In general anesthesia, gas exchange was altered by shunt and uneven ventilation perfusion ratios. Lung atelectasis was a cause of impaired oxygenation. High-flow nasal cannula oxygen therapy delivers adequately heated and humidified medical gas at up to 60 L/min of flow. It has physiological effects: reduction of anatomical dead space, positive end expiratory pressure (PEEP) effect, constant fraction of inspired oxygen, and good humidification. The hypothesis of this study is using high-flow nasal oxygen in intravenous general anesthesia could improve lung function and prevent lung atelectasis.

After general anesthesia, almost 90% patients have lung atelectasis. The lung atelectasis persisted even after the surgery, and caused post operative complication, for example: fever, pleural effusion, hypoxemia, pneumonia, and respiratory failure. So, how to improve lung function after the surgery is a important issue. High-flow nasal cannula oxygen therapy delivers adequately heated and humidified medical gas at up to 60 L/min of flow. It provides respiratory support: 1. Reduction of dead Space by clearance of expired air in the upper airways. 2. Delivering positive airway pressure. 3. delivering optimal humidity, which helps maintain function of the mucociliary transport system, clearing secretions and reducing the risk of infections. Many reports suggest that high-flow nasal cannula decreases breathing frequency and work of breathing and reduces intubation rate in critical ill patients. Although high-flow nasal cannula was used widely in intensive care unit (ICU) , there are no enough evidence in patients under general anesthesia. The hypothesis of this study is that using high-flow nasal oxygen in intravenous general anesthesia could improve lung function and prevent lung atelectasis. Liver tumor radiofrequency ablation was performed sometimes in patients under general anesthesia. Intravenous general anesthesia is one of the anesthesia choice. Oxygen mask is the traditional oxygen delivery system. Apnea, hypoxemia, and CO2 retention are common situation in intravenous general anesthesia. So high-flow nasal oxygen is an ideal oxygen delivery system in intravenous general anesthesia. This randomized control study will enroll patients receiving CT guided liver tumor radiofrequency ablation under general anesthesia. One group will receive high-flow nasal oxygen, the other group will receive traditional oxygen mask. The primary outcome is lung atelectasis area in CT scan. The secondary outcome is respiratory function (for example: arterial blood gas, lung injury biomarkers, saturation, postoperative pulmonary complication)

Inclusion Criteria:

• Patients with hepatic tumor undergoing CT guided radiofrequency ablation
• Age > 20 years old

Exclusion Criteria:

• Cardiac dysfunction, such as heart failure > NYHA class II, coronary arterial disease
• Impaired renal function, cGFR< 60 ml/min/1.73 m2
• Pulmonary disease