In the previous stage of the current study, 700 elderly patients who were admitted to the ICU after noncardiac surgery were recruited and randomized into two groups (dexmedetomidine group and placebo [normal saline] group). The results showed that prophylactic low-dose dexmedetomidine infusion during the early postoperative period ameliorated the subjective sleep quality and decreased the incidence of delirium (22.6% [79/350] with placebo vs. 9.1% [32/350] with dexmedetomidine; Odds Ratio 0.35, 95% Confidence Interval 0.22 to 0.54; p < 0.0001). The investigators hypothesize that low-dose dexmedetomidine infusion may also improve long-term outcome in this patient population.