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pressure over the likely range encountered (such as the lower three pressure settings that I
tested) has no relevant effects on the measurements in most individuals. Conespondingly
there was no difference in NO, CO2, flow or duration of exhalation across these readings.
However as two, possibly three, individuals did show an effect, this again suggested the need
also to standardise mouth pressure for exhaled NO measurements.
The background NO concentration is variable and occasionally can be extremely high. In
many of the published studies, the ambient NO level at the time of testing was not stated and I
felt this may be another reason for error in the results obtained between and within groups.
This could occur particularly if testing was done on different days pre and post an intervention
with a subject group when the background levels may differ over time. In three subjects, I
showed that there were large differences in the mean exhaled NO concentration obtained
depending on the concentration of the NO being inhaled. In this experiment, the exhaled NO
measured was higher when inhaling low reservoir NO than when inhaling high ambient
levels. Three studies documented inhalation from NO-free air (Alving, Weitzberg et al. 1993;
Schilling, Holzer et al. 1994; Steerenberg, Nierkens et al. 2000) with one having noted that
the ambient air contained high and variable amounts of NO concentration (6-192ppb) leading
them to prepare and recommend an NO free reservoir for inhalation (Schilling, Holzer et al.
L994). Another study mentioned that the ambient NO air was recorded and the absolute zero
was adjusted just before each measurement by flushing the NO analyser with NO free
certified air (Kharitonov, Logan-sinclair et al. L994). A number of studies recorded levels of
ambient air at 0-20ppb (Persson, Wiklund et al. 1993), 5-20ppb (Kharitonov, Yates et al.
Lgg6), 0-38ppb (Kharitonov, Yates et al. 1994; Jilma, Kastner et al. 1996; Kimberly,
Nejadnik et al. 1996) and 0-68ppb (Kharitonov, Robbins et al. 1995) but did not seem to
affect the readings and therefore were not taken into account. One goup had specifically
requested two volunteers to inhale an NO concentration of 800ppb, hold their breath for
fifteen seconds and then exhale into the NO analyser. As they documented no change in the
NO concentration before and after inhalation, they concluded that the inspired NO must
disappear from the respiratory tract within that time (Kharitonov, Yates et al. 1994).
While the effect of ambient NO was either not specifically tested or was not published at this
time by any of these other groups, some researchers did investigate whether inhalation via the
nose or the mouth made a difference to the exhaled NO levels measured. This was an area of
variability, now known to be the degree of contamination of exhaled NO by nasal and sinus
production. The measured NO levels have been found to be greatest in the paranasal sinuses
(Lundberg, Farkas-Szallasi et al. 1995) with an age-dependent increase in keeping with sinus
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