Can CPAP machines be repurposed to solve the ventilator shortage ?


 The worldwide shortage of medical-grade ventilators is a well-known issue, that has become one of the central topics during the COVID-19 pandemic. Given that these machines are expensive and have long lead times, one approach is to vacate them for patients in critical conditions while patients with mild to moderate symptoms are treated with stripped-down ventilators. We propose a mass-producible solution that can create such ventilators with minimum effort. The central part is a module that can be attached to CPAP machines and repurpose them as low-pressure ventilators. Here, we describe the concept and first measurements which underline the potential of our solution. Our approach may serve as a starting point for open-access ventilator technologies.


Introduction
The COVID-19 pandemic plunged many countries, such as Italy, Spain and the United States, into a health crisis due to the sudden demand for medical facilities, staff and equipment 1,2 . Since the search for a vaccine is still ongoing, treatments revolve around symptom control and life-sustaining measurements.
Patients with COVID-19-related respiratory failures often require prolonged mechanical ventilation [3][4][5] . Such a treatment, however, cannot be provided due to a long-term worldwide issue: the shortage of medical-grade ventilators 6 . These costly devices have long lead times because of their complexity.
Consequently, this shortage may continue to be the one of few central issues during a potential second wave of the disease.
In response to the urgent need for ventilators, many designs were proposed to serve as life-sustaining measurements or to vacate medical-grade ventilators for patients in critical conditions 7 . The lead time for each proposed design depends on the range of functionalities, cost and, more importantly, the available production facilities. One prominent approach focuses on machines that automatically compress and decompress bag valve masks [8][9][10][11][12] . Other groups pursue ventilator solutions that can be built from scratch with the expertise and the facilities from different industrial branches 13,14 .
In the presence of these developments, we propose a mass-producible solution that is built around continuous positive airway pressure (CPAP) machines. These home-level ventilators are commonly used to treat sleep apnea worldwide. Nowadays, many machines sit unused in households [15][16][17] and thus, there are potentially millions of CPAP machines around the globe that can be repurposed as stripped-down ventilators.
CPAP machines generate and maintain a constant positive airway pressure in the patient. To cover the fundamental functions for mechanical ventilation, however, we need to: provide two well-de ned airway pressure levels, i.e. the peak inspiratory pressure (P IP ) and the positive end-expiratory pressure (PEEP), to assist or replace the patient's breathing during the inhalation and exhalation phase, set the respiratory rate, set the I:E ratio, which is the ratio of the inhalation and the exhalation time, and allow patients to trigger and to cycle inhalation. Therefore, ventilation by means of CPAP machines requires a modi cation that regulates the constant air ow with time. We developed a module, referred to as CARL (for CPAP Apparatus Respiratory Life support), that can be attached to any CPAP machine and instantaneously repurpose it as a basic ventilator with minimum effort. We believe that our proposed solution can be used to treat patients with mild symptoms that do not require high-level pressures provided by medical-grade ventilators or to wean off recovering patients from the ventilator.
We would like to clarify that the ventilator presented in this report: has only been tested with electric and mechanic equipment, is subject to pre-/clinical trials and approval, and has yet to be examined and approved by a regulatory health institution.
Therefore, at this stage of development, we strongly discourage the use of our device on humans. Our module should only be reproduced for the purpose of further development and investigation.

Results
CARL is an electronic device that is placed in the ventilation pathway between the CPAP machine and the patient. It uses a microcontroller-activated valve to generate two airway pressure levels from the constant air ow provided by the CPAP machine. The computer assures that the periodic alteration between both pressure levels corresponds to a set respiratory rate and I:E ratio. CARL uses sensors to monitor the airway pressure, the ow and the ventilation volume. That way, users can easily adjust the ventilator settings to the needs of the patient which is comparable to adjustments with medical-grade ventilators.
In the following, we present measurements of the airway pressure, ow and ventilation volume for different ventilation parameters. As a representative patient model, we use an arti cial lung (details in 'Methods').

