key: cord-252101-77lnyjan authors: Mathanlal, Thasshwin; Israel Nazarious, Miracle; Mantas-Nakhai, Roberto; Zorzano, Maria-Paz; Martin-Torres, Javier title: ATMO-Vent: an adapted breathing atmosphere for COVID-19 patients date: 2020-09-26 journal: HardwareX DOI: 10.1016/j.ohx.2020.e00145 sha: doc_id: 252101 cord_uid: 77lnyjan The ongoing worldwide pandemic of coronavirus disease 2019 (COVID-19), has been one of the most significant challenges to humankind in centuries. The extremely contagious nature of the SARS-CoV-2 virus has put forth an immense pressure on the health sector. In order to mitigate the stress on the healthcare systems especially to battle the crisis of mechanical ventilators, we have designed a modular, and robust DIY ventilator, ATMO-Vent (Atmospheric Mixture Optimization Ventilator) which can be fully mounted within two days by two operators. The ATMO-Vent has been designed using low-cost, robust, Commercial Off The Shelf (COTS) components, with many features comparable to a full-fledged ventilator. ATMO-Vent has been designed based on the United Kingdom Medicines & Healthcare products Regulatory Agency (UK-MHRA) guidelines for Rapidly Manufactured Ventilator System (RMVS), yet scalable to the specific requirements of different countries. ATMO-Vent is capable of adjusting the Fraction of Inspiratory Oxygen (FiO(2)) levels, Tidal Volume (TV), frequency of breaths, Inspiratory/Expiratory ratio (I/E), Peak Inspiratory Pressure (PIP) and Positive End-Expiratory Pressure (PEEP). ATMO-Vent can operate in two modes - Continuous Mandatory Ventilation (CMV) using Volume-Controlled Ventilation (VCV) and in Assisted Control (AC) mode with pressure triggered by the patient. ATMO-Vent has undergone rigorous testing and qualifies under Class B Electric and Magnetic Compatibility (EMC) requirements of EN 55011 CISPR 11 standards. , OxVent of University of Oxford and King's College London [3] and ApolloBVM from Rice University [4] . The ATMO-Vent uses a linear actuator to actuate the BVM, which provides higher versatility in controlling the respiration parameters such as the Tidal Volume (TV), frequency of breaths, Peak Inspiratory Pressure (PIP) and Positive End-Expiratory Pressure (PEEP). The Graphical User Interface (GUI) of ATMO-Vent is very similar to that of the existing, commercial, fullfledged mechanical ventilator with the ability to display respiratory parameters such as Plateau Pressure, Mean Airway Pressure, Minute Ventilation, Resistance and Compliance which are crucial for healthcare professionals to determine the lung condition of the patient. Adhering to the RMVS guidelines, ATMO-Vent features a CMV Mode which minimally is a VCV in addition to an AC-VCV which can be pressure triggered by the patient. The former mode is used as invasive ventilation with intubation for people with ARDS, and the latter can be used as non-invasive positive pressure ventilation for people with mild to moderate respiratory discomfort. ATMO-Vent incorporates safety features such as audio-visual alarms when the frequency of breaths is below the set threshold, pressure in airway exceeds the PIP or if the set TV is not delivered to the patient. In case of airway pressure exceeding the set PIP, the linear actuator movement immediately ceases, and the solenoid actuating valves opens to vent any excess pressure. All these features are designed using commercial open-source electronics platforms such as Arduino and Raspberry Pi along with medical device compatible sensors. A robust construction along with a carefully designed software architecture in compliance with the RMVS guidelines adhering to modes of ventilation, infection control, safety alarms and biological contamination make ATMO-Vent a unique DIY solution to battle the current ventilator crisis with a minimal investment of time, financial and human resources. ATMO-Vent is a BVM based ventilator which has been completely designed using readily available COTS components. The materials used have been meticulously chosen such that there is a very minimal adaptive work needed to fit the ventilator application. The components used in-line with the patient's airway, has been constrained purely to medically approved and compatible elements. ATMO-Vent uses an efficient design construction utilizing minimal components without compromising the safety and operability of the ventilator. Based on the Harvard EdX