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Physiological variables also affect the results. Exhaled NO levels have been shown to be
higher in males than females, are reduced with cunent Smoking, recent ingestion of water'
alcohol or caffeine and increased with accidental gut contamination or after nitrogen-
compound rich meals. Levels of NO also increase with age throughout childhood' but not
adulthood, possibly in relation to pneumatisation of the sinuses. Relationships with height,
weight, surface area, menstrual cycle in non-asthmatic women' or a circadian rhythm has not
been demonstrated. However, there remains only one range of 'normal' values given for oral
or nasal NO.
By far the most work in exhaled and nasal NO has been undertaken in atopic and/or asthmatic
subjects with comparison to healthy controls. NO appears to correlate well with the
inflammatory cells, lung function and bronchial hyper-responsiveness in steroid naive
asthmatics and less well in those on IHCS or oral steroids. It does not correlate well with
.severity, of asthma as the NO result here is attenuated by treatment, and it is the requirement
for the higher levels of treatment with possible breakthrough symptoms that determine the
severity category in asthma guidelines(Anonymous 1997 GINA 20O2; GINA 2005; SIGN
2005). There is the potential for No measurements to contribute to diagnosis, management
when measured longitudinally in individuals (on any medication), and to wafii of loss of
Control. NO measurement can also be used as an outcome measure for ant-inflammatory
agents used in asthma treatment. However there are some cautions to be made. Firstly, it does
not appear useful to guide acute asthma treatment. Secondly, measurement of sputum
eosinophils appear as, or more, accurate than NO itself, and the changes in the eosinophil
counts may be a more accurate predictor of loss of control and a better target to tailor
treatment. It is increasingly apparent that, like most parameters we measure' it is likely to be
the pattern from a number of investigations along with an individual's history that best
describes their clinical disease rather than a single marker' The effect of atopy alone on
exhaled NO levels needs to be kept in mind with overlapping results between asthmatic and
atopic subjects. Unfortunately, smoking renders the test inaccurate and unhelpful' In addition'
the presence of high NO levels may be a marker for respiratory diseases that are steroid
sensitive no matter what the underlying disease is labelled.
In exercise, NO drops during single breath measurements, but the NO output is increased
when taking into account the increased rate of breathing. An inability to increase the NO
output during exercise was associated with more severe cardiac disease and, in elite athletes'
worse performance suggesting that the increased NO output may depend on the ability to
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