Pressure-related ventilation settings
Ventilation strategies for COVID-19-patients include a minimum airway pressure of at least 5 cmH 2 O that must be maintained during the exhalation phase to stabilise the alveoli 5,18-21 . Therefore, for our rst set of measurements we vary the P IP value while keeping the respiratory rate and the I:E ratio xed. P IP value is set and adjusted on the CPAP machine. Our measurements of the airway pressure, ow and volume as a function of time for two different P IP values can be observed in Figure 1a (P IP = 15 cmH 2 O) and 1b (P IP = 20 cmH 2 O). From the pressure curves, we can see a decrease of the PEEP value with the P IP value: when the latter is set to 20 cmH 2 O or 15 cmH 2 O, we get a PEEP value of 7.5 cmH 2 O or 5 cmH 2 O, respectively.
To investigate the impact of the CPAP machine model on the ventilation performance, we repeated the same measurement as above with another model. These measurements are also presented in Figure 1a and 1b. The comparison reveals minor differences between the ventilation curves, which are due to the machine's capability to maintain the set pressure. The values for the P IP and the PEEP, on the other hand, are unaffected by the CPAP machine model.

Time-related ventilation settings
In contrast to the pressure-settings, all time-related ventilation settings, i.e. the respiratory rate and the I:E ratio, are adjusted on CARL. For the rst measurement, we vary the I:E ratio while the P IP value and the respiratory rate are xed. The acquired curves of the airway pressure, ow and volume as a function of time are shown in Figure 2a. An evaluation of the inhalation and exhalation durations con rms the correct implementation of the I:E ratios with CARL.
Our measurements for ventilations with different respiratory rates while the P IP value and the I:E ratio are xed are presented in Figure 2b. The ventilation volume data shows a drop of the maximum ventilation volume as the respiratory rate increases. This is expected since the duration of the inhalation phase decreases as the respiratory rate increases. We found that the respiratory rate agrees with the value that is set on CARL over the duration of the measurement.

Patient-triggered/Patient-cycled ventilation
Patients who receive no sedation may breathe spontaneously during ventilation 22,23 . To support such events, the ventilator must be able to recognise the patient's breathing effort, so that they can trigger and/or cycle the inspiration phase themselves. Our implementation relies on air ow monitoring which is demonstrated in Figure 3. Here, the start of the inhalation and exhalation phase are marked as 'o' and 'x', respectively. When no breathing effort is detected, e.g. when patient's breathing ceases, the ventilation is provided in accordance to the parameters set on CARL. The latter functionality is known as 'backup ventilation function' in medical-grade ventilators.

Discussion
The previous results demonstrate that our module CARL can repurpose CPAP machines to basic ventilators which provide ventilation pressures up to 20 cmH 2 O. Such ventilators may serve as a temporary solution for patients with mild respiratory conditions or as solution to wean off recovering patients from the ventilator. CARL has a clear and easy-to-use interface that allows constant patient monitoring and instant access to the ventilation settings, i.e. the respiratory rate and the two I:E ratios.
Owing to CARL's modular design, we can develop additional components to include more functionalities without interfering with the ventilator design. One potential component is a module to supply oxygen to the patient at a given oxygen level. This will also imply the addition of an oxygenation measurement device.
Further adjustments of CARL's design could be made to improve the infection control of the module. The design should allow the user to easily clean its surface within a reasonable time frame. In our case, this would primarily concern the components that are in the ventilation pathway. However, since these components are affordable and easy to produce, it may be safer to dispose and replace them instead.
Owing to the working principle of CARL, our device could repurpose any continuous positive pressure machine to a basic ventilator. As such, CARL may be the starting point for an open-source ventilator solution. Such decisions, however, require testing the device carefully on animals or patients, which is beyond the scope of this report.

Summary And Outlook
The rapid dissemination of COVID-19 has led to a short-term shortage of ventilators worldwide. We believe that CPAP machines repurposed as stripped-down ventilators can help with the crisis in the shortterm. Our mass-producible solution could be used to vacate medical-grade ventilators for patients in critical conditions. However, in the worst case and after further development and regulatory approval, our solution may be used as a last resort to save the patient's life.
It is important to note that CARL must be rigorously examined and tested before it can be used on humans. This includes stress tests for both hardware and software, performance measurements under clinical conditions and a biocompatibility evaluation of all materials in the ventilation pathway.

