The inhaled Iloprost, approved for pulmonary hypertension, caused increase in oxygenation, and increase exercise tolerance in ARDS or chronic obstructive pulmonary disease. Inhaled iloprost has been researched in animal study, but not yet in human during one-lung ventilation. The investigators will enroll patients who diagnosed moderate to severe chronic obstructive pulmonary disease, preoperative pulse oximetry (SpO2) of below 95% at room air or PaO2 /FiO2 ratio< 150 mmHg after initiating one-lung ventilation. The primary outcome is pulmonary oxygenation expressed by PaO2 /FiO2 ratio. And secondary outcome is assessment of cardiac function including Tei-index during nebulizing iloprost. The investigators hypothesized inhaled iloprost will improve oxygenation and decrease the FiO2 in chronic obstructive pulmonary disease during one-lung ventilation.

Inclusion Criteria:

1. Chronic obstructive pulmonary disease was defined as FEV1/FVC ratio < 0.7 and an FEV 1 of 80% or less after inhalation bronchodilator : 1. Above 40 years of age.
2. American Society of Anesthesiologists (ASA) Physical Status II, III.
3. Preoperative SpO2 95 % or less at room air (spontaneous ventilation) or 4. PaO2 /FiO2 ratio< 150 mmHg after initiating one-lung ventilation

Exclusion Criteria:

1. Severe functional liver or kidney disease
2. Diagnosed HF (LV ejection fraction <50% , or wall motion abnormality)
3. Arrhythmia or received treatment with antiarrythmic drug .
4. Severe bradycardia(HR < 45 bpm) and AV block 6. pathologic esophageal lesion (esophageal stricture or varix ) 7. pregnancy 8. PaO2 /FiO2 ratio ≥ 150 mmHg after initiating one-lung ventilation