A microprocessor controlled ventilator controls a patient's breathing selectively. In a preferred aspect, the ventilator utilizes intratracheal pulmonary ventilation and a reverse thrust catheter to provide all patient inspiration. The microprocessor connects to a pneumatic subsystem which includes a variety of sensors and actuators and the catheter disposed in the patient's trachea. A feedback control loop provides a basis for controlling selected actuators within the subsystem to control the patient's breathing. The microprocessor provides the selective venting of expiration gases, and the selective mixing of treatment gases, such as anesthesia. The ventilator has a plurality of possible ventilating modes, each selectable by the user, thereby reducing the number of hospital equipment needed to all forms of ventilation required by a patients.