course on mechanical ventilation [5] , the modes of operation programmed in ATMO-Vent were implemented with an efficient GUI, resembling the interface of the commercially available Hamilton C1 ventilator. This was done in keeping mind of the ease of training to the clinicians, respiratory therapists and intensive care nurses to operate ATMO-Vent. Fig. 1 shows the architecture block diagram of ATMO-Vent. The block diagram is colour coded in six categories as follows: Fig.1 The Air-Oxygen Mixture Optimization circuit controls the Fraction of Inspiratory Oxygen (FiO 2 ). Compressed medical-grade air and oxygen are supplied into the circuit using the 8mm tubing. The polyurethane tube is FDA approved for use in the food industry. The circuit does not include the humidifier and the pressure regulating valve, which are commonly found in healthcare infrastructure. The flow rates of air and oxygen are controlled using the flow meters. The MR3000 flow meter from Brooks Instruments can control the flow of each gas from 2 LPM to 30 LPM with a full-scale accuracy of ± 4%. The flow meter can handle 6.9bar pressure of the gas, and it is crucial to ensure that the pressure setting on the pressure regulating valve at the source is below 6 Dead space is a critical issue which can limit the gas exchange in the lungs by accumulating carbon dioxide from the patient's exhaling breath in the long proximal tube between the BVM and the patient. In order to mitigate the dead space, ATMO-Vent uses a double duck-bill valve assembly (one at the output of the BVM and the other at the exhaust as close as possible to the patient) to reduce the proximal tube length. The exhaust of the duck-bill valve assembly close to the patient is fixed with a spring-loaded PEEP valve that maintains an end positive pressure in the airway after exhalation. This positive pressure prevents the alveoli of the lungs from collapsing after a breath. The PEEP valve is connected to the exhaust port using a PEEP adapter through a High-Efficiency Particulate Air (HEPA) filter. The filter ensures that any virus or microbe from the patient's airway does not spread out into the ambient atmosphere. The HEPA filter is crucial to prevent health care professionals from being infected by the patient. The second duck-bill valve within 15 to 20 cm from the patient's face, is the patient inflating valve which allows the flow of air and oxygen mixture from the BVM to ventilate the patient and at the same time closes the expiratory limb. During expiration, the valve closes the inspiratory limb and exhausts the exhaled air through the PEEP valve and HEPA filter assembly. Having the duck-bill valve close to the patient reduces the dead space, which prevents patients from re-breathing excess carbon dioxide, should the duck-bill valve be located far from the patient by increasing the proximal tube length. The Laerdal (duck-bill) and Ambu based patient valves are widely used as non-return valves in Bag Valve Mask (BVM) based resuscitators owing to their simplicity and reliability [6] . The validation of the duck-bill valve as a patient inflating valve has been discussed in the Validation and Characterization section of the article. Duckbill valves in Resuscitation bags may tend to perform poorly during Pre-Oxygenation and are less effective, with only 40% oxygen or less being delivered during spontaneous ventilation. This is because the Resuscitation bags using duck-bill inspiratory valves function differently during manual versus spontaneous ventilation [7] . ATMO-Vent is purely based on manual mechanical ventilation, where there is flow only during inspiration as opposed to the continuous flow observed in spontaneous ventilation, and hence the duck-bill valve should not be of concern regarding the Pre-Oxygenation. The linear actuator is the core of the ventilator. Various modes of actuation have been proposed in various DIY ventilator designs such as gripping arm design of E-Vent [2] , compressed air in OxVent [3] , and Rack and pinion mechanism in ApolloBVM [4] . The use of an electric linear actuator in ATMO-Vent provides a versatile approach to control the stroke length and stroke speed, which makes it possible to control the frequency of breaths and tidal volume, both parameters that are monitored and are adjustable in commercial mechanical ventilators. This controlled motion allows for a continuous volume of air