For the implementation of time-critical decision support al- gorithms in a clinical information system (CIS) a precise relation between medical interventions and effects needs to be established. We evaluated for selected drugs and infusions the relation in time between charted dose and effect on on-line hemodynamic variables. The time of the intervention was compared with the onset of the change of the hemodynamic variables as determined by new time series methods. The average time difference between intervention and calculated hemodynamic effect was 13.23 min (0-29) which did not differ significantly between different interventions. The marked lag between intervention and effect and the great variance of this lag pose an important problem for time-critical decision support. Even after optimizing data acquisition important factors will remain unaccounted for. Therefore, decision support systems may need extensive testing with real-world data before they are released into clinical practice. (supported by the Deutsche Forschungsgemeinschaft, Sonderforschungs-bereich 475 “Complexity Reduction in Multivariate Data Structures”)