-capillary junction with time. Hyperventilation during breath collection can be prevented by requiring the subject to breathe at a constant defined rate (10 breath/min) and at a constant tidal volume based upon height and body weight ([8]). Breath collected during tidal breathing will provide the average composition of all the breaths sampled. The resulting breath analyses can be normalized to minute ventilation per body mass (pmol/kg.min), normalized to minute ventilation per body surface area (pmol/m min) or normalized to oxygen consumption (pmol/ml of O ) or carbon dioxide production (pmol/ml of CO ). The latter method of normalization is preferred when cross-sectional breath data is compared for subjects with different body mass indices. This method of normalization assumes that there is no ventilation/perfusion mismatch. Finally, when standard techniques for breath collection and analysis and procedures for background correction have been adopted, then it should be possible to generate normal concentration ranges for diagnostic breath biomarkers as a function of gender, age, ethnicity, body mass index, pulmonary function, etc. These ranges will allow limits to be set that identify abnormal concentrations of breath biomarkers. Similarly, it will be possible to set limits for concentrations of breath biomarkers not normally present in breath so that breath biomarkers can be used to diagnose abnormal physiology, tissue injury or disease. Once this basic information on the molecules that have been already identified in breath has been obtained, then pioneering studies can be performed to identify new breath biomarkers of normal and abnormal physiologies. Clinical research in breath analysis must mature. The available literature in breath analysis to date is, almost uniformly, grossly lacking in the basic fundamentals. Quality clinical research, including small pilot studies, requires, at minimum, the calculation and justification of sample size, power, , and error ([9]). There should be an a priori justification for treating outcome variables as continuous versus dichotomous, accompanied by appropriate statistical analysis. Institutional review boards (i.e., local research ethics boards) are duty bound to reject clinical research proposals lacking these elements, and journals should not publish their results. Last, it is notable that most reports in breath analysis contain only a handful of “presumably healthy controls” compared to a handful of subjects who “presumably” have a disease state of interest. Quality clinical research requires precise characterization of study groups and much larger numbers. “Normal”
IEEE SENSORS JOURNAL, VOL. 10, NO. 1, JANUARY 2010
ranges for standard blood tests, for example, are often determined through the analyses of large databases containing thousands or tens of thousands of data points (e.g., [10]). These presumed normal ranges are then routinely challenged over the years by new analyses, expert opinion, and/or new developments. Definitions for normal blood pressure, for example, are routinely reconsidered by expert consensus panels convened by the National Institutes of Health ([11]). In conclusion, clinical breath analysis remains in its infancy, despite the fact that its potential has been recognized since antiquity. Recent advances in instrumentation are inspiring a renaissance in breath analysis. In particular, the wider availability of real-time, portable monitors is enabling breakthrough research that can more easily allow the analysis of large numbers of human subjects. Progress will require dynamic multidisciplinary teams consisting of 1) device makers, 2) breath analysis experts, and 3) clinical researchers and clinicians (including statistical support). Teams fulfilling these roles can anticipate breakthrough success; however, teams lacking any of these essential roles will not be successful. The International Association for Breath Research (IABR) must play a critical role in this task and coordinate its efforts with the European Respiratory Society and the American Thoracic Society.
REFERENCES [1] S. Kharitonov, K. Alving, and P. J. Barnes, “Exhaled and nasal nitric oxide measurements: Recommendations. The European Respiratory Society,” Eur. Respir. J., vol. 10, pp. 1983–1693, 1997. [2] “American Thoracic Society; Recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide in adults and children,” Amer. J. Respir. Crit. Care Med., vol. 160, pp. 2104–2117, 1999. [3] P. E. Silkhoff et al., “ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide,” Amer. J. Respir. Crit. Care Med., vol. 171, pp. 912–924, 2005. [4] T. H. Risby and S. S. Sehnert, “Clinical application of breath biomarkers of oxidative stress status,” Free Rad. Biol. Med., vol. 27, pp. 1182–1192, 1999. [5] J. K. Schubert, W. Miekisch, T. Bireken, K. Geiger, and G. F. E. Noldge-Schomberg, “Impact of inspired substance concentration on the results of breath analysis in mechanically ventilated patients,” Biomarkers, vol. 10, pp. 138–152, 2005. [6] J. D. Pleil, D. Kim, J. D. Prah, D. L. Ashley, and S. M. Rappaport, “The unique value of breath biomarkers for estimating pharmacokinetic rate constants and body burden from environmental exposure,” in Breath Analysis for Clinical Diagnosis and Therapeutic Monitoring, A. Amann and D. Smith, Eds. Singapore: World Scientific, 2005, pp. 347–359. [7] J. K. Furne, J. R. Springfield, S. B. Ho, and M. D. Levitt, “Simplification of the end-alveolar carbon monoxide technique to assess erythrocyte survival,” J. Lab. Clin. Med., vol. 142, pp. 52–7, 2003. [8] K. A. Cope, M. T. Watson, W. M. Foster, S. S. Sehnert, and T. H. Risby, “Effects of ventilation on the collection of exhaled breath in humans,” J. Appl. Physiol., vol. 96, pp. 1371–9, 2004. [9] S. B. Hulley, Designing Clinical Research, Textbook, 3rd ed. New York: Lippincott, Williams and Wilkins, 2007. [10] M. Lazo, E. Selvin, and J. M. Clark, “Brief communication: Clinical implications of short-term variability in liver function test results,” Ann. Intern. Med., vol. 148, pp. 348–52, 2008. [11] [Online]. Available: http://www.nhlbi.nih.gov/guidelines/hypertension/jnc7full.htm
SOLGA AND RISBY: WHAT IS NORMAL BREATH? CHALLENGE AND OPPORTUNITY
Steven F. Solga attended Deep Springs College before graduating Phi Beta Kappa from Cornell University, Ithaca, NY, in 1993. He attended medical school at Duke University, and then spent ten years at Johns Hopkins School of Medicine, Baltimore, MD, where he pursued internal medicine residency and gastroenterology fellowship. He became Assistant Professor of Medicine and Co-Director of Liver Transplantation at Johns Hopkins Hospital. Presently in private practice, he maintains a part time affiliation with the University. He is board certified in internal medicine, gastroenterology, and transplant hepatology. He has extensive interest in breath analysis.
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Terence H. Risby received the Ph.D. degree in chemistry from Imperial College of Science, Technology and Medicine, London, U.K. in 1970. He is currently an Adjunct Professor in the Department of Environmental Health at the Johns Hopkins University Bloomberg School of Public Health, Baltimore, MD. After postdoctoral fellowships at the University of Madrid and the University of North Carolina, he was appointed Assistant Professor of chemistry at the Pennsylvania State University. In 1979, he was appointed Associate Professor of Environmental Health Sciences at Johns Hopkins University and was promoted to Professor in 1987. When he took early retirement December 31, 2003, he was a Professor of toxicological sciences, Professor of pathology, and Professor of international health. He was also a member of the Johns Hopkins Center in Urban Environmental Health, the Johns Hopkins Center for Human Nutrition, and the Division of Clinical Chemistry in the Johns Hopkins Hospital. In his retirement, he has founded MediBreath, LLC, while maintaining a part-time appointment at Johns Hopkins.