resulting in a typical breathing cycle and highly customizable for patient's age and size. In contrast, other actuating mechanisms like the automated single-direction camming solution do not allow versatile control of the volume of air as needed for a typical breath cycle. The linear actuators used in ATMO-Vent do not involve the use of feedback circuit and use built-in limit switches to detect position of the actuator. The use of external sensors has not been considered as 1) it makes the design more complicated involving more sensors increasing risks of failure 2) cost of the development increases with the additional sensors. Hence, the displacement of the linear actuator in ATMO-Vent is controlled through an open loop system using PWM signals. The choice of the linear actuator is one of the most critical design factors to consider in building ATMO-Vent. ATMO-Vent uses an adult-sized BVM which has a height of approximately 120mm in the distended state. BVM, when operated by hand, provides a volume of approximately 500ml per squeeze. Tidal volume into the lung represents the average volume of air displaced between normal inhalation and exhalation when extra effort is not applied. It is approximately calculated by multiplying the bodyweight of the patient in kg, times 6ml per kg. [8] . In any case, the choice of TV is decided by the respiratory therapist and is an important parameter to be controlled and monitored. The stroke length decides the tidal volume delivered to the patient. The frequency of breath is another crucial parameter to be controlled, and patients with respiratory illness may demand a higher frequency of breaths. The stroke length and speed decide the maximum stroke speed of the linear actuator limits the minute volume delivered to the patient since the maximum frequency of breaths for a particular tidal volume that can be provided by the ventilator. The current linear actuator LD3 used in ATMO-Vent can provide a maximum stroke speed of 25mm/s, and this constrains the frequency of breaths in high tidal volume requirements. The 12V LD3 linear actuator has an inbuilt limit switch that switches off the motor when the end effector is in the highest position or viceversa. The stroke speed is controlled by VNH3SP30 motor driver module using Pulse Width Modulation (PWM). The master Arduino Mega 2560 generates the PWM signals to control the linear actuator. The Arduino Mega 2560 is chosen for its higher programmable memory of 256 KiloBytes (KB). The software algorithm is discussed in the Computing and Interface section. Pneumatic systems, including life-saving ventilators, need to have a strict measurement system to monitor and control the flow of gases. Ventilators have three basic parameters to be controlled and monitored -Pressure, flow rate and volume. ATMO-Vent uses an Arduino Mega 2560 as a slave microcontroller to monitor pressure and flowrate. The volume of the air-oxygen mixture is calculated from the latter by integrating the flowrate over the inhalation time period. The native 10-bit Analog to Digital Conversion (ADC) capability of Arduino Mega 2560 limits the resolution of the output voltages measured from the Pressure and flowrate sensors. This is mitigated by using an external 16-bit ADC such as ADS1115 interfaced to the Arduino using an I 2 C interface. The ADS1115 is a four-channel programmable gain amplifier with gain up to 16x. A gain of 1x is used with the pressure and flow measurements since the output voltages from the sensors are in the range of the 5.0V reference voltage used. The pressure is an important parameter to be controlled as it is commonly associated with Ventilator Induced Lung Injury (VILI) such as barotrauma. Excess pressure can be fatal to the patient by damaging the alveoli, or an under-pressure may not be able to overcome the resistance of the patient's lungs preventing the lungs from getting the required airflow. Hence, there needs to be tight monitoring and control of the pressure. SDP2000-L Differential Pressure sensor from Sensiron is used in ATMO-Vent connected to the proximal tube. The sensor has been designed specifically for ventilators, and its range of -100Pa to 3500Pa best fits the pressure monitoring requirements. The low limit Flow rate is also a critical parameter to be monitored as it is also associated with VILI. A higher flow rate leads to frictional losses occurring along the airway