MECHANICAL VENTILATION

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KEY QUESTIONS

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VENTILATOR

A medical device that provides mechanical ventilation by moving air in and out of patient's lungs, where oxygen from the air and carbon dioxide from the blood are exchanged.

During a normal breathing cycle, an inhalation phase and an exhalation phase can be distinguished. A ventilator can be used for the inhalation phase only if the patient's lungs are able to carry out the exhalation phase. This means that the lungs must be flexible enough to exhalate on their own, in a way similar to a balloon deflating.

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When all the conditions are met, doctors prescribe therapy and device parameters are adjusted accordingly. When a patient is attached to a ventilator, the machine replaces respiratory mucle function and provides ventilation. These machines are very important in situations where respiratory muscles are not strong enough, but also where breathing is difficult and where more air pressure is needed to inflate the lungs. As part of the therapy, doctors prescribe appropriate air composition (volumes of different gases in the mixture, usually a percentage of oxygen), breathing phases duration ratio, number of breathing cycles per minute, as well as inspiratory pressure or volume and expiratory pressure.

TECHNICAL REQUIREMENTS

  • 1
    The first and basic requirement for a ventilator is that the device is able to perform the therapy prescribed to save the patient's life. The settings must be easily accessible, easy to use and exact.
  • 2
    The device must be safe and able to endure at least 14 days of continuous operation.
  • 3
    The device must have a safety valve to ensure that the air pressure in the systems stays whitin safe limits in case of device failure, which could endanger the patient's life.
  • 4
    All the connectors must comply with existing standards for the device to be easily used within existing infrastructure.
  • 5
    The device must have sound and visual signaling in case of a pneumatic or electrical system fault, device switching off during operation or if the chosen therapy cannot be achieved.
  • 6
    If the device requires electricity for functioning, it must have a battery backup in case of main power supply failure. The backup battery should be able to supply power for at least 20 minutes of operation and hot swappable batteries can be utilised if battery supply is needed for an extended period, e.g. 2 hours for hospital transfer.
  • There are many other requirements as well as recommendations provided by official regulatory bodies (see, for example, a document issued by UK regulatory agencies on the gov.uk special page).

LUNGSTORS

You can learn more about ventilators only if you attempt to make one. We named our attempt LUNGSTORS and it is a blower-based ventilator. Within the answer to the question of what a ventilator is, a simple explanation of its operation is given. After consideration of all the technical requirements, we obtained the following device schematic.

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BLOWER

Within the ventilator pneumatic system, the purpose of the blower is to enable the flow of gas from a lower to a higher pressure point, from the gas mixing point to the patient's lungs, and that's why it is the most important part of the system. The blower used in this ventilator was specially designed to provide the required pressure and airflow. From the aspect of power flow, the blower converts electricity into mechanical work that is used for compressing and moving air.

The blower impeller is powered by a brushless, direct current electric motor and its angular velocity is controlled by special electronics. This type of electric motors is widely available today because of its use in small UAVs. In addition to the complex geometry of the case and impeller, the impeller must be properly balanced because it spins very fast. If faster response time is required, the effect of impeller inertia can be compensated using the dynamic braking. Prototyping can be quickly and easily done on a 3D printer.


ELECTRONICS

In addition to supplying electricity to the electric motor and controlling its angular velocity, the electrical system is responsible for sensor data acquisition, actuator control, and user interface. The user interface should provide an easy way of adjusting device settings and audible and visual information about device operation.

According to the previous, the system consists of three printed circuit boards. The first one for continuous power supply (PS BOARD), the second one for process control (PROCESS BOARD), and the last one for user interface (CONTROLS BOARD). When the main power supply is disconnected, the PS BOARD automatically switches to the battery backup supply. This battery is rechargeable and can be replaced to extend operation time without switching the device off.

The electrical system is based on a PROTORS_STM32F1 development board. It is small in size and uses a 32-bit microcontroller that can be programmed using the Arduino integrated development environment and the official Arduino_Core_STM32 library. The ST-LINK/V2 programmer is used to transfer the program to the microcontroller. This kind of approach (use of custom made electronics) enables the implementation of advanced functionality such as remote control and device performance monitoring.

