The present invention relates to the use of the arterial pressure waveform recorded invasively or non-invasively, to distinguish between compensatory vasoconstriction with low cardiac output on the one hand from vasodilation from more severe organ damage on the other, as a cause of hypotension in acute emergencies. The waveforms may be subjected to harmonic analysis and the moduli of their harmonic components compared whereby a hypotensive individual can be confirmed to have the higher (second and above) greater than the first harmonic is considered as having vasoconstriction as a cause of hypotension.