Measurement of Nitric Oxide Dynamics in Central and Peripheral Airways

Nitric oxide (NO) is an endogenous signaling molecule produced in low concentrations in many physiological processes, while higher concentrations are found in inflammation. NO can be measured in exhaled breath and increased concentrations are found in airway inflammation. Fractional exhaled NO concentration (FENO) is usually measured at exhalation flow rate of 50 ml/s (FENO50). FENO50 measurement is technically standardized and is in clinical use as a marker of airway inflammation, especially in the diagnosis and follow-up of asthma. However, FENO50 is insensitive in detecting inflammation in small airways and lung parenchyma and cannot distinguish alveolar and bronchial contributions to FENO.

Measuring FENO at multiple flow rates (extended FENO or eFENO) enables the calculation of flow-independent NO parameters (alveolar NO concentration (CANO), bronchial flux of NO (JawNO), bronchial mucosal NO concentration (CawNO) and bronchial wall NO diffusion capacity (DawNO)) with different mathematical models. This measurement is usually performed by applying the mathematical method of Tsoukias & George (T&G method) or the Högman & Meriläinen algorithm (HMA), which are both based on the two-compartment model of pulmonary NO dynamics. The major advantage of eFENO over FENO50 is the capability to partition the anatomical source and release mechanism of NO in the lungs, providing more accurate knowledge on NO exchange dynamics in the lower airways. However, the eFENO measurement lacks technical standardization and different methods and protocols are used. This impedes research and development of the method by causing notable interstudy variation. Additionally, further knowledge on the bronchial parameters (CawNO and DawNO) is needed to assess whether they would provide additional value over FENO50 that is already in clinical use.

The purpose of this thesis is to accumulate technical understanding of the eFENO measurement to make suggestions and improvements for future technical standards. This thesis investigated the effects of different analytical methods and flow rates on NO parameter estimation in different patient groups (22 subjects with asthma, 72 with COPD and 73 with asbestos exposure) and 91 healthy adults and 66 healthy children. Repeatability and normal variation of the NO parameters were investigated for the first time in the literature using a sample of 25 healthy adults. Additionally, in 23 children with asthma or asthma-like symptoms, the effect of ICS treatment on NO parameters was investigated, and NO parameters were evaluated at multiple time points after the commencement of ICS treatment for the first time in the current literature.

The eFENO measurement was found to be repeatable, and normal variation in the NO parameters was found to be quite low. JawNO and CANO were distinctly less variable than CawNO and DawNO. A small increase in JawNO was observed from the morning toward the afternoon; thus, scheduling repeated FENO measurements at the same time of the day is suggested until more knowledge is gained. Of the mathematical methods, it is recommended to use nonlinear HMA to accumulate knowledge on CawNO and DawNO. If the interest lies only in CANO, the T&G method is recommended. The advantages of the T&G method are higher feasibility and lower variation in the NO parameters, with the disadvantage of only two NO parameters being solved. Objective evidence was found supporting the current expert opinion that flow rates lower than 100 ml/s should not be used in the T&G method, as lower flow rates increase bias caused by the linear approximation, especially when DawNO is high. CANO was distinctly more prone to bias than JawNO. Additionally, it was found that the reduction in FENO50 after the commencement of ICS treatment seems to be caused by reduced CawNO, while DawNO remains unchanged.

In future guidelines, not only the lowest flow rate but also all of the flow rates to be used should be precisely defined, as different combinations of flow rates seem to provide different results. Once the eFENO measurement becomes standardized, its clinical value can be tested in trials, and ultimately, if useful, it could be taken into clinical use.