There is provided an apparatus for providing ventilatory assistance to a spontaneously breathing patient comprising: processing means (15) for computing a measure of the difference between a function of respiratory airflow and a target value, and a control mechanism (15) for causing a blower to deliver air to the patient at a pressure that is a function of said difference measure, the control mechanism (15) using a blower in a servo-controlled loop whose gain varies depending on the magnitude of said difference measure.
