The Science Journal of the American Association for Respiratory Care

2011 OPEN FORUM Abstracts


Joseph J. Bolton1, Richard Lin2, James McCarrick1; 1Respiratory Care, Children's Hospital of Philadelphia, Philadelphia, PA; 2Anesthesiology and Critical Care Medicine, Children's Hospital of Philaddelphia, Philadelphia, PA

1. Background: For home care ventilators such as the Pulmonetics LTV, supplemental O2 can be provided using a low pressure source with a fixed flow rate. It is commonly understood that the effective FiO2 is mainly dependent on th "input" O2 flow rate relative to the patient's minute ventilation. Operator manual nomograms help estimate the relationship. We wanted characterize this relationship more precisely and in this process, discovered that other variables also play a role. 2. Method: We used an LTV 1200 ventilator in volume mode connected to a test lung with an O2 analyzer on the inspiratory limb. Input O2 flows were varied between 0 and 16 l.p.m., with minute volumes from 1.5 to 9.0 l.p.m., and inspiratory times (IT) from 0.3 to 0.6 seconds. Tidal volumes (TV) varied from 50 to 400 mL and constrained IT. Measurements of FiO2 were made with the O2 conserve feature both on and off (bias flow is shut off with O2 conserve turned on). 3. Results: The relationship between the fixed liter flow and FiO2 with O2 conserve off is similar to that found in the operator's manual. However the relationship changes dramatically when O2 conserve is on. For minute volumes ranging from 0.75 to 9.0 l.p.m., using O2 conserve, the input O2 flows required to achieve an FiO2 of 0.5 were 0.5 to 6.0 l.p.m. With O2 conserve off, the required O2 input flows ranged from 4.0 to 7.0 l.p.m. Each increase in input O2 flow causes a larger increase in FiO2 when O2 conserve is used. Longer IT results in a slightly higher FiO2. For a given minute ventilation with O2 conserve off, a smaller tidal volume results in a slightly higher FiO2. With O2 conserve on, the relationship between input O2 flow and FiO2 is more complex. 4. Conclusions: We speculate that with O2 conserve off, the LTV bias flow of 10 l.p.m. was a significant factor influencing FiO2 especially for minute volumes less than 10 l.p.m. With O2 conserve on, changes in fixed liter flow had a much more dramatic effect on FiO2. The increase in FiO2 with increased IT or smaller tidal volumes is most likely due to the lower flow rate required to achieve the prescribed tidal volume.
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