An in vitro study of the effects of respiratory circuit setup and parameters on aerosol delivery during mechanical ventilation

IntroductionAerosol therapy is often prescribed concurrently during invasive mechanical ventilation (IMV). This study determines the effects of nebuliser position, circuit humidification source, and most importantly, lung health on the delivery of aerosol in simulated adult and paediatric IMV patients. Furthermore, the influence of closed suction catheters on aerosol delivery is also addressed.MethodsA vibrating mesh nebuliser was used to deliver Albuterol to simulated adult and paediatric IMV patients with differing states of lung health. Four different nebuliser positions and two types of humidification were analysed. Closed suction catheter mounts, a mainstay in IMV therapy, were incorporated into the circuits. The mean ± SD dose of aerosol (%) was assayed from a filter at the distal end of the endotracheal tube.ResultsNebuliser placement and circuit humidification source had no effect on the delivered dose (%) in adults, yet both significantly did in the simulated paediatric patients. The use of closed suction catheter mounts significantly reduced the delivered dose (%) in adults but not in paediatric patients. A simulated healthy lung state generated the largest delivered dose (%), irrespective of nebuliser position in the adult. However, different lung health and nebuliser positions yielded higher delivered doses (%) in paediatrics.ConclusionLung health and respiratory circuit composition significantly affect aerosol delivery in both adult and paediatric IMV patients. Nebuliser placement and respiratory circuit humidification source do not affect the delivered dose in adult but do in paediatric IMV patients

On the thermal and hydrodynamic characteristics of liquid-liquid Taylor flows

Two phase liquid-liquid flows offer significant heat and mass transfer enhancements over single phase flows and, as a result, have found use in numerous emerging technologies employing microfluidics. Such technologies include lab-on-chip devices for chemical and biological diagnostics, and biosensors. Liquid-liquid flows have also shown potential for use in high-heat flux removal systems. Although these flows are found in numerous applications, little is known about the complex fluid mechanics that govern them. Consequently, there is a need for a greater knowledge base to serve as a foundation for future system design and characterisation. This thesis presents a fundamental investigation of the hydrodynamic and thermal characteristics of liquid-liquid slug or Taylor flows confined to minichannel geometries. There were three principal aspects to this thesis, which encompassed the measurement of film thickness, pressure drop and heat transfer in liquid-liquid Taylor flows. Experiments were carried out using a number of different carrier fluids – while maintaining water as the dispersed phase throughout. Dimensionless slug length, Capillary and Reynolds numbers were varied over several orders of magnitude. High speed imaging was used in conjunction with microscopy to measure the mean slug velocity and liquid film thickness. Images of the dispersed slugs revealed that the thickness of the liquid film was not constant along the length of the slug. However, above a threshold dispersed slug length, a region of constant film thickness existed. The thickness of the film was found to be heavily dependent on the Capillary number. Analysis of the experimental data revealed that it fell into two distinct flow regimes: a visco-capillary regime and a visco-inertial regime. A modified Taylor’s Law is proposed for flows in the visco-capillary regime, while a novel correlation – based on the Capillary and Weber numbers – is put forward for flows in the visco-inertial regime. The pressure drop induced by the liquid-liquid flow regimes was measured using a differential pressure transducer, and the results were compared to the most referenced correlations in the literature. Comparisons highlighted a lack of robustness in the liquid-liquid pressure drop correlations. Interpretation of the data using liquid-gas Taylor flow correlations unearthed a threshold viscosity ratio, above which liquid-gas correlations may be used to model the flow. Below this threshold, a modification to an existing correlation is proposed, where the interfacial pressure drop is normalised by the volumetric channel fraction occupied by the carrier phase. A heat transfer facility was designed and commissioned to subject the flow to a constant wall heat flux boundary condition. Local temperature measurements were acquired using a high resolution infrared thermography system. Slug length and film thickness were found to have a significant effect on the local heat transfer rates, with enhancements up to 600% over conventional Poiseuille flow noted. Nusselt number oscillations were observed in the lower Capillary number flows. However, these oscillations damped out as the Capillary number, and hence film thickness, increased. Based on the characteristics identified, a novel correlation is proposed to model the flow in the thermal entrance and fully developed regions. The findings of this thesis are of fundamental and practical relevance for the design of systems and devices incorporating liquid-liquid Taylor flow regimes

Performance characterisation of the Airvo2^TM nebuliser adapter in combination with the Aerogen Solo^TM vibrating mesh nebuliser for in line aerosol therapy during high flow nasal oxygen therapy

