Methods for evaluation of helium/oxygen delivery through non-rebreather facemasks

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.

Comparison of pulsed versus continuous oxygen delivery using realistic adult nasal airway replicas

“… 6 , 7 , 17 , 18 These realistic airway replicas, derived from medical imaging, are more representative of human upper airways compared with highly simplified geometries that have been previously used for evaluating inhaled concentrations of medical gases. 16 , 19 Testing with multiple replicas can aid in accounting for variability in the airway geometry of different patients; that is, intersubject variability. 17 In addition, benchtop experiments permit precise control of simulated breathing patterns, eliminating a significant source of variance that may be present during in vivo human studies.…”

Pulmonary Delivery of Therapeutic and Diagnostic Gases

“…While the use of realistic, and idealized, upper airway replicas has been frequent in research related to pharmaceutical aerosol administration [e.g., Byron et al (53) , Grgic et al (54) , and Golshahi et al (55) ], previously, only highly simplified geometries have been used for evaluating delivery of medical gases. (56,57) For pulsed gas delivery, the use of realistic airway replicas allows in vitro testing to be conducted under more realistic triggering conditions. Chen et al observed instances of failure to trigger pulse delivery for four of the fifteen airway replicas they employed.…”

“…Similar to methods used in medical aerosol research, realistic or simplified upper airway geometries have been employed to evaluate the influence of gas delivery parameters on inhaled gas concentrations. (56,57) Analytical models have also been developed to estimate medical gas transport and uptake in the lungs. (80,81) Such tools could be valuable in the design of therapeutic gas delivery systems, and in evaluating contrasting modes of delivery.…”

High-Flow and Low-Flow Oxygen Delivery by Nasal Cannula Evaluated in Infant and Adult Airway Replicas

“…[21][22][23][38][39][40] Based on airway geometries segmented from medical images, these airway replicas are more closely representative of human upper airways than are highly simplified geometries previously used for evaluating inhaled concentrations of medical gases. 41 Most recently, Sabz et al 30 conducted an in vitro investigation to evaluate variation in F DO 2 that resulted from lowflow oxygen delivery through nasal cannulas to realistic nasal airway replicas derived from computed tomographies of infants < 3 months old. Tidal breathing through the airway replicas was controlled by using a lung simulator.…”