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SERVO VALVE

For the inhalation and the exhalation phase, two pneumatic valves are used to control the gas flow through the system. One of the technical requirements for the ventilator is that it is able to maintain positive expiratory pressure in the lungs. In addition to controlling the gas flow path, a servo valve also enables flow rate control by changing the pressure drop. In this way, the required operating mode can be achieved by simultaneously controlling the blower and the servo valves. One of the advantages of this approach is a reduction of the impeller inertia effect during acceleration.

Structurally, this servo valve consists of a sphere with an opening, a servo motor used to rotate the sphere, housing, and gaskets. A servo motor of this size and power usually consists of a DC electric motor, gears, a position sensor, and a control unit. Closed-loop control is achieved using the position sensor and this makes it possible to precisely orient the opening on the sphere. This type of actuator is often used in modelmaking for radio-controlled models. It is are widely available and the control signal can be easily generated using a microcontroller. Prototyping can be quickly and easily done using a 3D printer and by cutting gaskets.


FLOW METER

Static gas pressure and the gas flow rate measurements during inhalation and exhalation phases are needed for controlling machine operation. The system measures static pressure in two places using piezoresistive transducers (such as the popular NXP Semiconductors MPXV series differential pressure sensors) and the same kind of sensor is used to indirectly measure the flow rate.

Considering the pneumatic system characteristics, an orifice plate flow meter was chosen. This type of flow meter is very easy to make and the flow rate is easily calculated from the static pressure difference before and after the orifice. Prototyping can be quickly and easily done using a 3D printer. Calibration is needed to determine the mathematical relationship between pressure drop and flow rate.

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VIDEO

If you want to see all of our future videos as soon as we post them, please follow our YouTube channel. Following video is made using OpenShot Video Editor and Inkscape.

SOFTWARE USED

All of technical documentation given here is made using FreeCAD. Using the FreeCAD native file format, anyone can see how we designed all the models and can easily edit and adapt the design to personal needs. FreeCAD is an open-source parametric 3D modeler made primarily to design real-life objects of any size. The program is licensed under Lesser General Public Licence version 2 or superior (LGPL2+).

You can start learning FreeCAD within courses of the Aerospace Engineering Department at the Faculty of Mechanical Engineering, University of Belgrade (for more information contact us at info@protors.co).

freecadweb.org
KiCad
Electronics Design
Blender
Rendering and animation
Inkscape
Vector graphics
Arduino
Microcontroller programming
STM32
Library for STM32
QElectroTech
Pneumatic and electrical schemes

LICENSE

All of technical and other documentation is given under the terms of the license CERN-OHL-P version 2. Open source hardware is a hardware whose design is made publicly available so that anyone can study, modify, distribute, make, and sell the design or hardware based on that design. Open source hardware gives people a freedom to control their technology while sharing knowledge and encouraging commerce through the open exchange of designs.

RESPONSIBILITY

The development of medical devices is legal and one of the goals of this project is to encourage the development and further improvements, but their use is not legal before obtaining appropriate permits from the regulatory institutions. The use of such devices is done at your own risk and marketing or use for medical purposes is strictly prohibited. This project is a demonstration of a development process that can result in a life-saving medical device. It is important to be aware that the life of a patient often depends on this device. Any type of device failure during its use, due to mistakes in design, manufacture or maintenance is unacceptable.

Based on all the foregoing, the authors of this project deny any responsibility. The documentation provided is offered without warranty of any kind for functionality or use, and also without limitation for learning, modification, production and distribution of products based on this documentation.

WE ARE PROTORS

A team of mechanical engineers, professors and PhD students specializing in a variety of fields. PROTORS can provide engineering consulting and product development services to clients worldwide. Our joint knowledge and experience in solving complex and interdisciplinary engineering problems using state of the art methods is what makes us unique.
The scientific institutions and companies that helped us in the realization of the project:
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