High Frequency Jet Ventilation (HFJV) describes a technique to provide mechanical ventilatory support with frequent small volume breaths (greater than or equal to 100/min). The purpose of this research is to identify clinical conditions where this from of ventilation is most likely to benefit patients who respond poorly to conventional mechanical support. Since the main characteristics of HFJV is to maintain alveolar ventilation withou the development of high peak inspiratory pressure, its efficacy will be investigated in patients at high risk of pulmonary barotrauma, such as those requiring positive end-expiratory pressure (PEEP) and in patients with pathologic airway disruption. The use of HFJV as a cardiac assist device will also be investigated.