A device and method for controlling a level of ventilatory assist applied to a patient by a mechanical ventilator measures, during patient's assisted breath, an inspiratory volume Vassist produced by both the patient and the mechanical ventilator, an inspiratory volume Vvent contributed by the mechanical ventilator, and an inspiratory assist pressure Pvent produced by the mechanical ventilator. A first relation between pressure Pvent and volume Vassist and a second relation between pressure Pvent and volume Vvent are calculated. Using the first and second relations, a ratio is determined between pressure Pvent at volume Vvent and pressure Pvent at volume Vassist, with volume Vvent equal to volume Vassist, for a plurality of volumes Vvent and Vassist. Values of Pvent are multiplied by the corresponding calculated ratios to calculate a third relation between a predicted inspiratory pressure Ppred and volume Vassist. The mechanical ventilator is responsive to the third relation to control the level of ventilatory assist.
