Physical aspects of automatic respirators.
Objective of artificial ventilation is to maintain sufficient gas exchange with the lowest degree of mechanical burden to the lung that is possible. The basic principle of IPPV is a pressure gradient between the upper airways and the alveoli during inspiration, thus creating a positive gas flow to the lungs. Different technical arrangements can be chosen to overcome the resistances of the lung, depending upon whether the flow or the pressure is the underlying parameter for artificial ventilation. The driving system can be either compressed gas from a pipeline or an electrically driven pump or compressor. With the direct acting generator, the force of the driving mechanism directly inflates the lung, whereas the indirect acting generator has flexible coupling element between pump and patient. The switchover from inspiration to expiration is determined by the cycling parameter, uhich can either be the pressure, the flow, the volume or the time. The mode of cycling influences the reaction of the respirator to changing lung conditions. Only the volume and time cycled types maintain a constant ventilation volume with changing lung resistances. Lastly, the different behavior of respirators during expiration is discussed. An active support of expiration by means of a negative pressure has less meaning nowadays. On the other hand, a positive end expiratory pressure (PEEP) has turned out to be a very helpful tool in long-term ventilation.
D000403:Airway Resistance; D006801:Humans; D008168:Lung / Q000502:physiology; D012123:Pulmonary Ventilation; D012119:Respiration; D012121:Respiration, Artificial; D012122:Ventilators, Mechanical