1. A method for determining a beat-to-beat stroke volume or a cardiac output based on at least one measurement of arterial pressure data, the method comprising the steps of: computing a compliance or impedance in dependence of at least one measurement of arterial pressure data using a non-linear model; using said compliance or impedance in a pulse contour method for determining the beat-to-beat stroke volume or cardiac output based on the measured arterial pressure data; approximating parameters of the non-linear model based on a further model based on a person-specific characteristic; and using the approximated parameters for the non-linear model.