Exhaled nitric oxide

In medicine, exhaled nitric oxide (eNO) can be measured in a breath test for asthma or other conditions characterized by airway inflammation. Nitric oxide (NO) is a gaseous molecule produced by certain cell types in an inflammatory response. The fraction of exhaled NO (FE_NO)is a promising biomarker for the diagnosis, follow-up and as a guide to therapy in adults and children with asthma. The breath test has recently become available in many well-equipped hospitals in developed countries, although its exact role remains unclear.

Biology

In humans, nitric oxide is produced from L-arginine by three enzymes called nitric oxide synthases (NOS): inducible (iNOS), endothelial (eNOS), and neuronal (nNOS). The latter two are constantly active in endothelial cells and neurons respectively, whereas iNOS' action can be induced in states like inflammation (for example, by cytokines). In inflammation, several cells use iNOS to produce NO, including eosinophils. As such, eNO has been dubbed an "inflammometer".cite journal |author=Pijnenburg MW, De Jongste JC |title=Exhaled nitric oxide in childhood asthma: a review |journal=Clin. Exp. Allergy |volume=38 |issue=2 |pages=246–59 |year=2008 |month=February |pmid=18076708 |doi=10.1111/j.1365-2222.2007.02897.x |url=http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=0954-7894&date=2008&volume=38&issue=2&spage=246]

Although iNOS is thought to be the main contributor to exhaled NO in asthmatics,cite journal |author=Lane C, Knight D, Burgess S, "et al" |title=Epithelial inducible nitric oxide synthase activity is the major determinant of nitric oxide concentration in exhaled breath |journal=Thorax |volume=59 |issue=9 |pages=757–60 |year=2004 |month=September |pmid=15333851 |pmc=1747143 |doi=10.1136/thx.2003.014894 |url=http://thorax.bmj.com/cgi/pmidlookup?view=long&pmid=15333851] cite journal |author=Brindicci C, Ito K, Barnes PJ, Kharitonov SA |title=Effect of an inducible nitric oxide synthase inhibitor on differential flow-exhaled nitric oxide in asthmatic patients and healthy volunteers |journal=Chest |volume=132 |issue=2 |pages=581–8 |year=2007 |month=August |pmid=17550932 |doi=10.1378/chest.06-3046 |url=http://www.chestjournal.org/cgi/pmidlookup?view=long&pmid=17550932] studies in mice also point to a role for nNOS.cite journal |author=De Sanctis GT, MacLean JA, Hamada K, "et al" |title=Contribution of nitric oxide synthases 1, 2, and 3 to airway hyperresponsiveness and inflammation in a murine model of asthma |journal=J. Exp. Med. |volume=189 |issue=10 |pages=1621–30 |year=1999 |month=May |pmid=10330441 |pmc=2193630 |doi= |url=http://www.jem.org/cgi/pmidlookup?view=long&pmid=10330441] cite journal |author=De Sanctis GT, Mehta S, Kobzik L, "et al" |title=Contribution of type I NOS to expired gas NO and bronchial responsiveness in mice |journal=Am. J. Physiol. |volume=273 |issue=4 Pt 1 |pages=L883–8 |year=1997 |month=October |pmid=9357865 |doi= |url=http://ajplung.physiology.org/cgi/pmidlookup?view=long&pmid=9357865]

It was initially thought that exhaled NO derived mostly from the sinuses, which contain high levels of NO. It has subsequently been shown that the lower airways contribute most of the exhaled NO, and that contamination from the sinuses is minimal.

Medical use

Asthma

Patients with asthma have higher eNO levels than other people. Their levels also rise together with other clinical and laboratory parameters of asthma (for example, the amount of eosinophils in their sputum). In conditions that trigger inflammation such as upper respiratory tract infections or the inhalation of allergens or plicatic acid, eNO levels rise.cite journal |author=Vahlkvist S, Sinding M, Skamstrup K, Bisgaard H |title=Daily home measurements of exhaled nitric oxide in asthmatic children during natural birch pollen exposure |journal=J. Allergy Clin. Immunol. |volume=117 |issue=6 |pages=1272–6 |year=2006 |month=June |pmid=16750986 |doi=10.1016/j.jaci.2006.03.018 |url=http://linkinghub.elsevier.com/retrieve/pii/S0091-6749(06)00659-2] cite journal |author=Obata H, Dittrick M, Chan H, Chan-Yeung M |title=Sputum eosinophils and exhaled nitric oxide during late asthmatic reaction in patients with western red cedar asthma |journal=Eur. Respir. J. |volume=13 |issue=3 |pages=489–95 |year=1999 |month=March |pmid=10232414 |doi= |url=http://erj.ersjournals.com/cgi/pmidlookup?view=long&pmid=10232414] The eNO levels also tend to vary according to the results of lung function test results such as the degree of bronchial hyperresponsiveness. Furthermore, drugs used to treat asthma (such as inhaled glucocorticoids or leukotriene receptor antagonists) also reduce eNO levels.

