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When planning the protocol, it was obvious that the response time of the NO analyser limited
the possible respiratory manoeuvres. I next considered online measurement of repeated single
exhalations analogous to lung function testing versus exhaling into a reservoir that was then
fed into the analysers. I briefly explored the use of a reservoir, as this had three theoretical
advantages:
o it would allow the measurement of a whole exhaled breath, whereas the online
measurement which would control flow may mean that at low flows the subject would
not reach complete exhalation before requiring to take another breath,
o it would allow me as operator to put the collected sample through the machine at a
constant flow instead of requiring the individual subject's voluntary but imperfect
control,
o performing tidal breathing over a set time or exhaling into a bag may have been easier
when it came time to measure the children rather than relying on their being able to do
controlled breaths into the analysers.
At the time that I was commencing the experiments, there had been some studies into NO
levels in exhaled air where a reservoir system had been used. The problems that had been
detected and needed to be overcome involved No instability -
a topic that has been touched
on repeatedly. Firstly, although in O2-free and haemoglobulin-/ree solutions NO may be
stable, in the presence of 02, NO levels rapidly decrease by 50Vo in 8-20 minutes. Secondly,
NO is adsorbed by most plastics giving low and/or variable readings. NO also interacts with
transition metals and with the sulftrydryl groups contained in rubber. Reservoir systems that
had been used were polyvinyl chloride, rubber and fluoroethylene propylene' Preliminary
studies in one group demonstrated that at levels of 5-30ppb there was no NO loss in the
polyethylene reservoir at 6 and 12 hours post collection, although they commented that any
loss may have been within the noise of the measurement that they were getting at that time of
10Zo (Schilling, Holzer et al. Igg4). Another group had found that using polyethylene tubing
compared to Teflon tubing resulted in loss of NO presumably as it adsorbed to the former,
particularly at higher (>40ppb) levels of NO (Kharitonov, logan-Sinclair et al. 1994). One
group with a Douglas bag collection (an anaesthetic bag used to collect exhaled air, usually
made of rubber) described no perceivable problems, but two other groups found them
unreliable at higher NO concentrations (Kharitonov, logan-Sinclair et al. 1994)'
Subsequently Mylar foil bags or balloons were used and seem to hold NO levels stable for at
least 48 hours (Steerenberg, Nierkens et al. 2000). However, this research group also
discarded the first part of exhalation to get lower respiratory tract samples and found that it
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