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the direct to the indirect method led to an increase in flow rate from 440mUmin to 665
mUmin. This may have been enough to account for the different results.
Clearly, at the time of this research, there were major and unexplained differences between
results from different investigators as documented in Table 6.1. These results showed that the
levels of NO measured in exhalate were critically dependent on measurement conditions. This
may be related to expired flow rates but other factors may be important. It has been suggested
that orally-exhaled NO concentrations correlate with the inhaled ambient concentrations of
NO @otsch, Demirakca et al. 1996), although we found in a subsequent experiment a high
ambient NO rendered exhaled NO levels difficult to interpret @yrnes, Dinarevic et al. 1997).
Our current study was only undertaken if the ambient NO levels were low -
while we aimed
for less than l0ppb, on one occasion by the end of the session the ambient NO had drifted up
to llppb hence the range given from l-llppb in the results. As mentioned the greater NO
levels and wider variations seen in females may be due to possible effects of the menstrual
cycle (Kharitonov, Iogan-Sinclair et al. 1994) and this was not controlled for in the present
study. However, as the measurements were done sequentially on each subject on the same
day, conclusions as to the origin of NO from this study should not be affected.
As increasing research on NO in exhaled air was being undertaken in subjects in health and
different respiratory and vascular diseases at the time, it was increasingly important to
understand where and at what level NO was being generated so that abnormal results could be
properly interpreted.
NB: The results of this researchformed the basis of the following publications:
. Byrnes CA, Dinareyic S, Bzsst CA, Bush A, Shinebournes EA. Is nitric oxide produced at
airway or alveolar level? European Respiratory Joumal 1997 10 (5) 1021-1025
. Byrnes CA, Bush A and Shineboume EA "Methods to Measure Expiratory Nitric Oxide in
People" in "A Volume of Methods in Enrymology" edited by Lcster Packeter Academic
Press Inc. 1996.
t52