## 2 Design Considerations  
While the use of mechanical ventilation is a clinical practice that is normally only used on patients that require it as a life-sustaining measure, inappropriate ventilation of a patient could result in damage to the lungs that would also be life-threatening [11; 12; 13]. The breathing process might seem a relatively simple one at first glance. To first approximation it is not very different to inflating and deflating a balloon, however, that simplistic view is what physicists would call a spherical-cow-approximation. There are a number of subtleties that make respiration more complex. In the following, we provide a brief description of the breathing process based on [14]. Under normal circumstances, inhalation is driven by the unconscious motion of a series of muscles, mainly the diaphragm as the key muscle together with different other muscle groups of the chest wall, that increase the volume of the thoracic cavity. This leads to a decrease of the pressure in the lungs which, at its time, results in the flow of fresh air into the lungs through the airway. The subsequent relaxation of said muscles increases the pressure in the lungs that produces the exhalation. Therefore inhalation is usually produced by \"negative\" thoracic pressure, while exhalation is produced by \"positive\" thoracic pressure. When mechanical ventilation is applied, usually through an endotracheal tube, the inhalation is no longer driven by negative pressure produced by the patient, but by positive pressure provided by the ventilator that \"forces\" air into lungs, this is a complete game-changer in many senses. For instance, natural \"negative\" pressure inspiration cannot over inflate the lungs and its internal ramified structures, the alveoli, which can potentially be damaged by overpressure when \"positive\" pressure is applied. This could lead to damage of the lung, which is usually already compromised by the underlying condition, such as pneumonia caused by COVID-19. Therefore, the volume per inspiration (tidal volume) and the appropriate pressures across the breathing cycle must be limited. Unlike healthy subjects, who maintain a small volume of air in the lungs once the exhalation is over, critical patients in some cases produce a full exhalation. This causes the fluids that are in the lung to completely fill all the cavities, which means that it is more difficult to inflate the lung again in the following breath cycle, and making it more prone to injury by the mechanical ventilation. Therefore, a minimal positive pressure (positive end-expiratory pressure--PEEP) should be maintained in the lung at the end of the expiration in order to prevent the collapse of the lungs. It is worth mentioning that bag-valve resuscitators are single-use-products, thus, for each new patient, a new resuscitator is needed. Yet, hygienic standards are easy to follow, as the two mechanism prototypes themselves are not part of the pneumatic system itself. In addition, a connecting hose between the bag-valve system and the mask or endotracheal tube will be needed, which is normally not used in manual resuscitation. The total volume of that hose has to be significantly smaller than the tidal volume pumped in order to ensure appropriate exhaustion of the exhaled CO2 to the atmosphere. In addition to the purely pneumatic considerations, there are other aspects to be taken into account. For instance, forcing air into the lungs of a patient at a rate determined by the ventilator instead of the natural rhythm determined by the patient is extremely uncomfortable and requires deep sedation, which might complicate other aspects of the progress evaluation and treatment of the patient. Hence, a feedback mechanism, usually by pressure or flow, is desirable in order to trigger the breath cycle of the ventilator and thus improve the patient-ventilator synchrony.