Background: Many hospitals are converting to portable medical gas cylinders with an integrated regulator/flow meter as part of a handle. This single unit design is convenient for practitioners because there is no longer a need to change regulators out when a tank runs empty. Lost or broken regulators can become a large cost to a respiratory care department. Having a single, unified unit available allows the respiratory therapist to select a full tank for patient use. It is unclear if the flow delivered remains constant at various stages of fullness. The aim of this study is to determine if cylinder pressure has any effect on delivered flow. Methods: Portable oxygen cylinders with an integrated regulator and flow meter were included. The following information was recorded for each cylinder: tank temperature, ambient temperature and gas flow at the following settings 2, 4, 8, and 15 L/min. Flow was validated using an external Thorpe tube (ResOne, Bentonville, AR) for either 0 – 8 L/min or 0 – 15 L/min. connected to the flow output. Cylinders were classified by pressure; full >1,900 psig, partial 800 to 1,899 psig, and near empty < 800 psig. Results: 415 unique cylinders were examined. All data was tested for normal distribution using the Shapiro-Wilkes test. The Kruskal-Wallis rank sum test was used for all non-parametric testing. Tank pressure did have an effect on the efficiency of the delivered flow at all settings. Flow ranges were within 0.5 L/min for 2 and 4 L/min, 1.25 L/min for 8 L/min, and within 1.5 L/min for 15 L/min P < .01. Conclusions: Resistor type regulators are subject to variation in delivered flow with changes in driving pressure. Care must be taken in considering tank duration and needs of the patient. Devices which are dependent upon set liter flows, e.g., small volume nebulizers, may have their performance affected. Variation in flow output could have an impact on patient needs. SpO2 monitoring should be considered whenever using portable oxygen cylinders.Table 1: Variation in Measured Flow According to Cylinder Fill LevelFigure 1: Effect of cylinder pressure on measured flow
Evans et al. (Tue,) studied this question.