Clinical trials have looked at whether tailoring asthma therapy based on eNO values is better than conventional care, in which therapy is gauged by symptoms and the results of lung function tests.cite journal |author=Smith AD, Cowan JO, Brassett KP, Herbison GP, Taylor DR |title=Use of exhaled nitric oxide measurements to guide treatment in chronic asthma |journal=N. Engl. J. Med. |volume=352 |issue=21 |pages=2163–73 |year=2005 |month=May |pmid=15914548 |doi=10.1056/NEJMoa043596 |url=http://content.nejm.org/cgi/pmidlookup?view=short&pmid=15914548&promo=ONFLNS19] cite journal |author=Shaw DE, Berry MA, Thomas M, "et al" |title=The use of exhaled nitric oxide to guide asthma management: a randomized controlled trial |journal=Am. J. Respir. Crit. Care Med. |volume=176 |issue=3 |pages=231–7 |year=2007 |month=August |pmid=17496226 |doi=10.1164/rccm.200610-1427OC |url=http://ajrccm.atsjournals.org/cgi/pmidlookup?view=long&pmid=17496226] cite journal |author=Szefler SJ, Mitchell H, Sorkness CA, "et al" |title=Management of asthma based on exhaled nitric oxide in addition to guideline-based treatment for inner-city adolescents and young adults: a randomised controlled trial |journal=Lancet |volume=372 |issue=9643 |pages=1065–72 |year=2008 |month=September |pmid=18805335 |doi=10.1016/S0140-6736(08)61448-8 |url=http://linkinghub.elsevier.com/retrieve/pii/S0140-6736(08)61448-8] To date, the results in both adults and children have been modest and this technique can not be universally recommended.cite journal |author=Petsky HL, Cates CJ, Li AM, Kynaston JA, Turner C, Chang AB |title=Tailored interventions based on exhaled nitric oxide versus clinical symptoms for asthma in children and adults |journal=Cochrane Database Syst Rev |volume= |issue=2 |pages=CD006340 |year=2008 |pmid=18425949 |doi=10.1002/14651858.CD006340.pub2 |url=http://dx.doi.org/10.1002/14651858.CD006340.pub2] It has also been noted that factors other than inflammation can increase eNO levels, for example airway acidity.cite journal |author=Hunt JF, Fang K, Malik R, "et al" |title=Endogenous airway acidification. Implications for asthma pathophysiology |journal=Am. J. Respir. Crit. Care Med. |volume=161 |issue=3 Pt 1 |pages=694–9 |year=2000 |month=March |pmid=10712309 |doi= |url=http://ajrccm.atsjournals.org/cgi/pmidlookup?view=long&pmid=10712309] cite journal |author=Shin HW, Shelley DA, Henderson EM, Fitzpatrick A, Gaston B, George SC |title=Airway nitric oxide release is reduced after PBS inhalation in asthma |journal=J. Appl. Physiol. |volume=102 |issue=3 |pages=1028–33 |year=2007 |month=March |pmid=17110506 |doi=10.1152/japplphysiol.01012.2006 |url=http://jap.physiology.org/cgi/pmidlookup?view=long&pmid=17110506]

The fraction of eNO has been found to be a better test to identify asthmatics than basic lung function testing (for airway obstruction). Its specificity is comparable to bronchial challenge testing, although less sensitive.cite journal |author=Malmberg LP, Pelkonen AS, Haahtela T, Turpeinen M |title=Exhaled nitric oxide rather than lung function distinguishes preschool children with probable asthma |journal=Thorax |volume=58 |issue=6 |pages=494–9 |year=2003 |month=June |pmid=12775859 |pmc=1746693 |doi= |url=http://thorax.bmj.com/cgi/pmidlookup?view=long&pmid=12775859] cite journal |author=Miedinger D, Chhajed PN, Tamm M, Stolz D, Surber C, Leuppi JD |title=Diagnostic tests for asthma in firefighters |journal=Chest |volume=131 |issue=6 |pages=1760–7 |year=2007 |month=June |pmid=17400683 |doi=10.1378/chest.06-2218 |url=http://www.chestjournal.org/cgi/pmidlookup?view=long&pmid=17400683] This means that eNO might not be as useful to rule in a diagnosis of asthma as it might be useful to rule it out.

