The Anesthesia Machine - Anesthesia Ventilators
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collapsible bellows within a closed chamber
Conventional anesthesia machines are fitted with a mechanical ventilator that uses a collapsible bellows within a closed chamber. The bellows is compressed intermittently when oxygen or air is directed into the chamber, thereby pressurizing it. The ventilators are time-cycled flow (as opposed to pressure) generators, controlled both mechanically and electronically and pneumatically driven (requiring 10 to 20 L of driving gas per minute). Ventilator controls vary among makes and models. Some ventilators require setting of minute ventilation, rate, and inspiratory/expiratory (I/E) ratio to produce the desired tidal volume; other ventilators allow direct adjustment of the tidal volume, with I/E ratio dependent on the inspiratory flow rate (which is set independently). A portion of the fresh gas flow delivered by the machine adds to the set tidal volume during the inhalation phase. For example, an increase in total fresh gas flow from 3 to 6 L/minute will increase delivered minute ventilation by an additional 1 L/minute at an I/E ratio of 1:2 or by 1.5 L/minute at an I/E ratio of 1:1 (more inspiratory time in the latter). Although gas-driven ventilators can be safely driven with either oxygen or air, most often oxygen is chosen and is supplied by pipeline. Whether cylinder gases are used to drive the ventilator in the event of pipeline failure is usually determined by the user. If the machine is set up to drive the ventilator using cylinder oxygen, mechanical ventilation should be discontinued in the event of pipeline failure to conserve oxygen supplies.