Can CPAP machines be repurposed to solve the ventilator shortage?

doi:10.21203/rs.3.rs-83143/v1

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Can CPAP machines be repurposed to solve the ventilator shortage?

https://doi.org/10.21203/rs.3.rs-83143/v1

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published 09 Jun, 2021

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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.

Pulmonology

Mechanical Engineering

covid19

emergency ventilator

CPAP

ventilator shortage

mechanical ventilation

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^3–5. Such a treatment, however, cannot be provided due to a long-term worldwide issue: the shortage of medical-grade ventilators⁶. 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 conditions⁷. 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 ^8–12. 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^15–17 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-defined 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 modification that regulates the constant airflow 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.

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 airflow 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 flow 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, flow and ventilation volume for different ventilation parameters. As a representative patient model, we use an artificial lung (details in ‘Methods’).

3.1 Pressure-related ventilation settings

Ventilation strategies for COVID-19-patients include a minimum airway pressure of at least 5 cmH₂O that must be maintained during the exhalation phase to stabilise the alveoli^5,18–21. Therefore, for our first set of measurements we vary the P_IPvalue while keeping the respiratory rate and the I:E ratio fixed. P_IP value is set and adjusted on the CPAP machine. Our measurements of the airway pressure, flow and volume as a function of time for two different P_IP values can be observed in Figure 1a (P_IP = 15 cmH₂O) and 1b (P_IP = 20 cmH₂O). 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 cmH₂O or 15 cmH₂O, we get a PEEP value of 7.5 cmH₂O or 5 cmH₂O, 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.

3.2 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 first measurement, we vary the I:E ratio while the P_IP value and the respiratory rate are fixed. The acquired curves of the airway pressure, flow and volume as a function of time are shown in Figure 2a. An evaluation of the inhalation and exhalation durations confirms the correct implementation of the I:E ratios with CARL.

Our measurements for ventilations with different respiratory rates while the P_IPvalue and the I:E ratio are fixed 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.

3.3 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 airflow 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.

The previous results demonstrate that our module CARL can repurpose CPAP machines to basic ventilators which provide ventilation pressures up to 20 cmH₂O. 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.

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 short-term. 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.

6.1 Ventilation specifications for a minimally acceptable ventilator

Our set of ventilation specifications 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 definition of the ventilation variables and modes we use the terminologies and the guidelines provided by Charburn et al. ²⁴.

Ventilation Variable	Description	Values
Triggering mechanism	A method to initiate the inspiration phase.	time-triggering, flow-triggering
Cycling mechanism	A method to end the inspiration phase.	time-cycling, flow-cycling
Breath control variable	A parameter that describes the mechanism to assist the patient’s breathing.	pressure-control (PC)
Breath sequence	A pattern of mandatory and/or spontaneous breaths.	intermittent mandatory ventilation (IMV)
Targeting Scheme	A method used by the ventilator to achieve a specific ventilation pattern.	set-point (s)
Ventilation Mode	A term to describe the set of ventilation operations based on the selected breath control variable, breath sequence and targeting scheme.	PC-IMVs,s
I:E ratio	The ratio of the inspiration (I) and the expiration (E) phase during ventilation.	1:1, 1:2
Respiratory rate	The amount of breaths per minute (bpm). Here, it is the number of mandatory breaths per minute. The number of spontaneous breaths per minute cannot be lower than the number of mandatory breaths per minute.	8 - 30 bpm
Positive end-expiratory pressure (PEEP)	The airway pressure at the end of the exhalation phase.	at least 5 cmH₂O
Peak inspiratory pressure	The airway pressure set at the CPAP machine.	15 - 20 cmH₂O
Maximum airway pressure	The upper pressure limit in the patient’s airways.	30 cmH₂O

Table 1: Specifications for a minimally acceptable ventilator to treat COVID-19 patients with mild or moderate symptoms.

In addition to the ventilation specifications, 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.

Table 2: Design requirements for CARL to ensure the patient’s safety during ventilation.

6.2 CARL

For the development of our module CARL, we aim for a mass-producible solution that can be manufactured worldwide. To fulfil 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 airflow stops and the airway pressure drops to zero. A flexible tube is used to bypass the valve and to allow for a continuous minimum airflow to the patient. That way, a PEEP of at least 5 cmH₂O 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 cmH₂O, 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 FiO₂value, can be extracted by connecting a patient monitor to the provided connector.

To measure the airway pressure and the flow, we use a flow sensor (No. 281637, Hamilton Medical) that is connected to two implemented pressure sensors (AMS5915, Analog Microelectronics GmbH). The flow 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.

6.3 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 qualified 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 defined and implemented into our code.

6.4 Measurement of ventilation quantities

All pressure and flow 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 amplifier (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 microfilter attached to it. The filter 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 artificial lung (Test Lung 190, Maquet). All measurements are taken with two different CPAP machines from Löwenstein Medical: prisma SOFT and prisma25.

Acknowledgements

We acknowledge financial support from the Minister for Higher Education, Research and the Arts of the German State of Hessen and from the Sparkasse Marburg-Biedenkopf. We also thank Dr. Tobias Breuer, Anna Lena Isenberg, Sven Heuer, Philipp Bergen, Lea Eller, Meral Kara, Fritz Winter Eisengießerei GmbH & Co. KG and Conrad Electronic for their support throughout this research.

Author contributions

GK, KK proposed the concept for CARL; ECC proposed the study supported by MK, MK coordinated the workgroup; CB, JH, PL, AP, DG contributed to the development of the software framework; GK, CB, JH, CS, CSo contributed to the implementation of the electronic components; PM, NH, BLT, HB, GK, KK contributed to the development of the CARL prototype; KK, JN, JH performed and evaluated all measurements; RH, PW, TW, BB provided medical experitise; KK, JH, PL, CB, JN contributed to the documentation; JN, GS, MK provided coordination between the medical, mechanical/software developers and test experts; The manuscript was written by JN, ECC. All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

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Download PDF

Journal Publication

published 09 Jun, 2021

Read the published version in Scientific Reports →

Version 1

posted

You are reading this latest preprint version

Can CPAP machines be repurposed to solve the ventilator shortage?

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

3.1 Pressure-related ventilation settings

3.2 Time-related ventilation settings

3.3 Patient-triggered/Patient-cycled ventilation

Discussion

Summary And Outlook

Methods

6.1 Ventilation specifications for a minimally acceptable ventilator

6.2 CARL

6.3 Interface/GUI

6.4 Measurement of ventilation quantities

Declarations

Acknowledgements

Author contributions

Competing interests

References

Status:

Journal Publication

Version 1