An apparatus for determining a physiological parameter of a patient, such as cardiac output, comprises a pressure sensor device providing readings of an arterial blood pressure, memory means storing said readings as a pressure curve over time, evaluation means determining a mean arterial blood pressure from the pressure curve and determining the physiological parameter using the mean arterial blood pressure is provided. The evaluation means determine a spectral density of said pressure curve and/or a variance of said arterial blood pressure. The evaluation means also determine at least one model parameter representing an effective value of a heart beat using the mean arterial blood pressure and said spectral density and/or variance. The effective value is selected from an effective amplitude of the heart beat, an effective duration of the heart beat and an effective area under the heart beat. The physiological parameter is determined using at least one of the model parameters.
