key: cord-0726736-319mnakn
authors: Dillon, Tom; Ozturk, Caglar; Mendez, Keegan; Rosalia, Luca; Gollob, Samuel Dutra; Kempf, Katharina; Roche, Ellen Tunney
title: Computational Fluidic Modeling of a Low‐Cost Fluidic Oscillator for Conversion of a CPAP Machine into an Emergency Use Mechanical Ventilator
date: 2021-01-23
journal: Adv Nanobiomed Res
DOI: 10.1002/anbr.202000112
sha: 76063d76067d3feedb3d2cf62090cd77bf6aca6a
doc_id: 726736
cord_uid: 319mnakn

This paper presents the computational fluidic modeling of a fluidic oscillator for the conversion of continuous positive airway pressure (CPAP) machines into emergency pressure support mechanical ventilators by providing a periodic pressure output to patients. The design addresses potential ventilator shortages resulting from the ongoing COVID‐19 pandemic, or future pandemics by converting a positive pressure source into a mechanical ventilator with a part that is (i) inexpensive, (ii) easily manufactured without the need for specialized equipment, (iii) simple to assemble and maintain, (iv) does not require any electronics, and (v) has no moving components that could be prone to failure. A Computational Fluid Dynamics (CFD) model is used to assess flow characteristics of the system, and a prototype is developed and tested with a commercial benchtop respiratory stimulator. The simulations show clinically relevant periodic oscillation with outlet pressures in the range of 8‐20 cmH(2)O and end‐user‐tunable frequencies in the range of 3‐6 seconds (respiratory rate (RR) of 10‐20 breaths per minute). The prototype can respond to disrupted oscillations, an analogue for patient‐initiated breaths. The fluidic oscillator presented here functions at physiologically‐relevant pressures and frequencies, demonstrating potential as a low‐cost, readily deployable means for converting CPAP machines into emergency use ventilators. This article is protected by copyright. All rights reserved.

This paper presents the computational fluidic modeling of a fluidic oscillator for the conversion of continuous positive airway pressure (CPAP) machines into emergency pressure support mechanical ventilators by providing a periodic pressure output to patients. The design addresses potential ventilator shortages resulting from the ongoing COVID-19 pandemic, or future pandemics by converting a positive pressure source into a mechanical ventilator with a part that is (i) inexpensive, (ii) easily manufactured without the need for specialized equipment, (iii) simple to assemble and maintain, (iv) does not require any electronics, and (v) has no moving components that could be prone to failure. A Computational Fluid Dynamics (CFD) model is used to assess flow characteristics of the system, and a prototype is developed and tested with a commercial benchtop respiratory stimulator. The simulations show clinically relevant periodic oscillation with outlet pressures in the range of 8-20 cmH2O and end-user-tunable frequencies in the range of 3-6 seconds (respiratory rate (RR) of 10-20 breaths per minute). The prototype can respond to disrupted oscillations, an analogue for patient-initiated breaths. The fluidic oscillator presented here functions at physiologically-relevant pressures and frequencies, demonstrating potential as a low-cost, readily deployable means for converting CPAP machines into emergency use ventilators.

This article is protected by copyright. All rights reserved

The coronavirus disease 2019 (COVID-19) pandemic has placed a tremendous burden on the healthcare system worldwide and still continues to rise with recent new variants being reported and causing dramatic rises in case numbers and hospitalization rates. There is still a very serious concern regarding the insufficient supply of ventilators to support critically ill patients. In the United States, a country that spends 18% of GDP on annual health expendituresmore than twice the average among developed countriesit was estimated that there would be a shortage of at least 45,341 ventilator units during the first peak of the outbreak. [1] Although this shortage did not transpire, cases are currently rising rapidly and there could still be a need for emergency ventilators during this pandemic, or indeed in future pandemics. While vaccines are being rolled out, and multiple measures such as social distancing, mask wearing and restrictions are being implemented, [2] there remains a potential need for more ventilator units. This need is particularly pressing in developing countries where resources are less abundant, vaccines may not be imminently available and ventilator cost is prohibitive; it was reported in May 2020 that there are at least ten countries in Africa that have no ventilators. [3] To meet the demand for emergency ventilators in a timely and cost-efficient manner, fluidic oscillators can be utilized as a means to convert continuous positive airway pressure (CPAP) machines into ventilators. Fluidic oscillators are based on the bi-stable states of a jet of fluid inside a specifically designed flow chamber and can be harnessed to produce self-excited oscillating fluid flow. Combined with a CPAP machine -of which there are millions available in the United States alone -fluidic oscillators can be used to create functional emergency use ventilators without the need for complex moving parts. [4] The A.R.M.E.E (Automatic Respiration Management Exclusively for Emergencies) utilizes one such device, based on a fluidic oscillator developed by the US Army in 1965. [5] However, this device is restricted to a maximum oscillation period of 1 second, which limits the minimum achievable respiratory rate, a crucial metric used by physicians to manage patients on mechanical ventilation. [5] We were motivated to pursue this work following discussions with Vent19 (http://www.vent19.com) who had proposed cVent19 -an emergency use ventilator made from a fluidic oscillator with 3d-printable parts without any special materials or electronic components and a CPAP machine. In this paper we design, model, fabricate and test an alternative fluidic oscillator design, capable of oscillating in the range of 3-6 seconds period (respiratory rate (RR) of 10-20 breaths per minute (bpm)) with output pressures in the range of 11-18 cmH2O. These clinical metrics fall within the guideline recommended ranges for emergency use ventilators to treat patients with COVID-19 respiratory failure. [6] Our design, which affords physicians greater control over respiratory rate compared to currently available fluidic oscillator approaches for mechanical ventilation, can be deployed for low-cost, rapidly manufactured conversion of CPAP machines into emergency use ventilators to meet the pressing demand during this global pandemic and beyond. [5] The overall concept for the CPAP-ventilator conversion is highlighted in Figure 1 . For our design to be clinically applicable, we targeted oscillation periods on the order of 3-6 seconds (RR 10-20 bpm) according to the consensus guidelines for emergency use ventilators. [6] The switching time for an oscillator is related to the characteristic time of the vortex chamber; that is, the time taken to fill the vortex chamber with fluid for a given mass flow rate:

