Measurement of pulmonary exhaled nitric oxide: Relationship to exhalation rates and inline versus bag collection techniques

Introduction: Nitric oxide (NO) produced by cells within the respiratory tract, is detectable in exhaled air and plays an important role in the pathophysiology of airway disease. However, values from various laboratories differ widely, reflecting differences in measurement technique. Standardization of measurement techniques is required, if exhaled nitric oxide (ENO) values are to be used clinically and comparison of results from different laboratories is to be possible. We evaluated the effects of exhalation rates, and collection techniques on ENO values. Methods: Ten adolescents (15-18 years old) with normal lung function (FVC >90% predicted Crapo Standard) performed the study. ENO measurements were obtained inline using a Sievers Model 280 Chemiluminescence Analyzer and by collecting exhaled air simultaneously in mylar bags and analyzed within 2 hours. Exhalation was done against an expiratory pressures of >10- cm H2O, to ensure vellum closure and eliminate nasal NO contamination. ENO values at 8 different flow rates (42, 27, 19.3, 11.3, 6.2, 3.2, 1.17, and 0.7 ml/sec) obtained by resistors in the circuit were tested. Flow rates were Data Is expressed as mean ±standard deviation. Differences (p<.05) were sought between bag and inline collections using students t-tests and flow rates using Pearson correlation coefficients. Results: ENO values increased as flow rate decreased (20±1 at 42 ml/sec to 101±16 at 0.7 ml/sec). There were no differences and good correlation between inline and bag values for flow rates of 42 ml/sec (20±1 vs. 21 ±1 p=0.601, r=0.82 p=0.004), 27 ml/sec (24±1 vs. 25±2 p=0.829, r=085 p=0.001) and 19 ml/sec (37±2 vs. 34±1 p=0.083, r=0.97, p=0.000). However, at the lower flow rates, inline and bag ENO values differed significantly (p=0.000 to 0.003) and showed weak correlation (r=.33 to 0.74, p=0.01 to 0.3). Conclusion: Inline and bag collection techniques yield similar results and good correlation at flow rates 19 to 42 ml/sees. These rates seem to provide optimal amplification of the ENO signal with <5% variation in values. ENO values should be interpreted in relationship to exhalation rates and method of collection.