A Novel Ventilator Design for COVID-19 and Resource-Limited Settings

There has existed a severe ventilator deficit in much of the world for many years, due in part to the high cost and complexity of traditional ICU ventilators. This was highlighted and exacerbated by the emergence of the COVID-19 pandemic, during which the increase in ventilator production rapidly overran the global supply chains for components. In response, we propose a new approach to ventilator design that meets the performance requirements for COVID-19 patients, while using components that minimise interference with the existing ventilator supply chains. The majority of current ventilator designs use proportional valves and flow sensors, which remain in short supply over a year into the pandemic. In the proposed design, the core components are on-off valves. Unlike proportional valves, on-off valves are widely available, but accurate control of ventilation using on-off valves is not straightforward. Our proposed solution combines four on-off valves, a two-litre reservoir, an oxygen sensor and two pressure sensors. Benchtop testing of a prototype was performed with a commercially available flow analyser and test lungs. We investigated the accuracy and precision of the prototype using both compressed gas supplies and a portable oxygen concentrator, and demonstrated the long-term durability over 15 days. The precision and accuracy of ventilation parameters were within the ranges specified in international guidelines in all tests. A numerical model of the system was developed and validated against experimental data. The model was used to determine usable ranges of valve flow coefficients to increase supply chain flexibility. This new design provides the performance necessary for the majority of patients that require ventilation. Applications include COVID-19 as well as pneumonia, influenza, and tuberculosis, which remain major causes of mortality in low and middle income countries. The robustness, energy efficiency, ease of maintenance, price and availability of on-off valves are all advantageous over proportional valves. As a result, the proposed ventilator design will cost significantly less to manufacture and maintain than current market designs and has the potential to increase global ventilator availability.

Resistance Characterisation Figure S2: Resistance of the SmartLung as a function of flow rate, rated at (A) 5 cmH2O/(l/s), (B) 20 cmH2O/(l/s), (C) 50 cmH2O/(l/s), and (D) EasyLung rated at 25 cmH2O/(l/s). A custom MATLAB script was implemented to calculate resistance from internal pressure traces recorded by the prototype and external flow rate measurements recorded by a Sensirion SFM3000 series flow meter. Blue, red and green markers represent triplicate measurements for each configuration.
Numerical Modelling Background Figure S3: Subsection of the Simscape model showing the patient inflow and outflow pathways. Addition of a support or relief valve, Valve C', in a branch downstream of Valve C, to match the inhalation flow rate to the characteristic obtained from the calibration of the prototype Valve C and its connectors. Adjustment of the downstream pressure of Valve D for the same purpose.
At Valve C, a branch was added downstream with a Valve C' of the same connected to a pressure source , which is calculated as follows. For set pressures and , the difference between the expected flow rate in l/min in the experimental circuit and the ISO flow rate through Valve C is Valve C' should supply, or remove, Δq from the circuit, and functions according to the ISO standard, so should satisfy In the above equation, is the sign of , treated as positive for flow directed into the system. This second-degree polynomial in the pressure ratio, of discriminant Δ, has the positive root At Valve D, the downstream pressure , which is normally atmospheric, was adjusted in a similar fashion such that = .
Supplementary Material 6 ISO Test Traces Figure S4 Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 1, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2). Figure S5: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 2, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2).
Supplementary Material 8 Figure S6: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 3, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2). Figure S7: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 4, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2). Figure S8: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 5, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2). Figure S9: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 6, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2).
Figure S10: Recorded traces of pressure, flow rate and volume over six seconds for ISO Test 7, when using compressed gas supplies or an oxygen concentrator (related to Figure 7 and Table 2).

Inlet Valve Flow Coefficicents
Figure S11: Maintenance of tidal volume for ISO 80601-2-12-2020 cases for , ≥ 0.02 at a gas supply pressure of 4 bar (A) and for , ≥ 0.04 at a gas supply pressure of 1.3 bar (B).

Effects of Patient Circuit on Flow Rate Calibration
During inhalation: The flow rate is calculated based on Psys and Pres (Equation 1), which are both internal, and hence is independent of the patient circuit.
During exhalation: The only resistance that affects the exhaled flow rate, , is RD ( Figure  1B). As there is no flow in the inspiratory branch of the patient tubing during exhalation, the pressure Psys is equal to the value at the Y-piece, downstream of the endotracheal tube, patient HME filter, etc. Hence, the only components that could affect RD are the patient tubing connecting the Y-piece to the ventilator and the filter on the exhalation port of the ventilator. To investigate how the tubing and filters affect the calibration constants in Equation 5, multiple calibrations were run under various scenarios. The results confirm that the flow rate measurement is robust to different patient circuits ( Figure S12). Figure S12: Calibration of the flow-pressure relationship exhalation pathways with various additional components in the system. (A) Ventilator only, (B) 2m of 22mm ID ventilator tubing, (C) A high-efficiency HME filter on the exhalation port (Intersurgical Filta-Therm), (D) A sterile HME filter on the exhalation port (Intersurgical Inter-Therm), (E) 2 sterile HME filters in series on the exhalation port (to provide additional resistance), (F) A wet sterile HME filter on the exhalation port (to mimic a build-up of secretions, although in practice these would be unlikely to pass). (G) Shows the best fit lines of Equation 5 to each case (same colour coding as A-F) and the 95% prediction bounds for the ventilator only case.     Table S5: Statistics for Durability analysis (see Figure 8). Correlation coefficient evaluates whether the performance varied in time.