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RATIONALE Sedation Practice in Intensive Care Evaluation (SPICE-III) trial reported significant heterogeneity in mortality with dexmedetomidine treatment. Supplemental propofol was commonly used to achieve desirable sedation. OBJECTIVES to quantify the association of different infusion rates of dexmedetomidine or propofol, given in combination, with mortality and if this is modified by age. METHODS AND MEASUREMENTS We included 1177 patients randomized in SPICE-III to receive dexmedetomidine and given supplemental propofol, stratified by age (>65 or ≤65 years). We used double stratification analysis to produce quartiles of steady infusion rates of dexmedetomidine, while escalating propofol dose and vice versa. We used Cox proportional hazard and multivariable regression, adjusted for relevant clinical variable to evaluate the association of sedative dose with 90-day mortality. MAIN RESULTS Younger patients 598/1177(50.8%) received a significantly higher dose of both sedatives compared with older patients, to achieve comparable sedation depth. On double stratification analysis, escalating infusion rates of propofol to 1.27 mg/kg/h at a steady dexmedetomidine infusion rate (0.54 mcg/kg/h) was associated with reduced adjusted mortality in younger, but not older patients. This was consistent with multivariable regression modelling [hazard ratio: 0.59(95% Confidence Interval 0.43-0.78),P<0.0001], adjusted for baseline risk and interaction with dexmedetomidine dose. In contrast, among younger patients using multivariable regression, escalating dexmedetomidine infusion rate was associated with increased adjusted mortality [HR:1.30(95%CI 1.03-1.65), P=0.029]. CONCLUSION In patients ≤ 65 years sedated with dexmedetomidine and propofol combination, preferentially increasing the dose of propofol was associated with decreased adjusted 90-day mortality. Conversely, increasing dexmedetomidine may be associated with increased mortality. Clinical trial registration available at www. CLINICALTRIALS gov, ID: NCT01728558.