COPD is often associated with the airflow limitation and the pulmonary hyperinflation. During the one-lung ventilation (OLV), COPD patients' physiologic dead space increases and hypercapnia may develop. In addition, an alveolar overdistention by the pulmonary hyperinflation would divert pulmonary blood flow and impair arterial oxygenation. Dexmedetomidine has both vasoconstricting and vasodilatatory effects on a peripheral vasculature and a smooth muscle but its effects on bronchial smooth muscle is unknown. The investigators will evaluate the effects of dexmedetomidine (bolus of 1.0 μg kg-1 over 10 min followed by a continuous infusion at 0.5 μg kg-1 h-1 infusion during one-lung ventilation) as bronchodilator in COPD patients when administered during surgery with the one lung ventilation. If result successfully shows the effect of dexmedetomidine as a predominantly a vasodilator on bronchial tree, the investigators study can support the dexmedetomidine as a vasodilator for pulmonary vessels and bronchial smooth muscle and as an excellent adjunct to anesthetic agents in COPD patients during the OLV.

Inclusion Criteria:

1. Above 40 years of age
2. American Society of Anesthesiologists (ASA) Physical Status I, II, III.
3. COPD diagnosed , criteria ( on preop. PFT : FEV1 < 80%, FEV1 / FVC < 70% after bronchodilator)
4. thoracic surgical procedure

Exclusion Criteria:

1. severe functional liver or kidney disease
2. diagnosed HF ( NYHA class >3)
3. arrhythmia or received treatment with antiarrythmic drug .
4. exceed BMI > 30 kg/ m2
5. bradycardia (HR <50 bpm)