Impact of Differences Between Minute Ventilation Demand and O₂ Mask Flow on FIO₂ Achieved With Simple and Non-Rebreather O₂ Masks: A Mechanical Simulation Study

Abstract

Background: In an accompanying abstract we reported FIO₂ achieved with a simple O₂ mask (SM) or non-rebreather O₂ mask (NRM) varied according to O₂ flow and level of minute ventilation (V_E). Here we test the premise that air entrainment signified by mismatching of O₂ flows with both peak and mean inspiratory flows (VI-peak and VI-mean) result in lower FIO₂ values.

Methods: As a brief review, our model consisted of 3 V_E levels: normal (6 L/min: V_T 350 mL x 17), ARDS spontaneous breathing (8 L/min: V_T 350 mL x 24), and hyperpnea (14 L/min: V_T 550 mL x 26). O₂ flows were 10, 12, 15, and 20 L/min. Twelve paired measurements each were made between FIO₂ and corresponding measures of: 1) O₂ flow, 2) VI-peak, 3) VI-mean, and 4) differences between O₂ flow and both VI-peak and mean. Paired data were analyzed by linear regression. Alpha was set at 0.05.

Results: Relationships between O₂ flow and FIO₂ differed markedly between SM (R²: 0.48, P = .013) and NRM (R²:0.28, P = .08), that was reflected in how FIO₂ corresponded with differences between O₂ flow and both VI-peak and VI-mean (Table). Using a cut-off difference of ≤ 10 L/min for both devices, 67% of SM pairings had a FIO₂ ≥ 0.60, whereas 100% of NRM pairings had a FIO₂ ≥ 0.70.

Conclusions: With SM the association between FIO₂ and either O₂ flow or mismatches between VI-peak or VI-mean differed little; whereas with NRM the mismatch between VI-peak or mean with O₂ flow had a marked impact on FIO₂. The stronger association with VI-mean reflects the influence of V_T and mismatch duration (ie, VI-mean = V_T ÷ T_I). The weak association between O₂ flow and FIO₂ with NRM (and its strong relationship with flow differences) underscores the importance of an O₂ reservoir in reducing the impact of air entrainment.