Exhaled nitric oxide and carbon monoxide in respiratory diseases

Breath tests have gained increasing interest in recent years mainly driven by the unmet clinical need to monitor airway diseases and to obtain information on unravelled aspects of respiratory disorders. A prototype of such measurement reaching clinical significance besides its use as a research tool is the measurement of exhaled nitric oxide (NO). It took hardly more than a decade after the discovery that exhaled breath contains NO for this measurement to be approved for clinical practice to monitor anti-inflammatory treatment in asthma. Recent studies demonstrate that using exhaled NO measurement to guide anti-inflammatory treatment in asthma may help clinical decision making. A similarly small molecule present in exhaled breath is carbon monoxide, which is not only a biomarker of cigarette smoking but has also been suggested to reflect ongoing oxidative stress/antioxidant defense. The scope of this review is the exciting field of exhaled monoxides. Since several other biomarkers have also been studied in the exhaled breath this review will provide a brief introduction to them.