Abstract
BackgroundInhalation of low-density helium/oxygen mixtures has been used both to lower the airway resistance and work of breathing of patients with obstructive lung disease and to transport pharmaceutical aerosols to obstructed lung regions. However, recent clinical investigations have highlighted the potential for entrainment of room air to dilute helium/oxygen mixtures delivered through non-rebreather facemasks, thereby increasing the density of the inhaled gas mixture and limiting intended therapeutic effects. This article describes the development of benchtop methods using face models for evaluating delivery of helium/oxygen mixtures through facemasks.MethodsFour face models were used: a flat plate, a glass head manikin, and two face manikins normally used in life support training. A mechanical test lung and ventilator were employed to simulate spontaneous breathing during delivery of 78/22 %vol helium/oxygen through non-rebreather facemasks. Based on comparison of inhaled helium concentrations with available clinical data, one face model was selected for measurements made during delivery of 78/22 or 65/35 %vol helium/oxygen through three different masks as tidal volume varied between 500 and 750 ml, respiratory rate between 14 and 30 breaths/min, the inspiratory/expiratory ratio between 1/2 and 1/1, and the supply gas flow rate between 4 and 15 l/min. Inhaled helium concentrations were measured both with a thermal conductivity analyzer and using a novel flow resistance method.ResultsFace models borrowed from life support training provided reasonably good agreement with available clinical data. After normalizing for the concentration of helium in the supply gas, no difference was noted in the extent of room air entrainment when delivering 78/22 versus 65/35 %vol helium/oxygen. For a given mask fitted to the face in a reproducible manner, delivered helium concentrations were primarily determined by the ratio of supply gas flow rate to simulated patient minute ventilation, with the inspiratory/expiratory ratio playing a secondary role. However, the functional dependence of helium concentration on these two ratios depended on the mask design.ConclusionsLarge differences in mask performance were identified. With continued refinement, the availability of reliable benchtop methods is expected to assist in the development and selection of patient interfaces for delivery of helium/oxygen and other medical gases.
Highlights
Inhalation of low-density helium/oxygen mixtures has been used both to lower the airway resistance and work of breathing of patients with obstructive lung disease and to transport pharmaceutical aerosols to obstructed lung regions
Inhalation of helium/oxygen (He/O2) mixtures has been explored as a means to lower the airway resistance and work of breathing of patients suffering from obstructive lung disease [1,2,3,4,5,6], and as a carrier gas to transport pharmaceutical aerosols to obstructed lung regions [7,8,9,10,11,12]
Little difference was observed between the average helium concentrations measured for the Laerdal and Simulaids face models, and concentrations measured using either of these models were in reasonable agreement with mean values measured in vivo in healthy adults
Summary
Inhalation of low-density helium/oxygen mixtures has been used both to lower the airway resistance and work of breathing of patients with obstructive lung disease and to transport pharmaceutical aerosols to obstructed lung regions. Inhalation of helium/oxygen (He/O2) mixtures has been explored as a means to lower the airway resistance and work of breathing of patients suffering from obstructive lung disease [1,2,3,4,5,6], and as a carrier gas to transport pharmaceutical aerosols to obstructed lung regions [7,8,9,10,11,12] These effects stem from the physical properties of He/O2 mixtures, in particular their low density compared to air [13,14,15,16]. While face models have been examined for testing aerosol drug delivery through facemasks [19,20], little information is available on the suitability of face models for testing medical gas delivery interfaces
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