The apparatus comprises a pressure sensor providing readings of a blood pressure of the patient and storage means for storing the readings as a pressure curve over time. Blood pressure is measured. The frequency difference between heart rate and breathing rate is used to separate the respiratory effect from the heart activity. In particular, the Fourier transform of the blood pressure and the spectral density are used to determine the contribution of each frequency. The respiratory and cardiac power spectra are determined. The ratio between both powers is calculated as the quotient of the integrals over the cardiac power spectrum and the respiratory power spectrum. A parameter usable to characterize volume responsiveness is determined using above ratio and a correction factor.
