About 150 children who were not adequately sedated ( no evidence of adequate sedation within 30 min after administration of Initial dose of chloral hydrate) were stratified into four age groups as follows: 1-6 month, 7 -12 month, 13 -24 month, and 25-36 month. The intranasal dexmedetomidine dose was determined by the success or failure of rescue sedation achieved by the previous patients, according to Dixon's up-down sequential allocation method. Successful sedation was defined as a MOAA/S(modified Observer Assessment of Alertness and Sedation) of between 0 and 3. The EC50 were estimated from the up-and-down sequences using the method of Dixon and Massey and logistic regression. Patients' sedation status, sedation induction time, time to Wake up, blood pressure, heart rate, and oxygen saturation were recorded.