A recently introduced semi-invasive monitoring system consists of an algorithm that provides beat-to-beat measurement of CI by analysis of the arterial blood pressure tracing. By using the autocalibration mode this software calculates the individual aortic compliance and systemic vascular resistance by taking patient data like age, height, weight and gender into account. After estimation of beat-to-beat stroke volume, a subsequent multiplication by the heart rate delivers pulse contour cardiac index. Furthermore, this device offers the opportunity for external calibration by a reference technique, e.g. pulmonary or transpulmonary thermodilution.