epithelium leading to tissue damage [9] . Modern Intensive Care Unit (ICU) ventilators provide a flow rate varying between 50 LPM to 60 LPM. Due to the unavailability, during the pandemic situation, of a commercial flow rate sensor with this lower range of flow measurement apart from the sensors specifically manufactured for ventilators such as Sensiron SFM3000, we explored new possibilities that can be used for DIY ventilators. Flow rate sensors for air medium operate by the pressure difference across venturimeter, ultrasonic principle, or hot-wire anemometer. The hot-wire anemometer method is widely used in automobile engines as a Mass Air Flow (MAF) sensor. The robust, low-cost, efficient and readily available MAF sensor is chosen for ATMO-Vent. The MAF sensors have a very high measurement range of flow up to few thousand kg/hr, while the small petrol engine cars with TDIH engines having the least range of 480 kg/hr. Moreover, they cannot be directly interfaced to the 18mm or 30mm flex hoses used in BVM, owing to their larger diameter. Hence, a custommade 3D-printed part (Flow Meter Adaptor) was designed for ATMO-Vent to couple the MAF sensor to the existing 18mm flex hose used in ATMO-Vent. The sensor operates at 12V and provides a 0 to 5V analogue output depending on the flow rate. Fig. 2 Fig. 2 (Right). This commercial calibration curve provided by the manufacturer cannot be directly used to calibrate the MAF sensor, as they have been calibrated for the larger diameter engine inlet manifold. The calibration curve has been scaled down to match with the diameter of the 3D printed component holding the MAF sensor core. Fig. 3 (Left) shows the MAF sensor core attached to the 3D printed MAF core holder and Fig. 3 (Right) shows the scaled-down calibration curve. Equations (2) to (5) are used to convert and calculate the scaled-down calibration curve. The calculation has been done in MATLAB code, which is also attached to the article. where subscript '1' denotes the original engine inlet manifold, and '2' denotes the adapted configuration with the 3D printed MAF sensor holder for ATMO-Vent. The output voltage of the flow meter is obtained from the raw ADC measurements using the equation (6) . The calibration function between the measured output voltage and the equivalent airflow is given by equation (7) . The coefficients are obtained by curve fitting with a R 2 value of 0.9476. As an alternate method, the flow rate can also be calculated using a venturimeter with a differential pressure sensor. The pressure drop between the entrance and the throat of the venturimeter provides the flow rate. The SDP2000-L differential pressure sensor is again used for this venturimeter. The block diagram of the venturimeter is shown in Fig. 4 . The flow rate is calculated from the raw ADC output of the SDP2000-L pressure sensor as follows using equation (8) and (9): )) 60000 (9) Where, A 1 is the Area of the entrance, A 2 is the Area of the throat, and rho is the density of air = 1.225 kg/m 3 The MAF sensor is the preferred flow meter owing to its higher accuracy and a high-frequency response allowing measurement of turbulent flows. In either case, the sensor is connected to the output of the BVM between the first and the second duck-bill valve such that they measure the inspiration flow rate. Either sensor can detect a reverse flow when placed along the proximal tube, but the measurements would not be accurate, and hence it is placed in the inhalation phase. The full bill of materials is shown in Total cost: GBP 1000 approx. Step 1 (Fig. 7 Step 2 (Fig. 8) Step 3 (Fig. 9) Fig. 9 . The third step of ATMO-Vent building -Air-Oxygen mixture circuit Step 4 (Fig. 10) Fig. 10 (top) shows the components before assembly into the PC cabinet. The six modules follow the same colour code nomenclature, as indicated at the beginning of the article. Fig. 10 (bottom) Fig. 10 (Bottom) . Step 5 (Fig. 11) : With all the components connected, the PC cabinet side door is fixed and secured using its screws. The ATMO-Vent is now ready for operation. Fig. 11 (top) shows the fully assembled ATMO-Vent with side door opened and Fig. 11 (bottom) shows the side and front view of ATMO-Vent with the side door fixed. Fig. 11 . The final step of ATMO-Vent building Step -I (Fig. 12) : Connect the Peripherals -Monitor, Keyboard and Mouse to ATMO-Vent. Step -II (Fig. 13) : Turn on the Power Switch from