The effect of nasal oxygen flow and catheter position on the accuracy of end-tidal carbon dioxide measurements by a pharyngeal catheter in unintubated, spontaneously breathing subjects.

Reliable recordings of carbon dioxide concentrations during spontaneous respiration can be obtained from a catheter positioned in the hypopharynx. The present study investigated the possible influence on end-tidal carbon dioxide measurement of nasal oxygen administration, position of the sampling catheter and mouth breathing. The study demonstrated that not only can reliable capnographic tracings be obtained from a thin catheter placed in the unintubated airway, but the subject may also receive up to 6 l.min-1 of oxygen via the nasal route without interference with the accuracy of the measurements. Furthermore, the exact position of the sampling catheter tip in the airway is not critical, which means that it may be placed where it causes the patient least discomfort. Finally, mouth breathing caused a mean decrease in end-tidal carbon dioxide of 0.25 kPa compared to nose breathing.