Main outcome measures: The maximum cephalad sensory block level; the change of continuous cerebral oxygen desaturation (ScO2) over time; the incidence of hypotension and nausea and vomiting; the change of invasive arterial pressure; the consumption of ephedrine; the incidence of shivering; the onset time to T8 dermatome; the quality of anesthesia (efficacy of motor block and sensory block)