position 0 to 1. Step -III (Fig. 14) Step -IV (Fig. 15) Step -V (Fig. 16) : Turn on the Gas Supply and make sure to check that the pressure regulating valve is indicating a reading below 6.9 bar. Step -VIII (Fig. 17) Step-IX: Set the PEEP pressure by rotating the knob on the spring-loaded PEEP valve. Note down the PEEP setting as this would be used in the next step to define the negative trigger pressure. Step -X: Fig. 18 shows the GUI with all the values filled in for the AC Mode. Step -XI: In the case of CMV ventilation, select the Continuous Mandatory Ventilation Tab. The settings of CMV are very similar to AC ventilation except for the Trigger settings. CMV is a mandatory mode where the ventilator provides inhalation and exhalation sequence with the programmed frequency, tidal volume, Inspiratory pause, I/E setting and maximum PIP that can be encountered. In CMV mode, the maximum inspiration time is by default restrained to one second. Thus, the downward operation of the linear actuator will occur until the set tidal volume is reached or for one second, whichever precedes. All the values must be entered in the respective text boxes and double-checked before proceeding to Step XII. Fig. 19 shows the GUI with all the values filled in for CMV mode. Step -XII: This step is more of a medical procedure and is out of the scope of this article. In the case of NPPV, fasten the BVM mask on the patient and ensure that it has a tight seal around the patients' nose and mouth. In the case of invasive ventilation, intubation has to be performed by a trained healthcare professional. Once the necessary procedure is done, it must be ensured that the proximal tubing is securely connected to the patient. Step -XIII: With the proximal tube double-checked in place, the start button on the GUI is pressed. In the case of CMV mode, the ventilator would start immediately and ventilate the person. In case of Assisted control modes, the ventilator would wait for the patient to trigger the inhalation sequence and then would ventilate the patient. Modifications to the respiration parameters is allowed during the ventilator operation after the Start button is pressed, by entering the new parameters and pressing the Update button on the GUI. ATMO-Vent has three crucial alarm features respective to CMV or AC mode, namely PIP alarm, frequency alarm and tidal volume alarm. Volume alarm: This alarm can fire in both CMV and AC mode. ATMO-Vent being a volume-controlled ventilator is driven by tidal volume as the primary control variable. There can be scenarios when the needed tidal volume is not being delivered to the patient. This can be due to any leaks in the proximal tube or increased fluid accumulation in the lungs. The alarm is indicated by the low tidal volume highlighted in red, and the buzzer is producing a high-frequency buzzing sound. Again, this alarm can only be turned off manually by a healthcare professional by clicking the bell button after assessing the situation. A demonstration scenario with Continuous Mandatory Ventilation (CMV) mode was executed on a Mannequin with the input respiration parameters that resulted in the following output as provided in Table 2 . Fig. 20 shows the pressure, flow rate and volume plots during the 10 consecutive breaths during the ATMO-Vent operation. The validation of a duck-bill valve as a patient inflating valve is discussed in this section. The second duckbill valve used in ATMO-Vent along the proximal tube is fundamentally the patient inflating valve. During manual ventilation, the air-oxygen mixture is forced through the valve base, opening the duck-bill valve and air-oxygen mixture is delivered to the patient's lungs. This force also seals the valve base to the exhalation port preventing the fresh air-oxygen mixture from venting through the exhalation port. During exhalation, the valve base returns to its former position, and, thus, exhaled gases are vented through the exhalation port. To validate the optimal operation of the duck-bill valve in ATMO-Vent, a simple experiment is performed to ensure that the expiratory limb of the exhalation circuit is closed despite higher pressures generated by lung resistance. The experimental setup is shown in Fig. 21 The electromagnetic compatibility (EMC) tests for radiated emission, conducted emission and radiated immunity were so far carried out at the anechoic chamber facility (Fig. 