Effective Inspired Oxygen Concentration Measured Via Transtracheal and Oral Gas Analysis

BACKGROUND: The fraction of inspired oxygen (F_IO2) 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 F_IO2 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 F_IO2 was derived from plots of the fractional concentrations of carbon dioxide versus oxygen.

RESULTS: We found excellent correlation between the effective F_IO2 values from tracheal and oral sampling (r = 0.960, P < .001). With 97% oxygen via nasal cannula, effective F_IO2 increased by 2.5% per liter of increased flow (P < .001); effective F_IO2 reached 32.7% at 5 L/min while P_aO2 increased by 12 mm Hg per liter of increased flow. In 4 subjects with a transtracheal catheter, effective F_IO2 increased 5.0% (P < .001) per liter of increased flow, and P_aO2 increased by 13 mm Hg per liter of increased flow, whereas in the same 4 subjects using nasal cannula for oxygen delivery, P_aO2 increased by only 6 mm Hg per liter of increased flow.

CONCLUSIONS: Exhaled gas sampled at the mouth accurately reflected the effective F_IO2 in the trachea. In relation to inspired oxygen flow, the effective F_IO2 was lower than is conventionally thought. Compared to nasal cannula, transtracheal catheter approximately doubled the effective F_IO2 at a given flow rate. Accurate knowledge of F_IO2 should aid clinicians in managing patients with acute and chronic lung diseases.