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BACKGROUND: The fraction of inspired oxygen (FIO2) is quoted for different oxygen delivery systems, but variations in inspiratory flow and tidal volume make precise measurement difficult. We developed a reliable method of measuring the effective FIO2 in patients receiving supplemental oxygen.
METHODS: Ten subjects with chronic hypoxemia breathed through a mouthpiece with a sampling probe connected to a mass spectrometer. Four of the 10 subjects had transtracheal catheters that allowed direct sampling of tracheal gas. We used oxygen concentrations of 47% and 97%, and flow rates between 1 L/min and 8 L/min. We also compared oxygen delivery via nasal cannula and transtracheal catheter. Effective FIO2 was derived from plots of the fractional concentrations of carbon dioxide versus oxygen.
RESULTS: We found excellent correlation between the effective FIO2 values from tracheal and oral sampling (r = 0.960, P < .001). With 97% oxygen via nasal cannula, effective FIO2 increased by 2.5% per liter of increased flow (P < .001); effective FIO2 reached 32.7% at 5 L/min while PaO2 increased by 12 mm Hg per liter of increased flow. In 4 subjects with a transtracheal catheter, effective FIO2 increased 5.0% (P < .001) per liter of increased flow, and PaO2 increased by 13 mm Hg per liter of increased flow, whereas in the same 4 subjects using nasal cannula for oxygen delivery, PaO2 increased by only 6 mm Hg per liter of increased flow.
CONCLUSIONS: Exhaled gas sampled at the mouth accurately reflected the effective FIO2 in the trachea. In relation to inspired oxygen flow, the effective FIO2 was lower than is conventionally thought. Compared to nasal cannula, transtracheal catheter approximately doubled the effective FIO2 at a given flow rate. Accurate knowledge of FIO2 should aid clinicians in managing patients with acute and chronic lung diseases.
- Correspondence: Christopher B Cooper MD, Exercise Physiology Research Laboratory, David Geffen School of Medicine, University of California, Los Angeles, 10833 Le Conte Avenue, 37-131 CHS, Los Angeles CA 90095-1690. E-mail: .
This study was partly supported by a grant from Oxycare. The authors have disclosed no other conflicts of interest.
- Copyright © 2010 by Daedalus Enterprises Inc.