Methods for determining a cardiovascular parameter, e.g., a parameter reflecting fluid or volume responsiveness, using a modified waveform dataset are described. The waveform dataset corresponds to a signal, for example, from an arterial blood pressure, or any signal proportional to, or derived from the arterial pressure signal. These methods involve identifying individual cardiac cycles in the waveform dataset, measuring the waveform characteristics for the individual cycles, then determining if the individual cardiac cycles are regular cardiac cycles or irregular cardiac cycles. Once any irregular cardiac cycles are detected, a respiratory parameter is measured. Next, a modified waveform dataset containing the waveform characteristics of the regular cardiac cycles and the waveform characteristics of the irregular cardiac cycles is created wherein the waveform characteristics of the irregular cardiac cycles are replaced with estimated waveform characteristics. Finally, a cardiovascular parameter is determined using the modified waveform dataset.
