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OBJECTIVES To determine the 50% and 95% effective dose of intranasal dexmedetomidine sedation for transthoracic echocardiography in children with cyanotic and acyanotic congenital heart disease. DESIGN A prospective, nonrandomized study. SETTING A tertiary care teaching hospital. PARTICIPANTS Patients younger than 18 months with known or suspected congenital heart disease scheduled for transthoracic echocardiography with sedation. INTERVENTIONS Patients were divided into a cyanotic group (blood oxygen saturation <85%) or an acyanotic group (blood oxygen saturation ≥85%). This study used Dixon's up-and-down method sequential allocation design. In both groups, the initial dose of intranasal dexmedetomidine was 2 μg/kg and the gradient of increase or decrease was 0.25 μg/kg. MEASUREMENTS AND MAIN RESULTS The 50% effective dose (95% confidence interval) of intranasal dexmedetomidine sedation for transthoracic echocardiography was 3.2 (2.78-3.55) μg/kg and 1.9 (1.69-2.06) μg/kg in the cyanotic and acyanotic groups, respectively. None of the patients experienced significant adverse events. CONCLUSION The 50% (95% confidence intervals) effective doses of intranasal dexmedetomidine sedation for transthoracic echocardiography were 3.2 (2.78-3.55) μg/kg and 1.9 (1.69-2.06) μg/kg in children with cyanotic and acyanotic congenital heart disease, respectively.