Difference in the duration of sensory block between dexmedetomidine and placebo assessed using an alcohol swab. Dots represents the duration of the two blocks in each volunteer with ropivacaine plus placebo along the x-axis and ropivacaine plus dexmedetomidine along the y-axis. The black reference line indicates no difference in block duration. The blue lines indicate the defined minimal relevant difference of 4 h.