Other conditions

The role for eNO in other conditions is even less well established compared to asthma.

Since asthma can be a cause of chronic coughing (it may even be the sole manifestation, such as in cough-variant asthma), studies have looked at whether eNO can be used in the diagnosis of chronic cough.cite journal |author=Fujimura M, Ohkura N, Abo M, "et al" |title=Exhaled nitric oxide levels in patients with atopic cough and cough variant asthma |journal=Respirology |volume=13 |issue=3 |pages=359–64 |year=2008 |month=May |pmid=18399857 |doi=10.1111/j.1440-1843.2008.01273.x |url=http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=1323-7799&date=2008&volume=13&issue=3&spage=359] cite journal |author=Oh MJ, Lee JY, Lee BJ, Choi DC |title=Exhaled Nitric Oxide Measurement is Useful for the Exclusion of Non-asthmatic Eosinophilic Bronchitis in Chronic Cough Patients |journal=Chest |volume= |issue= |pages= |year=2008 |month=June |pmid=18583518 |doi=10.1378/chest.07-2541 |url=http://www.chestjournal.org/cgi/pmidlookup?view=long&pmid=18583518] cite journal |author=Sato S, Saito J, Sato Y, "et al" |title=Clinical usefulness of fractional exhaled nitric oxide for diagnosing prolonged cough |journal=Respir Med |volume= |issue= |pages= |year=2008 |month=July |pmid=18614345 |doi=10.1016/j.rmed.2008.04.018 |url=http://linkinghub.elsevier.com/retrieve/pii/S0954-6111(08)00166-2] cite journal |author=Hahn PY, Morgenthaler TY, Lim KG |title=Use of exhaled nitric oxide in predicting response to inhaled corticosteroids for chronic cough |journal=Mayo Clin. Proc. |volume=82 |issue=11 |pages=1350–5 |year=2007 |month=November |pmid=17976354 |doi= |url=http://www.mayoclinicproceedings.com/Abstract.asp?AID=4509&Abst=Abstract&UID=]

Exhaled NO is minimally increased in chronic obstructive pulmonary disease, but levels may rise in sudden worsenings of the disease (acute exacerbations) or disease progression. Early findings indicate a possible role for eNO in predicting the response to inhaled glucocorticoids and the degree of airway obstruction reversibility.

Children with cystic fibrosis have been found to have low eNO levels. In subjects with bronchiectasis (a state of localized, irreversible dilatation of part of the bronchial tree) not due to cystic fibrosis, high levels have been found. Sarcoidosis could also feature increased eNO. Low levels have been found in primary ciliary dyskinesia, bronchopulmonary dysplasia, and pulmonary arterial hypertension. In the latter condition, "inhaled" NO is used as a diagnostic test of the response of the pulmonary arteries to vasodilators (agents that relax the blood vessels).

Measurement techniques

The most widely used technique to measure eNO is with a chemical reaction that produces light; this is called a chemiluminescence reaction. The NO in the breath sample reacts with ozone to form nitrogen dioxide in an excited state. When this returns back to its ground state, it emits light in quantities that are proportional to the amount of exhaled NO.

The subject can exhale directly into a measurement device ('online' technique), or into a reservoir that can afterwards be connected to the analyser ('offline' technique).cite journal |author= |title=ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide, 2005 |journal=Am. J. Respir. Crit. Care Med. |volume=171 |issue=8 |pages=912–30 |year=2005 |month=April |pmid=15817806 |doi=10.1164/rccm.200406-710ST |url=http://ajrccm.atsjournals.org/cgi/pmidlookup?view=long&pmid=15817806] With the former technique, the early and later NO in the breath sample can be analysed separately. The test requires little coordination from the subject, and children older than 4 can be tested successfully.cite journal |author=Napier E, Turner SW |title=Methodological issues related to exhaled nitric oxide measurement in children aged four to six years |journal=Pediatr. Pulmonol. |volume=40 |issue=2 |pages=97–104 |year=2005 |month=August |pmid=15965893 |doi=10.1002/ppul.20249 |url=http://dx.doi.org/10.1002/ppul.20249]

