A system which generates a warning signal in the event of looming pulmonary edema and/or looming decompensation. The system has an impedance detection unit for determining impedance values, which represent a transthoracic impedance course, and an impedance analysis unit (78), which is connected to the impedance detection unit. The impedance analysis unit (78) is implemented to determine the degree of modulation for a particular impedance course detected by the impedance detection unit.