They use alpha-2 agonists for sedation all the time in veterinary medicine - but it doesn't look like it has a role here in the Emergency Department.
This is a small case-series out of Australia in which they gave dexmedetomidine (Precedex) to the acutely behaviorally challenged - a high-risk population in the Emergency Department, both for the patient and for staff. Patients became eligible for dexmedetomidine if they had acute behavioral disturbance requiring physical and chemical restraint. In this hospital, their protocol was to use droperidol 10mg IV for chemical sedation, then a second 10mg dose, and then they became eligible for second-line agents.
Their study population is thirteen patient enrollment over 21 months constituting a heterogenous mix of toxicologic and psychiatric agitation. Five of the thirteen patients received an IV loading dose only, and the remaining eight received loading dose and infusion. Of the five who received the loading dose, 2 had effective sedation without adverse effects - and the other 3 were not sedated and one became hypotensive. Of the other eight, three had effective sedation, one of which developed hypotension and atrial fibrillation. The other five had only transient or no sedation, four became hypotensive, and two were intubated for persistent agitation.
So, in all, five of the thirteen had adequate sedation using dexmedetomidine as rescue after initial attempts at chemical sedation - but seven had adverse effects. The authors then conclude that, while it provides an additional, reasonable alternative for sedation, monitoring and managing the adverse effects would be too resource intensive.
Seems reasonable enough.
"Dexmedetomidine in the emergency department: assessing safety and effectiveness in difficult-to-sedate acute behavioural disturbance"