High flow oxygen (HFO) therapy is a well-established treatment in respiratory disease. Concurrent aerosol delivery can greatly expediate their recovery. The aim of this work was to complete a comprehensive characterisation of one such HFO therapy system, the Airvo2TM, used in combination with the Aerogen SoloTM vibrating mesh nebuliser. Representative adult, infant, and paediatric head models were connected to a breathing simulator via a collection filter placed at the level of the trachea. A tracheostomy interface and nasal cannulas were used to deliver the aerosol. Cannula size and gas flow rate were varied across the full operating range recommended by the manufacturer. The tracheal and emitted doses were quantified via UV-spectrophotometry. The aerosol droplet diameter at the exit of the nares and tracheal interface was measured via cascade impaction. High gas flow rates resulted in low emitted and tracheal doses (%). Nasal cannula size had no significant effect on the tracheal dose (%) available in infant and paediatric models. Higher gas flow rates resulted in smaller aerosol droplets at the exit of the nares and tracheostomy interface. Gas flow rate was found to be the primary parameter affecting aerosol delivery. Thus, gas flow rates should be kept low and where possible, delivered using larger nasal cannulas to maximise aerosol delivery.</p

In-line aerosol therapy via nasal cannula during adult and paediatric normal, obstructive, and restrictive breathing

High-flow nasal oxygen therapy is being increasingly adopted in intensive and home care settings. The concurrent delivery of aerosolised therapeutics allows for the targeted treatment of respiratory illnesses. This study examined in-line aerosol therapy via a nasal cannula to simulated adult and paediatric models with healthy, obstructive and restrictive lung types. The Aerogen Solo vibrating mesh nebuliser was used in combination with the InspiredTM O2FLO high-flow therapy system. Representative adult and paediatric head models were connected to a breathing simulator, which replicated several different states of lung health. The aerosol delivery was quantified at the tracheal level using UV-spectrophotometry. Testing was performed at a range of supplemental gas flow rates applicable to both models. Positive end-expiratory pressure was measured pre-, during and post-nebulisation. The increases in supplemental gas flow rates resulted in a decrease in aerosol delivery, irrespective of lung health. Large tidal volumes and extended inspiratory phases were associated with the greatest aerosol delivery. Gas flow to inspiratory flow ratios of 0.29-0.5 were found to be optimum for aerosol delivery. To enhance aerosol delivery to patients receiving high-flow nasal oxygen therapy, respiratory therapists should keep supplemental gas-flow rates below the inspiratory flow of the patient.</p

In-line aerosol therapy via nasal cannula during adult and paediatric normal, obstructive, and restrictive breathing

High-flow nasal oxygen therapy is being increasingly adopted in intensive and home care settings. The concurrent delivery of aerosolised therapeutics allows for the targeted treatment of respiratory illnesses. This study examined in-line aerosol therapy via a nasal cannula to simulated adult and paediatric models with healthy, obstructive and restrictive lung types. The Aerogen Solo vibrating mesh nebuliser was used in combination with the InspiredTM O2FLO high-flow therapy system. Representative adult and paediatric head models were connected to a breathing simulator, which replicated several different states of lung health. The aerosol delivery was quantified at the tracheal level using UV-spectrophotometry. Testing was performed at a range of supplemental gas flow rates applicable to both models. Positive end-expiratory pressure was measured pre-, during and post-nebulisation. The increases in supplemental gas flow rates resulted in a decrease in aerosol delivery, irrespective of lung health. Large tidal volumes and extended inspiratory phases were associated with the greatest aerosol delivery. Gas flow to inspiratory flow ratios of 0.29-0.5 were found to be optimum for aerosol delivery. To enhance aerosol delivery to patients receiving high-flow nasal oxygen therapy, respiratory therapists should keep supplemental gas-flow rates below the inspiratory flow of the patient.</p

In vitro evaluation of disposable transport ventilators with combination aerosol therapy

Background The COVID-19 pandemic has highlighted the need for alternative short-term, reliable means to aid in the treatment of patients requiring ventilatory support. Concurrent aerosol drug delivery is often prescribed to such patients. As such, this study examines one such short-term option, the disposable gas-powered transport ventilator to effectively deliver aerosol therapy. Factors such as aerosol generator type, patient breathing pattern, humidification and nebuliser position within the respiratory circuit were also examined.Methods Aerosol drug delivery characterisation was undertaken using two different disposable transport ventilators (DTVs). Two different nebuliser types, a closed circuit vibrating mesh nebuliser (VMN) and an open circuit jet nebuliser (JN), at different locations in a respiratory circuit, proximal and distal to an endotracheal tube (ETT), with and without passive humidification, were evaluated in simulated adult and paediatric patients.Results Placement of a nebuliser proximal to the ETT (VMN: 25.19%–34.15% and JN: 3.14%–8.92%), and the addition of a heat and moisture exchange filter (VMN: 32.37%–40.43% and JN: 5.60%–9.91%) resulted in the largest potential lung dose in the adult patient model. Irrespective of nebuliser position and humidification in the respiratory circuit, use of the VMN resulted in the largest potential lung dose (%). A similar trend was recorded in the paediatric model data, where the largest potential lung dose was recorded with both nebuliser types placed proximal to the ETT (VMN: 8.12%–10.89% and JN: 2.15%–3.82%). However, the addition of a heat and moisture exchange filter had no statistically significant effect on the potential lung dose (%) a paediatric patient would receive (p&gt;&gt;0.05).Conclusions This study demonstrates that transport ventilators, such as DTVs, can be used concurrently with aerosol generators to effectively deliver aerosolised medication in both adult and paediatric patients