Mechanical ventilation can be defined as the technique through which gas is moved toward and from the lungs through an external device connected directly to the patient. The clinical objectives of mechanical ventilation can be highly diverse: To maintain gas exchange, to reduce or substitute respiratory effort, to diminish the consumption of systemic and/or myocardiac O2, to obtain lung expansion, to allow sedation, anesthesia and muscle relaxation, and to stabilize the thoracic wall, etc. Ventilation can be carried out by negative extrathoracic pressure or intermittent positive pressure. According to the cycling mechanism, positive-pressure ventilators are classified as pressure-cycled, flow-cycled, or mixed, and according to the type of flow in continuous-flow ventilators, as intermittent flow or constant basic flow. Finally, high-frequency ventilators are classified according to their high-frequency mechanism as intermittent positive pressure, oscillatory high-frequency and high-frequency jet ventilators.