Abstract: A method and apparatus are described for utilizing a source of vascular pulse waveform data from a patient for the purpose of measuring pulsus paradoxus. The arterial pulse waveform data source described is a pulse oximeter plethysmograph but can be any similar waveform data source, including an intra-arterial transducer, external blood pressure transducer, or plethysmograph. Through incorporation of measurements of values, such as an area under a pulse waveform curve, that are time-domain functions of a change in amplitude of the pulse waveform over a duration of the waveform, embodiments of the present invention represent a significant improvement upon previously described methods of measuring pulsus paradoxus.