## 2 Materials and Methods
### Design Overview
#### 2.1.1 Inspiratory Branch  
The gas connections (RS1, RS2) are used to connect the ventilator to the hospital's compressed air and oxygen supply, which typically operates at 4-5 bar. If CoroVent is used with gas cylinders, a standard medical pressure regulator must be installed on the cylinder. The air supply begins with a medical quick connector attached to a hose, which delivers air to the ventilator. The oxygen supply follows the same principle, using a dedicated oxygen quick connector. All materials in the oxygen circuit are oxygen-compatible. Inside the ventilator, the supply hoses are connected to pressure regulators (RV1, RV2), which reduce the hospital pipeline pressure to the ventilator's operating pressure of 200 \(\pm\) 2 kPa at a flow rate of 50 L min\({}^{-1}\) for both air and oxygen.  
After the pressure regulators, sensors (P1, P2) are placed in both branches to monitor the operating pressure. Based on the sensor data, the following alarms are provided: low air or oxygen pressure, large difference between the operating pressures of the two gases, and malfunction of one or both ON/OFF valves.  
Inspiratory flow generation is then provided by two fast ON/OFF valves (DV1, DV2) connected directly after the pressure regulators. Both ON/OFF valves use pulse width modulation (PWM) with constant switching frequency of 10 Hz. By setting the duty cycle (\(D\)) and the ON/OFF ratio of the air and oxygen valves, it is possible to generate the desired inspiratory flow rate while controlling the oxygen fraction in the ventilatory gas mixture, as depicted in Figure 3A-C. The sum of the ON/OFF valve duty cycles (\(D_{\text{sum}}\)) must not exceed 100% so that \(FiO_{2}\) can be set over the entire range from 21% to 100% even at maximum flow. In this ventilator, a constant inspiratory flow was selected. With proper timing, the ON/OFF valves also control the respiratory rate and the inspiratory-to-expiratory time ratio and generate the inspiratory hold.  
As shown in Figure 4A, a series of short electrical pulses at the ON/OFF valve with \(D=50\%\) creates high pressure oscillations forming an inspiratory flow. Acoustic filters (F1, F2) are then used to reduce these oscillations caused by the opening and closing of the ON/OFF valves. As shown in Figure 2, each acoustic filter consists of two throttle valves and a compliance between them [28]. The result is a smoothed pressure waveform behind the acoustic filter (Figure 4B).  
Figure 3: Electrical pulses and set duty cycles (\(D\)) on air ON/OFF valve (\(D_{\text{air}}\)) and oxygen ON/OFF valve (\(D_{\text{O2}}\)) at (**A**) maximum inspiratory flow (\(D_{\text{sum}}=100\%\)) and 1:1 ratio between the valves (\(FiO_{2}=60\%\)), (**B**) half inspiratory flow (\(D_{\text{sum}}=50\%\)) and 3:1 ratio between the valves (\(FiO_{2}=40\%\)), (**C**) maximum inspiratory flow (\(D_{\text{sum}}=100\%\)) with only air ON/OFF valve in operation (\(FiO_{2}=21\%\)).  
The air and oxygen branches are then joined (IS) and interconnected to a port for connection of the patient circuit. The IS coupling also interconnects these pneumatic circuits with a safety relief valve (BV) which is used to protect the patient from airway pressures exceeding 6 kPa (approximately 60 cmH\({}_{2}\)O) according to ISO standard [22]. The drawing of the safety relief valve design and its cross-section are shown in Figure 5A. An analog pressure gauge M1 indicates the pressure at the inlet to the patient circuit, but the precise _P_aw pressure is displayed on the ventilator screen and measured by the CoroQuant. The M1 pressure gauge provides the ventilator operator with information about the pressure throughout the entire respiratory cycle and is only an additional modality.