volunteers and patients with chronic obstructive pulmonary
disease,3 who additionally showed an increase in
exercise capacity. The airway pressure generated is directly
related to the flow administered; tracheal pressure
of up to 5 cm H2O has been recorded with a flow of
50 L/min.3 In the present study, however, airway pressure
was not measured. Moreover, the reported effect
was observed with higher flows than we used in the
majority of patients in this study. Thus, it is unlikely
that a continuous positive airway pressure effect would
have contributed to improved oxygenation in our patients.
Also, the heated humidifier system may indirectly
affect oxygenation. Active humidification improves
mucociliary function, facilitates secretion clearance,
and decreases atelectasis formation, which improves
ventilation-perfusion ratio and oxygenation. However,
in the present study it is unlikely that those factors
could have affected oxygenation after only 30 min. On
the other hand, heated humidification systems may attenuate
the development of bronchial hyper-response
symptoms,4 so humidified HFNC may be particularly
beneficial in patients with respiratory infection, chronic
obstructive pulmonary disease, or bronchial asthma.
The pronounced reduction in respiratory rate in the
present study is also important, particularly because it
was not associated with changes in the PaCO2 or pH. This
finding is consistent with other reported data1,2 and may
have contributed to the improvement in dyspnea and
comfort.