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General Project Description
In face of the urgent need to offer pulmonary ventilation devices that can be mass produced, due to COVID-19 disease, we present our proposal in the public domain for the development of a simple and easily replicable Pulmonary Ventilator. It is not a certified device for use, however, eventually, it can be used in patients with COVID-19, as a last option of respirator.
In Figure 1 is depicted a diagram of the device assembly, which uses a hybrid ventilation mode with constant flow limited to pressure. The VExCO can be divided into subsystem of control and monitoring electronics, and subsystem of high and low pressure, which consists of:
- Pressure regulators connected to the gas lines (compressed air and medical O2), ball valves, retainers, and a blender.
- A solenoid valve, which adjusts the flow of the driving pressure during inspiration.
- A pop-off valve to prevent the pressure inside the system from exceeding levels considered dangerous.
- One solenoid valve (Normally Closed) that will be activated by a programmed timer circuit to establish the respiratory rate and the TI: TE ratio (ratio between inspiratory time and expiratory time - wave duty cycle).
- Independent needle valves.
- One Heat and Moisture Exchanger (HMEF) filter to prevent contamination, it connects the device to the patient.
Figure 1: Project Diagram (the flow signal was measured between the HMEF filter and the mechanical model).
Prototype Operation
The pressure regulators are adjusted to the value of 1 kgf/cm2 during product assembly, and its manometer is visible so that the user can check the line pressures. The value of O2 percentage in the mixture is determined by combinations of the two ball valves' positions, one for each gas. These valves are robust and work only in the open or closed positions, allowing the adjustment of 3 O2 concentrations: 21 %, 60 % and 100 %. These ball valves are also visible for user adjustment and are labeled. The concentration table is also identified on the body of the prototype. Retention valves prevent gases from returning to the line in the event of improper adjustment of the regulators or failure to supply one of the gas lines. The blender reservoir has a volume of 750 ml and supplies the low pressure pneumatic circuit, providing the peak flow required for the respiratory system. This reduces the demand on the high pressure network, oscillations and transients.
Upstream of the compressed air ball valve, the circuit has a bifurcation to produce the Positive End-Expiratory Pressure (PEEP) and driving pressure adjustment signals, which are fluxes adjusted by independent needle valves, which add up and passes through a pneumatic resistance. The passage of this flux through the resistance produces a pilot pressure from the Ins/Exp valve. The flux for the PEEP remains constant throughout the breathing cycle, while the driving pressure adjustment flow is switched during inspiration. Thus, during expiration there is a small pilot pressure (PEEP) and during inspiration there is a higher pilot pressure (PEEP plus driving pressure).
From the reservoir output, another solenoid valve controls the switching of the inspiratory flow, whose amplitude can also be adjusted by another needle valve. This flux is applied to the patient circuit via one of the one-way valve inputs. The flux also passes through the Ins/Exp valve, through the HMEF filter and goes to the patient. If the pressure in the patient's airways becomes greater than the PEEP plus the driving pressure, the Ins/Exp valve opens gradually, allowing air to escape into the environment, and keeping the pressure approximately constant inside the respiratory system. During expiration, the pilot pressure of this Ins/Exp valve is reduced by closing one of the pilot circuit solenoid valves, so the valve must open in order to allow the relaxed expiration. When the airway pressure in the respiratory system becomes lower than the pressure defined by the pilot circuit as the PEEP pressure, the Ins/Exp valve closes. In the prototype assembly, the maximum inspiratory flow is initially adjusted to 0.6 l/s, enough to allow 500 ml of volume during ventilation with a frequency of 30 cycles per minute and an I: E ratio of 1: 1 (estimated volume for a person with 80 kg of body mass predicted in protective ventilation). This flux is smaller than the peak flux demanded by conventional PCV (Pressure Controlled Ventilation) mode, but high enough for the pressure limiter to act, even at high frequencies, to maintain the stipulated driving pressure.
If a fault condition raises the pressure inside the system to levels considered dangerous, a pop-off valve, installed in the body of the one-way valve, is activated, limiting the pressure to 60 cmH2O. In addition, spontaneous breathing cycles are allowed through the one-way valve whenever the prototype is not in a mandatory inspiration. The microprocessor control system is also capable of alarming in other fault conditions such as disconnection of the patient, ventilation different from the one set on the panel, lack of pressure in the line, lack of power in the electrical network, among others.
Figure 2 shows the flow rate waveforms (also known as flow in the medical field) and pressure. The flow rate is in L/s unit and the pressure in cmH2O. In the tests, the solenoid switching was manual, so the respiratory rate did not remain constant. The signal excursion (pressure variation at inspiration) is regulated by the flow that passes through the pneumatic valve.
Figure 2: Flux and pressure waveforms.
Normal values for physical quantities
- The pressure source is common in hospitals and is usually in the range of 2 to 4 kgf/cm2.
- It is desired that the maximum pressure in the respiratory system does not exceed 40 cmH2O.
- Frequencies are in the order of 10 cycles per minute.
- TI:TE ratio around 1:2 or 1:1.
- PEEP up to 15 cmH2O.
- Maximum inspiratory flow (flow rate) around 30 L/min.
- Model of lung/respiratory system: resistance around 20 cmH20/L/s and complacency of 0.05 L/cmH2O.
There are several sets of specifications made public by different entities, such as the examples below:
- https://www.gov.uk/government/publications/coronavirus-covid-19-ventilator-supply-specification/rapidly-manufactured-ventilator-system-specification;
- https://www.who.int/emergencies/what-we-do/prevention-readiness/disease-commodity-packages/dcp-ncov.pdf?ua=1.
First Results
Initial tests were conducted using the mechanical model of respiratory system TTL® (Michigan Instruments, USA) to simulate the patient, with parabolic resistance n° 20 and nominal complacency of 0.05 L/cmH2O (similar results were found for a model with resistance of 10 cmH2O/L/s and complacency of 0.04 L/cmH2O, as suggested in the literature for a model of this type of injury - Acute Respiratory Distress Syndrome (ARDS)). The signals were measured at the airway entrance of the simulated patient. The PEEP levels obtained with this assembly cover a range of the histogram shown in [1]. The tidal volume (breathed at each respiratory cycle) was around 500 mL, as the pressure excursion chosen was around 10 cmH2O (in this ventilatory mode the volume is a consequence of the mechanical characteristics of the respiratory system and the chosen pressure excursion, but protects the lung against excessive pressure that can aggravate lung injury). The flow rate signal (or flow, in clinical jargon) - with a typical shape of the Pressure Controlled Ventilation (PVC) mode - and the pressure signal at the entrance of the airways (mouth) are shown in Figure 2 ( the units are L/s and cmH2O respectively). In the tests, the solenoid switching was manual, so the respiratory rate did not remain constant.
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