ff ect of measurement conditions on measured levels of peak exhaled nitric oxide
and these may reflect airway inflammation. It Background – It is possible to measure has been shown that, when compared with nitric oxide (NO) levels in exhaled air. The normal subjects, the mean levels of NO in absolute concentrations of exhaled NO obexhaled air are significantly increased in asthtained by separate workers in similar matic subjects who are on bronchodilator therpatient groups and normal subjects with apy as their only treatment. 5–7 Exhaled NO apparently similar techniques have been decreases significantly in asthmatic subjects on very different. A study was undertaken to regular inhaled topical corticosteroids when determine whether changes in measurecompared with those on bronchodilator therapy ment conditions alter the concentration of only. Nitric oxide decreases with regular ciexhaled NO. garette smoking, 8 9 with ingestion of alcohol, Method – NO concentrations measured by and may be decreased in subjects with primary a chemiluminescence analyser (Dasibi ciliary dyskinesia. Levels have also been Environmental Corporation) and carbon shown to increase with viral upper respiratory dioxide (CO2) measured by a Morgan tract infections. There is a variation between capnograph were analysed in single results seen in men and women with an inexhalations from total lung capacity in fluence of time of the menstrual cycle affecting healthy volunteers (mean age 35.9 years). the levels obtained in the latter. Ten subjects performed five exhalations at However, the absolute mean concentrations four different expiratory flow rates, at four of exhaled NO obtained by separate workers different expiratory mouth pressures, and in similar patient groups and normal subjects before and after drinking hot (n=5) or using apparently similar techniques have been cold (n=5) water. Three subjects perquite disparate. In part this could be explained formed five exhalations on a day of high by the different sites of measurement and the background NO (mean NO level 134 ppb) degree of contamination of the exhaled air by before and after a set of five exhalations the nasal and sinus passages. By comparing made while both the subject and analysers the concentrations of NO exhaled during tidal were sampling from a low NO/NO-free breathing through either nose or mouth, Alving reservoir system. et al 6 suggested that the major contribution Results – The mean peak concentration of came from the nasal space with a minor adNO decreased by 35 ppb (95% CI 25.7 to dition from the lower airways. Lundberg et al 43.4) from a mean (SE) of 79.0 (15.5) ppb demonstrated a decreasing concentration of at an expiratory flow rate of 250 ml/min to exhaled NO when sampling progressively down 54.1 (10.7) ppb at 1100 ml/min. The mean the respiratory tract at the nose, mouth and, peak concentration of NO did not change in four tracheotomised patients, below the vocal significantly with change in mouth prescords. By isolating the nasal passages from sure. The mean (SE) peak NO conthe rest of the respiratory tract with voluntary centration decreased from 94.4 (20.8) ppb closure of the soft palate Kimberly et al to 70.8 (16.5) ppb (p=0.002, 95% CI 12.9 showed that the release of NO in the nasal to 33.1) with water consumption. The passages was approximately seven times greater mean NO concentration with machine and than the rest of the respiratory tract. Higher subject sampling from the low NO reslevels have now been demonstrated in the paraervoir was 123.1 (19.4) ppb, an increase nasal sinuses with an age dependent increase from results obtained before (81.9 in keeping with sinus pneumatisation. Gerlach (10.2) ppb, p=0.001, 95% CI −19.9 to et al measured NO concentrations in the nasoPaediatric −62.7) and after (94.2 (18.3) ppb, p=0.017, Department, Royal pharynx and trachea and found higher levels 95% CI 6.0 to 51.8) sampling with high Brompton Hospital, during inspiration than expiration, which sugSydney Street, London ambient NO. gests that there may be absorption of NO by SW3 6NP, UK Conclusions – The measurement of exC A Byrnes the lower respiratory tract but does not exclude haled NO must be performed in a carefully S Dinarevic the possibility of an exhaled component coming C A Busst standardised manner to enable different from pulmonary synthesis. E A Shinebourne teams of investigators to compare results. The variation in NO levels obtained makes A Bush (Thorax 1997;52:697–701) it difficult to compare results presented by Correspondence to: Dr A Bush. different groups. In a preliminary study in nor