A patient-specific model can show changes in cardiac stroke volume or cardiac output, such as to predict heart failure or to indicate cardiac remodeling. The patient-specific model can be derived from a surrogate indication of a cardiac stroke volume, such as a physical activity level, and features obtained from a thoracic impedance waveform, such as mean or peak-to-peak impedance values. In an example, several models corresponding to different patient physical activity levels can be determined.