where ∆ is the characteristic time, and ℎ are the diameter and height of the vortex chamber, respectively, is the specific volume of the fluid, and ̇ is the mass flow rate. [7] To improve upon the A.R.M.E.E. ventilator oscillation period of ~1 second, we use a larger vortex chamber (88 mm vs. 22 mm) and a central outlet to keep the vortex captive inside a chamber. [5] The height of the vortex chamber is relatively small (2.4 mm), which serves to maintain adequate pressures at the patient outlet. An input flow rate of 30 L/min was selected based on standard CPAP specifications for treating patients with COVID-19 respiratory failure. [6] Critical dimensions for the oscillator can be found in Figure S1 (Supporting Information).

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The nozzle-diverter region exploits the Coanda effect (the tendency of a fluid to remain attached to walls), so that nozzle flow is not divided between the patient and exhaust outlets throughout the respiratory cycle, but instead oscillates from one to the other. [7, 8] The high-velocity, low-pressure air in the feedback channel (FC) pulls nozzle flow towards the vortex chamber during the inspiration ( Figure 1A) . analysis of the oscillator. A -turbulence model was selected, owing to its capability of capturing adverse pressure gradients and turbulent dissipative effects. [9] The mesh geometry contained more than 170,000 tetrahedral elements. A flow rate of 30 L/min was applied at the CPAP inlet, and the SC and EC outlets were set to atmospheric pressure. A no-slip condition was defined at the oscillator walls. The convergence criterion for the residuals of mass, momentum, and energy equations was set to 10 -4 for each simulation. The physical quantities such as pressure, flow rate, and velocity were also monitored for the convergence. An illustration of both the mesh and boundary conditions utilized can be found in Figure S3 (Supporting Information).

A first-order Windkessel model, which describes the lungs as a resistance element, R, connected in series with a capacitance element, C, was implemented to capture the variation in alveolar pressure over the respiratory cycle. The alveolar and pleural pressures, as well as the PEEP, are strongly coupled with the dynamics of the oscillator, as described in Equation 2: [10] ( )

where is pressure, is time, and are lung compliance and resistance, respectively ( = 30 ml/cmH2O, = 3 cmH2O/L/s, according to clinical data from patients with COVID-19), [11, 12] and is flow, and the subscripts , and denote alveolar, PEEP, and pleural pressures, respectively ( Figure S4 , Supporting Information). An adjustable RR of 10-20 breaths per minute was identified as a key design criterion for the oscillator. [6] A grub screw at the EC (EC, Figure 3A ) was implemented to allow tunability of the RR. The effect of EC screw depth on the oscillation period was studied in the CFD model. The other two set-screws on the FC and SC were utilized to enable adjustments in PEEP and PIP.

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Control of PIP is paramount for pressure-controlled ventilation, as elevated values may increase the risk of mechanically-induced barotrauma. [6, 13] CFD analysis was conducted to determine the effects of the feedback channel and side channel depth on PIP (FC and SC, Figure 3A ).