Ventilation speci cations for a minimally acceptable ventilator
Our set of ventilation speci cations is aimed at COVID-19 patients with mild symptoms as they may not require all functionalities provided by a medical-grade ventilator. A summary of the ventilation variables supported by our system is presented in Table 1. For the de nition of the ventilation variables and modes we use the terminologies and the guidelines provided by Charburn et al. 24 .  In addition to the ventilation speci cations, we also implemented functionalities in CARL's design that ensure the patient's safety during therapy and the safe operation of CARL. These functionalities are summarised in Table 2.

Function Description Contamination protection
Protection of patient and medical equipment from pathogens.

Constant monitoring (settings)
Visual display of airway pressure, tidal volume and current ventilation settings. Constant monitoring (hardware and software) Visual and acoustic signals for hardware and software failures during operation.

CARL
For the development of our module CARL, we aim for a mass-producible solution that can be manufactured worldwide. To ful l this criterion, we developed a design that only uses easy-accessible hardware and electronic components. Our current design for CARL is presented in Figure 4, where the essential components are optimised for mass-production using plastic milling. We also developed and tested a version that can be built with a 3D printer. Subsequent tests show, however, that the results strongly depend on the 3D printer model and, thus, we focus on the plastic-milled version as it yields superior and more consistent results.
CARL serves as an intermediate module that we place in the ventilation pathway between the CPAP machine and the patient (see Figure 5). The airway pressure that is generated by the CPAP machine in the ventilation pathway is regulated with a rotatable valve: when the valve is open, the airway pressure increases to the peak inspiratory pressure; when it is closed, the air ow stops and the airway pressure drops to zero. A exible tube is used to bypass the valve and to allow for a continuous minimum air ow to the patient. That way, a PEEP of at least 5 cmH 2 O can be maintained even when the valve is closed. To meet the set respiratory rate and I:E ratio during ventilation, the valve is attached to a computer-controlled motor. The patient receives air through a mask or a catheter. Any excess air during the patient's exhalation phase can escape through an exhalation valve that is placed between CARL and the patient.
We also designed a pop-off valve and a low-pressure valve to contribute to the patient's safety. The former component makes sure that the airway pressure does not exceed 30 cmH 2 O, e.g. in the event of a cough. The low-pressure valve guarantees that patients can inhale spontaneously in the event of a hardware or software failure. Additional ventilation parameters, e.g. the FiO 2 value, can be extracted by connecting a patient monitor to the provided connector.
To measure the airway pressure and the ow, we use a ow sensor (No. 281637, Hamilton Medical) that is connected to two implemented pressure sensors (AMS5915, Analog Microelectronics GmbH). The ow data is evaluated to determine the ventilation volume and to trigger and to cycle inspiration. As such, we can monitor and evaluate the patient's breathing behaviour during mechanical ventilation.
We use an Arduino Uno and an Arduino Nano to control all electric components in CARL. To implement the ventilation modes in Table 1, we use our own code that is written in C++ and only employs standard libraries. All electronic parts are protected by a professional casing, to comply with safety precautions while in operation.

Interface/GUI
The use of clear labels is essential for the safe use of CARL by the medical staff. To achieve this, all descriptions use standard terms that are recognised by quali ed medical personnel. Figure shows a photo of CARL's user interface. To operate CARL, the user can set the desired I:E ratio and the respiratory rate. The current ventilation parameters and ventilation quantities are displayed on the screen. The red LED next to the description "Alarm" implies an error during operation. These errors can be de ned and implemented into our code.

Measurement of ventilation quantities
All pressure and ow measurements that are acquired with CARL are validated with an external setup which consists of a pneumotachometer heater control (Model 3850, Hans Rudolph Inc.) and a DC bridge ampli er (MIO-0501, FMI GmbH). To acquire and evaluate the data from this setup we use the Embla RemLogic Software.
To release the excess gas during ventilation, we use an exhalation valve (Whisper Swivel II, Philips Respironics) with a micro lter attached to it. The lter reduces the concentration of released air contaminants in the environment and thus, the overall infection risk for the medical staff. As a patient model, we use an arti cial lung (Test Lung 190, Maquet). All measurements are taken with two different CPAP machines from Löwenstein Medical: prisma SOFT and prisma25.