23) ATMO-Vent has been carefully designed in consideration with the minimal clinical requirements of ventilators, using the UK-MHRA RMVS document as a guideline. The design also ensures that the materials and components used are readily available in the commercial market and could be assembled with minimal investment of time and resources. The design is modular, and components could be substituted with alternatives found in the local market specific to the installation of the ventilator. ATMO-Vent's modes of operation have been tested using a mannequin and the electronics integrity with EMC/EMI testing in an anechoic chamber in consideration with the IEC 60601 specifications. In the following version of ATMO-Vent, a higher stroke speed linear actuator will be used to achieve a higher tidal volume with a set frequency, and further tests with a lung simulator will be performed in order to quantify the output respiration parameters accurately. The future line of work of action of ATMO-Vent are: Robustness for long-sustained operation needs to be tested: when applied in clinical use, mechanical ventilators may be typically needed for a couple of weeks, and this requires about a million cycles of assisted ventilation. During this time the configuration of the ventilator needs to be adapted, for diagnosis and evaluation, and be accommodated to the evolving condition of the patient which may get worse or improve and eventually lead to retire the supporting ventilation. Thus, both the mechanical, electrical and software robustness needs to be tested on a long-time test. Certify the equipment for its use in healthcare facilities. Although the design of this ventilator complies with the UK Medicines & Healthcare products Regulatory Agency (UK-MHRA) guidelines, the equipment needs to be certified according to the industrial and medical standards ISO 13485:2016(E). Although its design is in everything equal to existing commercial ventilators, before its operation in clinical use, it needs to be tested with trained healthcare personnel, to make sure that the information displayed in the screen, and the operating software interface is intuitive and similar to what is customarily used. The ventilator should also be subjected to Closed Suctioning Test to ensure continuous ventilator operation during the suctioning procedure. The closed suction is done to remove tracheal secretions through the endotracheal tube in mechanically ventilated patients. ATMO-Vent can be used in the future as support for other respiratory diseases. Oxygen therapy coupled with mechanical ventilation is meant to support patients so that an adequate oxygen saturation (>88%) in arterial blood is maintained. The ability of ATMO-Vent to provide non-invasive assisted ventilation support with control over the fraction of inspired oxygen can help patients who are in the development stage of respiratory distress. This shall ensure that patients who develop ARDS could be attended with a full-fledged ventilator. Besides this, and beyond its clinical use, ATMO-Vent will be miniaturized and used for its future usage as 1) portable life-support equipment for long-inhabited environments without rapid access to hospitals (emergency clinics in rural environments, ships, camping sites, migrant-settlements, military or scientific bases in remote regions); 2) life support system for Space applications using space-qualified components. The designs and other information (the "Design") made available in this article is at an early stage of development. Accordingly, specific results are not be guaranteed, and the Design provided here is provided "AS IS" and without any express or implied warranties, representations or undertakings Rapidly Manufactured Ventilator System (RMVS) Guidelines from UK-MHRA ApolloBVM HarvardX: COV19x -Mechanical Ventilation for COVID-19 The Patient Inflating Valve in Anaesthesia and Resuscitation Breathing Systems Efficacy of Preoxygenation with Tidal Volume Breathing: Comparison of Breathing Systems Should A Tidal Volume of 6 mL/kg Be Used in All Patients Role of Strain Rate in the Pathogenesis of Ventilator-Induced Lung Edema 3D printing for airway disease The authors declare that they have no known competing financial interests or personal relationships that could inappropriately influence or bias the content of the paper. This work does not involve the use of any human or animal subjects.