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subjects with respiratory disease, is to cough and an equally common response is to give the
person water to drink. This may be important if the levels of NO were then reduced. Exhaled
NO measured in the standard way did decrease significantly from 93.7ppb to 70.8ppb when
60mls of water was drunk between 20 and five seconds before the exhalation, and was not
different whether that water temperature was hot or cold. Many studies did not comment on
consumption of food or drink prior to the testing. One group reported asking the subjects not
to consume caffeine within two hours before the testing procedure (Kharitonov, O'Connor et
al. 1995; Kharitonov, Yates et al. 1996). Another goup did mention having a water absorber
installed in the proximal expiratory port in collections into a reservoir system (Schilling,
Holzer et al. 1994). The mechanism of the fall in the exhaled NO that I documented is
unknown. One group reported in two papers that the gastrointestinal tract had higher levels of
NO present which could be measured when 'belching' occurred (Lundberg, Weitzberg et al.
1994; Herulf, Ljung et al. 1998). The levels cited were far greater than those measured during
exhalation -
so it is possible that the gastro-intestinal tract may contribute to the exhaled NO
levels and was reduced by drinking water. Another possibility is that of quenching of the NO
gas. One study showed that prolonged sampling of wet gas resulted in an increased time for
the initial calibration to reach stable values from five to 20 minutes. They also demonstrated a
reduction in exhaled levels with increased water vapour and concluded that a decrease of NO
readings in water-saturated samples may have contributed to variation across studies (van der
Mark, Kort et al.1997).
In conclusion, several factors critically affect the measured mean peak concentration of
exhaled NO. An increase in expiratory flow rate resulted in a decreased concentration of NO.
Although there was no difference at increasing expiratory mouth pressures for most of the
subjects, individuals showed decreased exhaled NO concentrations and across all subjects
there was a decreased duration of exhalation and COz measurement. A high inspired
background NO made the exhaled NO concentrations difficult to interpret, and drinking water
just prior to measurement decreased the exhaled NO obtained. These findings confirmed that
the measurement of exhaled NO concentrations in humans should be performed in a standard
manner for the levels to have any meaning and to enable different teams of investigators to
compare results. These findings may have accounted for some of the discrepant results
reported in the literature. Since these results were published there has been a considerable
increase in the body of literature on exhaled NO with further studies in all these areas and
these will be covered to the present day in Chapter 9.
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