Ventilator comprising a controllable respiratory gas source for predetermining ventilation parameters in the respiratory gas supply of a patient with periodic respiration, comprising: a connecting tube to a patient interface, a statistics module, means (21, 22) for establishing a current measure of a ventilation parameter, storage means for storing a measure of a preceding ventilation parameter in a recallable manner, comparison means for deriving a measure for the change in the current ventilation parameter compared to the preceding ventilation parameter, means for establishing a current measure of the respiration, storage means for storing a measure of the preceding respiration in a recallable manner, comparison means for deriving a measure for the change in the current respiration compared to the preceding respiration, comparison means for deriving a measure for the change in the respiratory exertion from at least one of the measures (31, 41), a control apparatus which controls at least one ventilation parameter dependent on the measure, characterized in that the control unit, by way of the statistics module, determines changes in the trend in relation to one or more ventilation parameters (30, 30') over a period of time, wherein the changes in the trend are stored in the memory, wherein the ventilator establishes a measure for the current respiration by virtue of the respiration of the patient being recorded by means of a flow and/or pressure sensor, wherein the measure for the current respiration is established from the pressure and/or flow profile, and this measure of the current respiration is stored in the memory, wherein, by calculation, a measure for the change in the respiration is established from the values of the measures of the respiration of the last and second-to-last breaths, wherein a measure is established for the current ventilation by virtue of the ventilation being recorded by the instrument by means of the flow and/or pressure sensor and this measure of the current ventilation is stored in the memory and, by calculation, a measure is established for the change in the ventilation from the values of the measures of the respiration of the last and second-to-last breaths, wherein, by calculation, a measure is established for the change in the respiratory exertion of the patient from the values of the measures of the change in the ventilation and change in the respiration, wherein the control apparatus, as ventilation parameter, controls at least the IPAP pressure and/or the EPAP pressure for the subsequent breath dependent on the change in the respiratory exertion in the preceding breaths. a connecting tube to a patient interface, a statistics module, means (21, 22) for establishing a current measure of a ventilation parameter, storage means for storing a measure of a preceding ventilation parameter in a recallable manner, comparison means for deriving a measure for the change in the current ventilation parameter compared to the preceding ventilation parameter, means for establishing a current measure of the respiration, storage means for storing a measure of the preceding respiration in a recallable manner, comparison means for deriving a measure for the change in the current respiration compared to the preceding respiration, comparison means for deriving a measure for the change in the respiratory exertion from at least one of the measures (31, 41), a control apparatus which controls at least one ventilation parameter dependent on the measure, characterized in that the control unit, by way of the statistics module, determines changes in the trend in relation to one or more ventilation parameters (30, 30') over a period of time, wherein the changes in the trend are stored in the memory, wherein the ventilator establishes a measure for the current respiration by virtue of the respiration of the patient being recorded by means of a flow and/or pressure sensor, wherein the measure for the current respiration is established from the pressure and/or flow profile, and this measure of the current respiration is stored in the memory, wherein, by calculation, a measure for the change in the respiration is established from the values of the measures of the respiration of the last and second-to-last breaths, wherein a measure is established for the current ventilation by virtue of the ventilation being recorded by the instrument by means of the flow and/or pressure sensor and this measure of the current ventilation is stored in the memory and, by calculation, a measure is established for the change in the ventilation from the values of the measures of the respiration of the last and second-to-last breaths, wherein, by calculation, a measure is established for the change in the respiratory exertion of the patient from the values of the measures of the change in the ventilation and change in the respiration, wherein the control apparatus, as ventilation parameter, controls at least the IPAP pressure and/or the EPAP pressure for the subsequent breath dependent on the change in the respiratory exertion in the preceding breaths.