Reference range

The upper normal level of eNO in different studies ranges from 20 to 30 parts per billion. However, several major features influence the reference values. Men have higher eNO values than women. Smoking notoriously lowers eNO values, and even former smoking status can influence results. The levels are higher in people with an atopic constitution (a tendency towards allergies).cite journal |author=Travers J, Marsh S, Aldington S, "et al" |title=Reference ranges for exhaled nitric oxide derived from a random community survey of adults |journal=Am. J. Respir. Crit. Care Med. |volume=176 |issue=3 |pages=238–42 |year=2007 |month=August |pmid=17478616 |doi=10.1164/rccm.200609-1346OC |url=http://ajrccm.atsjournals.org/cgi/pmidlookup?view=long&pmid=17478616] The fraction of eNO is also flow-dependent (higher at lower flow rates and vice versa), so measurements are normally measured at 50 ml/s. Age or height could also considerably confound eNO values in children.cite journal |author=Buchvald F, Baraldi E, Carraro S, "et al" |title=Measurements of exhaled nitric oxide in healthy subjects age 4 to 17 years |journal=J. Allergy Clin. Immunol. |volume=115 |issue=6 |pages=1130–6 |year=2005 |month=June |pmid=15940124 |doi=10.1016/j.jaci.2005.03.020 |url=http://linkinghub.elsevier.com/retrieve/pii/S0091674905005920] The magnitude of these effects lies in the order of 10%, so even single cut-off values might be useful.cite journal |author=Turner S |title=Exhaled nitric oxide in the diagnosis and management of asthma |journal=Curr Opin Allergy Clin Immunol |volume=8 |issue=1 |pages=70–6 |year=2008 |month=February |pmid=18188021 |doi=10.1097/ACI.0b013e3282f3b4b0 |url=http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?an=00130832-200802000-00013]

History

Until the 1980s, nitric oxide, a product of fossil fuel combustion, was thought only to play a role the detrimental effects of air pollution on the respiratory tract. In 1987, experiments with coronary arteries showed that nitric oxide was the long sought endothelium-derived relaxing factor. After scientists realised that NO played a biological role, its role as a cell signalling molecule and neurotransmitter became clear from abundant studies.cite journal |author=Turner S |title=The role of exhaled nitric oxide in the diagnosis, management and treatment of asthma |journal=Mini Rev Med Chem |volume=7 |issue=5 |pages=539–42 |year=2007 |month=May |pmid=17504190 |doi= |url=]

NO was first detected in exhaled breath samples in 1991.cite journal |author=Gustafsson LE, Leone AM, Persson MG, Wiklund NP, Moncada S |title=Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans |journal=Biochem. Biophys. Res. Commun. |volume=181 |issue=2 |pages=852–7 |year=1991 |month=December |pmid=1721811 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/0006-291X(91)91268-H] In 1992, NO was voted molecule of the year by the scientific journal "Science".cite journal |author=Koshland DE |title=The molecule of the year |journal=Science (journal) |volume=258 |issue=5090 |pages=1861 |year=1992 |month=December |pmid=1470903 |doi= |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=1470903] In 1993, researchers from the Karolinska Institute in Sweden were the first to report increased eNO in asthmatics.cite journal |author=Alving K, Weitzberg E, Lundberg JM |title=Increased amount of nitric oxide in exhaled air of asthmatics |journal=Eur. Respir. J. |volume=6 |issue=9 |pages=1368–70 |year=1993 |month=October |pmid=7507065 |doi= |url=]

Today, NO is not only used in breath tests but also as a therapeutic agent for conditions such as pulmonary arterial hypertension and possibly for the acute respiratory distress syndrome.

Footnotes

Further reading

*cite journal |author=Taylor DR, Pijnenburg MW, Smith AD, De Jongste JC |title=Exhaled nitric oxide measurements: clinical application and interpretation |journal=Thorax |volume=61 |issue=9 |pages=817–27 |year=2006 |month=September |pmid=16936238 |doi=10.1136/thx.2005.056093 |url=http://thorax.bmj.com/cgi/pmidlookup?view=long&pmid=16936238