Patient discomfort can arise when a delay exists between the desire to inhale, and the oscillator's switchover to inspiration. The patient's intention to inhale is detected in modern ventilators by a moderate decrease in alveolar pressure (i.e. a pressure trigger). [6, 14, 15] If the ventilator does not respond to this trigger, the patient will take a breath later than they are comfortable with. The active pressure component contributed by the patient is represented in

by the pleural pressure, . Here, the patient trigger is modeled as a forced step function of magnitude -3 cmH2O with first-order dynamics and the time constant , defined as =

. [16] The effects of triggering on the oscillator's dynamics during the expiration phase were characterized, and the results are presented in the following sections. respectively. [6] The demonstration of flow streamlines during the inspiration and expiration can be found in Figure S2 (Supporting Information). A video illustrating the inspiration and expiration phases of the fluidic oscillator is also available in the supplementary video (Video S1, Supporting Information).

As seen in Figure 3B , increasing the EC screw depth reduces the rate of pressure decay between PIP and PEEP, facilitating an adjustable oscillation period of 3-5 seconds (RR 12-20 breaths per minute). The positive linear relationship was observed between exhaust screw depth, d, and oscillation period, T ( Figure S5 , Supporting Information). Variations in PIP and PEEP under different EC screw depths were found to be 1 cmH2O and 3 cmH2O, respectively, which are small relative to the overall oscillation amplitude of 13 cmH2O. Hence, the EC screw allows for robust, predictable control of the oscillation period.

This article is protected by copyright. All rights reserved Figure 3C demonstrates that the simultaneous opening of the FC screw and closing of the SC screw increases PIP. The clinician can check the instructions and the information chart for details about calibration screws and incrementally adjust screw depths to achieve the desired PEEP and PIP outputs. As seen in Figure 4 , each calibration screw has a distinct impact on the respiratory characteristics. Both computational and experimental findings confirm these key functions of calibration screws during the respiratory cycle. Figure 3D illustrates that the oscillation period can vary dynamically from cycle to cycle based on a patient trigger. Our data corroborate that a premature transition from the expiratory phase to the inspiratory phase is possible following a small decrease in alveolar pressure, representing a patient-initiated breath. In the cycle immediately following the trigger, PIP, PEEP, and oscillation period all remain consistent.

Based on our computational results, a prototype was fabricated and tested with a clinical lung simulator (ASL 5000 Breathing Simulator) ( Figure S6 , Supporting Information). As illustrated in Figure 3E , the prototype is able to achieve stable, constant-amplitude oscillations between (11.66 ± 0.37) cmH2O and (17.15 ± 0.33) cmH2O at the lung compliances tested (C = 10, 30, 50 ml/cmH2O, based on the range of lung compliances observed in patients with COVID-19), [11, 12] with similar waveform shapes to those seen in silico , and showing minimal variation over a period of 60 seconds (n=15 cycles for each waveform). Moreover, our experimental results demonstrate that the prototype responds successfully to the patient trigger, as inspiration begins immediately following a patient-initiated pressure drop by the lung simulator. Figure 3E also demonstrates that the prototype is functional for the range of COVID-19 lung compliances tested, while highlighting that higher compliances produce a slower rate of pressure decay between PIP and PEEP. In order to account for this effect, the physician can simply vary the EC screw depth as the patient's lung compliance improves or worsens over the course of their intubation period.

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The main contributions of this work are as follows; (i) we present a low-cost fluidic oscillator that is capable of converting a CPAP machine into an emergency use mechanical ventilator, (ii) CFD modeling demonstrated that our design is able to achieve physiologically relevant respiratory pressures and allows greater control of the respiratory rate than previous fluidic oscillator-based solutions, (iii) with a respiratory simulator, we showed that a prototype of our design responds successfully to patient-initiated spontaneous breaths, while also remaining functional over a wide range of lung compliances that represent varying levels of lung pathology.

Given that our aim was to design an emergency ventilator for under-resourced communities, a key design criterion was simplicity. As such, we sought to avoid complex electromechanical systems that are expensive, potentially difficult to operate, and more prone to failure. Instead, we dedicated our efforts to understanding the oscillator's fluid dynamics using simulation and visualization in CFD. We provide an in-depth explanation of the design's operation at each point throughout the respiratory cycle (see description of Figure 2 ). This knowledge in turn allowed us to better optimize the design, which is evident in the clinically relevant respiratory rates that are achievable (tunable oscillation period between 3-5 s). This represents a substantial improvement on other emergency ventilator designs seen in the literature (such as the emergency A.R.M.E.E.

ventilator) that cannot attain the same metrics (maximum 1 s period [5] ).

To ensure our simulations most accurately depicted the real-world physics of mechanical ventilation, we coupled a first-order Windkessel model of lung respiratory mechanics to our CFD model that can be adjusted for a range of COVID-19 lung compliances. We provide further quantification of user considerations through implementation of a patient "trigger", which is translated to an induced pressure drop in the Windkessel model. The effect of this trigger was also simulated experimentally, and similar results were obtained to those of the CFD. To our knowledge, these modeling approaches have not been applied to CFD simulation of mechanical ventilators to date.

