Study objectives: The measurement of exhaled nitric oxide (eNO) in asthmatic patients is increasingly being used to aid diagnosis and management. To standardize the measurement techniques, the American Thoracic Society and European Respiratory Society guidelines were published, but these were based mainly on expert opinions without strong clinical evidence on many aspects. We investigated the effect of three different factors on the on-line measurement of eNO. In a clinical setting, we evaluated the effect of prior spirometry, the use of nasal clips, and the influence of submaximal inspiratory effort on on-line eNO readings. Recommended guidelines on these factors have been published but have been supported by scanty research data.
Methods: Three groups of stable asthmatic patients (30 patients in each group) had eNO measurements made on-line (NIOX; Aerocrine AB; Solna, Sweden) before and 5 min after performing spirometric maneuvers, without and with a nasal clip, or with maximal and then submaximal inspiratory efforts.
Results: There were no significant differences in mean eNO levels among all three groups, before and after spirometry (68.2 vs 66.0 parts per billion [ppb], respectively; mean difference, 2.2 ppb; 95% confidence interval [CI], −0.4 to 4.9; p = 0.090), without use of a nasal clip compared with its use (46.7 vs 45.6 ppb, respectively; mean difference, 1.1 ppb; 95% CI, −0.7 to 2.8; p = 0.234), and maximal or submaximal inspiratory effort (52.6 vs 51.2 ppb, respectively; mean difference, 1.4 ppb; 95% CI, −0.3 to 3.0; p = 0.096).
Conclusion: We conclude that on-line eNO measurements in clinical practice are not significantly affected by prior spirometry maneuver, use of a nasal clip, or submaximal inspiratory effort.