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Figure 6.11b: COz levels at peak NO and at peak CO2 measured by the t-piece system
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Comparisons of the COz conc€Dtrations in percent of total expired gases at the time of peak NO and
at peak COz in measurements by the direct system (n=12, each mean of 5 exhalations).
6.7 Discussion: The origin of the exhaled nitric oxide
The study demonstrated that NO from a single exhalation could be measured by two methods;
either by direct connection to the NO analyser or by using a t-piece side arm sampling system.
However the results of NO were significantly different with an approximate halving of the
peak levels from the direct to t-piece technique, while there was no difference seen in the COz
results. These methods gave similar individual coefficients of variation as other groups
(Kharitonov, Yates et al. 1994; Monis, Caroll et al. 1995; Jilma, Kastner et al. 1996;
Massaro, Mehta et al, 1996; Hogman, Stromberg et al. 1997). The NO results from the female
subjects showed greater variability, and this has been noted previously as possibly being an
effect of the menstrual cycle in one (Kharitonov, lngan-Sinclair et al. 1994) but not a second
paper (Jilma, Kastner et al. 1996). Our subjects were chosen to minimise any other
confounding factors most notably being non atopic, non-asthmatic and having normal lung
function. As noted in the total group of subjects from which all the studies were done, there
were two smokers (two cigarettes per day) which was not ideal but I believed controlling for
atopy was more important when excluding subjects. As it happened, neither subject took part
in this direct versus t-piece experiment but were involved in later experiments.
Previous researchers had been unable to agree whether NO was of alveolar or airway origin
(Alving, Weitzberg et al. 1993; Borland, Cox et al. 1993; Persson, Wiklund et al. 1993). It is
known that the vast bulk of COz is of alveolar origin, having diffused across the
alveolar:capillary membrane from the pulmonary circulation. If COz and NO were both of
alveolar origin, the time to reach their respective peaks and the ratio of peak NO to COz
would remain the same even if measurement conditions were varied. I thus measured exhaled
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