A new ventilator is described which is capable of interposing controlled breaths synchronized with the patient's own breathing rhythm. This ventilation pattern is called "intermittent assisted ventilation" (IAV). It differs from intermittent mandatory ventilation (IMV) in that each ventilator cycle is triggered by the patient. IAV constitutes a new approach to the problems during the critical period of weaning from mechanical ventilation. Further, this new ventilator provides means for continuous display and recording of airway gas flow and pressures and expired minute volume (EMV) during different types of ventilation, e.g. controlled ventilation, intermittent assisted, and spontaneous ventilation.