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washout characteristics were noted to vary from breath to breath (Hall, Reinmann et al.2002)'
This group then divided the exhalation to assess whether one part of the expiratory signal
exhibited less intra-subject variability and therefore could potentially be the most
discriminatory. Firstly, based on the exhalation pattern of COz, they looked at the NO levels
which corresponded to phase one (from the convective airways), phase two (progressive
washout of the airways with alveolar gas) and phase three (alveolar compartment when
exhaled COz is high). Secondly, they divided the tidal volume into four quarters of exhalation
time. The most variable periods were phase one, and the first and fourth quarters'
Correspondingly, the second and third quarter time periods had about the same variability as
phase two, which was the best, followed by phase three. The group concluded that in the
absence of COz measurement as a guide, the reported result should come from the second and
third quarter of the exhalation (Hall, Reinmann et al' 2002)'
Similar to studies in other age groups, flow dependency of exhaled NO levels have been
demonstrated in infants by both methods. This was demonstrated in the original single breath
study comparing a second flow of 100mls/s as well as the standard of 5omls/s in three
subjects showing a significant difference in NO results (Wildhaber, Hall et al' 1999)'
Similarly, low to higher NO levels were obtained at expiratory flows of 50, 25 and l5mls/s in
five full term healthy infants (Martinez, Weist et al. 2003). Breath-by-breath analysis for tidal
breathing also showed that higher expiratory flow rates were associated with lower exhaled
NO levels (Franklin, Turner et a:.2004). In a large prospective healthy birth cohort of 98
infants the tidal breathing parameters of flow, breathing rate and expiratory time, measured at
age one month, all led to varying exhaled NO results. This group also came to the conclusion
that the third expiratory quartile of exhaled NO during tidal breathing was the most
reproducible (Frey, Kuehni et al.2004). In a direct comparison in 7l infants, the peak flow
was 128mls/s during tidal breathing and llmls/s during single breath measurements
(Franklin, Turner et al. 2004). This may also explain why single breath results versus tidal
breathing results usually have higher absolute exhaled No levels.
Other parameters have been seen to affect NO levels in infancy. Measurements in twelve
infants showed that ambient NO concentrations as low as 5-10 ppb compared to the inhalation
of NO free air resulted in an increase exhaled level (Franklin, Turner et al. 2004).In the 98
infants at one month, a gender difference, with boys having higher levels at 17 '7ppb than girls
at 14.6ppb, was noted. This remained significant after adjusting for weight and minute
ventilation (Frey, Kuehni et al.2004). In thirteen preterm infants the exhaled NO was almost
absent and gradually increased over the first 48 hours, while in eleven term infants there was a
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