A computer-implemented method of determining a cardiovascular parameter comprising the steps: receiving a waveform dataset corresponding to an arterial blood pressure, or a signal proportional to, or derived from, the arterial blood pressure signal; identifying individual cardiac cycles in the waveform dataset; measuring waveform characteristics for the individual cardiac cycles; determining if the individual cardiac cycles are regular cardiac cycles or irregular cardiac cycles; characterized in that measuring a respiratory parameter which is a respiratory-induced variation in the waveform dataset caused by respiration; creating a modified waveform dataset containing the waveform characteristics of the regular cardiac cycles and the waveform characteristics of the irregular cardiac cycles wherein the waveform characteristics of the irregular cardiac cycles are replaced with estimated waveform characteristics, wherein replacing the waveform characteristics of the irregular cardiac cycles with estimated waveform characteristics comprises: determining, based on the waveform characteristics of regular cardiac cycles, pseudo-starting points and/or pseudo-ending points for the irregular cardiac cycles which have longer durations than regular cycles, and directly measuring or calculating waveform parameters/characteristics of the irregular cardiac cycle based on the pseudo-starting points and/or pseudo-ending points; and calculating a cardiovascular parameter using the modified waveform dataset, wherein the cardiovascular parameter is a parameter reflecting preload dependence and fluid responsiveness, such as stroke volume variation, pulse pressure variation, systolic pressure variation, stroke volume, cardiac output, systemic vascular compliance, cardiac flow, cardiac flow velocity, vascular compliance, or vascular elastance. receiving a waveform dataset corresponding to an arterial blood pressure, or a signal proportional to, or derived from, the arterial blood pressure signal; identifying individual cardiac cycles in the waveform dataset; measuring waveform characteristics for the individual cardiac cycles; determining if the individual cardiac cycles are regular cardiac cycles or irregular cardiac cycles; characterized in that measuring a respiratory parameter which is a respiratory-induced variation in the waveform dataset caused by respiration; creating a modified waveform dataset containing the waveform characteristics of the regular cardiac cycles and the waveform characteristics of the irregular cardiac cycles wherein the waveform characteristics of the irregular cardiac cycles are replaced with estimated waveform characteristics, wherein replacing the waveform characteristics of the irregular cardiac cycles with estimated waveform characteristics comprises: determining, based on the waveform characteristics of regular cardiac cycles, pseudo-starting points and/or pseudo-ending points for the irregular cardiac cycles which have longer durations than regular cycles, and directly measuring or calculating waveform parameters/characteristics of the irregular cardiac cycle based on the pseudo-starting points and/or pseudo-ending points; and calculating a cardiovascular parameter using the modified waveform dataset, wherein the cardiovascular parameter is a parameter reflecting preload dependence and fluid responsiveness, such as stroke volume variation, pulse pressure variation, systolic pressure variation, stroke volume, cardiac output, systemic vascular compliance, cardiac flow, cardiac flow velocity, vascular compliance, or vascular elastance.