[Measurement of exhaled nitric oxide: methodology]

Abstract

Introduction: The singular relationship between the exhaled nitric oxide (NO) fraction and the expiratory flow rate has both technical (subject of international guidelines) and theoretical (modelling of pulmonary NO exchange) implications.

State of the art: Guidelines recommend the measurement of exhaled NO at a single, defined, expiratory flow rate (V') against a positive expiratory pressure to ensure velum closure, providing a fraction of exhaled NO, FE(NO,V'). With some oversimplifications concerning the relationship between FENO and V', NO exchange parameters independent of the expiratory flow rate can be calculated based on a two-compartment model: maximum conducting airway NO flux (J'awNO), alveolar NO concentration (CalvNO), and in some conditions, airway NO diffusing capacity (DawNO) and epithelial NO concentration of conducting airways (CawNO).

Perspectives: Technical progress has provided the pulmonologist with simple equipment to allow the determination of the NO output from the respiratory tract. The two-compartment model provides the physiologist with a non-invasive technique for evaluating the contribution of alveolar space and conducting airways.

Conclusion: The measurement of exhaled NO allows the non-invasive evaluation of a key mediator involved in the regulation of biological processes.