However, there are some limitations associated with the current design. The slight discrepancy observed between the pressure oscillation obtained in silico (8-20 cmH2O) and in vitro (11-17 cmH2O ) may be attributed to discretization errors associated with the meshing of the computational model. Additionally, the idealized first-order Windkessel model implemented in ANSYS may not represent complex second-order resistive losses that would arise in a physical system. The lung simulator utilizes a variable piston that moves under changes in the tidal volume,

where both viscous and frictional losses could be present.

We do not envision use of our ventilator when fully electronic, commercial ventilators are available that offer on-demand, digital control of clinical parameters and multiple modes of ventilation (pressure-controlled and volume-controlled). We recommend oscillator use primarily as an emergency ventilator in scenarios where there are no other ventilator options available, for example, due to cost, demand, or lack of electricity in emergency settings, or when patients are heavily sedated and cannot breathe on their own. When compared to similar emergency ventilators designed for use in these settings, our device supports a longer oscillation period, which affords greater flexibility and control of respiratory rate. Further investigation is required to understand the effect of less predictable respiratory patterns, such as coughing and wheezing, that might take place mid-respiratory cycle or early exhalation stage. Moreover, the ability of our proposed device to adapt and recover from such disturbances has yet to be studied. While the oscillator responds robustly when patient-triggered breaths are introduced, an important measure for patient comfort, we recognize that the inspiratory rise time of ~0.7 seconds is long compared to the settings used in traditional pressure-controlled ventilation (0-0.4 seconds), which could increase the work of breathing and cause patient-ventilator dyssynchrony.

In summary, this work introduces a low-cost, rapidly deployable emergency ventilator using a novel fluidic oscillator design. Clinically relevant values for PEEP, PIP, and oscillation period were achieved according to consensus guidelines for emergency use ventilators, with model input parameters taken from COVID-19 patient data. Additionally, the design facilitates precise, This article is protected by copyright. All rights reserved independent control of these variables guided by CFD studies. The oscillator responds robustly when patient-triggered breaths are introduced, which could afford greater patient comfort during the ventilation. Finally, the operational concept of the oscillator was validated by the prototype testing, and PEEP and PIP values between 11-17 cmH2O were achieved. A summary of the primary simulation and experimental results can be found in Table S1 (Supporting Information). Further tuning of the prototype may be necessary to extend this range to the 8-23 cmH2O as seen in the simulation. Further research may also explore the full range of design parameters (e.g. FC, SC, and EC screw depth) that determine the oscillator's functional performance. Breathing Simulator. An additional 3 openings were cut above the FC, SC and EC channels, and screws for variable occlusion were placed into each ( Figure 3E) . These screws are used to alter flow resistance as predicted in silico. 3D CAD files of oscillator design are available in Supporting

Information.

Prototype testing set-up: Tests on the respiratory simulator -ASL 5000 Breathing Simulator (IngMar Medical) -were conducted with a constant input flow rate of approximately 30 L/min measured using a digital mass flow meter (SFM300, Sensirion). Pressures and volumes were recorded directly by the ASL simulator. A muscle (diaphragmatic) pressure profile simulating a patient-initiated breath every 4 seconds was applied in the ASL settings. To simulate the respiratory mechanics of COVID-19 patients, simulator compliance was varied between 10-50 mL/cmH2O. [17] COVID-19 causes decreased lung compliance (i.e. stiffening of the lungs) compared to healthy lung This article is protected by copyright. All rights reserved compliance of 100-400 mL/cmH2O. [18] The airway resistance and trigger amplitude were chosen to align with those used in the in silico model ( = 3 cmH2O/L/s and 5.5 cmH2O, respectively). [11, 12, 19] Statistical Analysis: The results are reported as mean values ± SD in the main text and supplementary table (Table S2 ). Cycle number (n) is 15 for each waveform. Data processing was performed using Matlab software (Mathworks Inc., MA, USA).

Supporting Information is available from the Wiley Online Library or from the author. ∆ and ∆ ranges are indicated on the graph also. Tuning PIP and PEEP through simultaneous variation of FC and SC screw depth (C). Controlling the oscillation period in response to a patient trigger (D). Note the small decrease in pressure at ~4.5s induced by the patient. The muscle-induced pressure profile applied by the ASL lung simulator to be used for in vitro experimental testing, simulating a patient trigger initiated at regular timepoints T (E). Corresponding experimentally measured pressure profiles for a range of COVID-19 lung compliances from 10-50 ml/cmH2O (F). 

The computational modelling of a fluidic oscillator designed to convert continuous positive airway pressure (CPAP) machines into emergency mechanical ventilators is presented. The oscillator addresses potential ventilator shortages using a part that is inexpensive and easily manufactured. The fluid dynamics model is used to assess system performance, and a prototype is tested using a benchtop respiratory simulator.

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