A monitoring arrangement 100 is configured to predict a rapid symptomatic drop in a subject's blood pressure, e.g. during a medical treatment or when operating aircraft. To this aim, a pulse shape parameter (pps) with respect to a peripheral body part (105) of the subject (P) is repeatedly registered by means of a pulse oximetry instrument (110) adapted to detect light response variations in blood vessels. A respective pulse magnitude measure is calculated based on each of a number of received pulse shape parameters (pps), and a statistical dispersion measure is calculated based on the thus-calculated pulse magnitude measure. It is investigated whether or not the statistical dispersion measure fulfils a decision criterion relative to a reference measure. An output signal (α) is generated if the decision criterion is found to be fulfilled.