Estimation of oxygen concentration in the airway during inspiration and
pre-inspiration with a nasal cannula
Abstract
BACKGROUND: In low flow oxygen therapy, FIO2 is difficult to measure in
spontaneously breathing patients due to room air dilution and dead space
rebreathing, especial in impairments respiratory mechanics. This study
determined oxygen concentrations with different tidal volumes and
respiratory rates among different lung mechanics and provided equations
to estimate oxygen concentrations during standard nasal cannula oxygen
therapy. METHODS: Two Training & Test Lung models were used in this
study. One simulated spontaneous breathing, whereas the other included
an expiratory gas modification bellow. Three lung mechanics [normal
(R5/C60), restrictive (R20/C80) and obstructive (R5/C40)] were
designed, and spontaneous breathing settings for different tidal
volumes(VT) and respiratory rates(f) were simulated by the mechanical
ventilator. The nasal cannula used flows of 1, 3 and 5L/min; peak
inspired oxygen concentration (FO2 insp.) and pre-inspired oxygen
concentration (FO2 pre-insp.) were measured. RESULTS: Increased VT
caused a decreased FO2 insp. and FO2 pre-insp., except at 1L/min oxygen
flow with a high f (30breaths/min). Multiple regression analysis showed
oxygen flow rate, VT and f as the most important factors in predicting
oxygen delivery during nasal cannula therapy. Therefore, we provided
equations to predict oxygen concentration for managing patients with
acute and chronic lung diseases. CONCLUSIONS: Our study suggested that
under low-flow nasal cannula therapy, various lung mechanics and
respiratory patterns in the normal, restrictive and obstructive lung
models will affect the oxygen concentration.