The present study is to evaluate the effect of positive end-expiratory pressure (PEEP) in predicting fluid responsiveness in patients undergoing one-lung ventilation.

Appropriate fluid challenge is crucial during lung surgery under one-lung ventilation. Applying PEEP in mechanically ventilating patients changes dynamic hemodynamic parameters (stroke volume variation, pulse pressure variation), which is more pronounced in hypovolemic patients. The aim of the present study is to evaluate the mechanically ventilating patients under one-lung ventilation requiring fluid resuscitation by applying PEEP to predict fluid responsiveness.

The investigators hypothesized that applying PEEP in one-lung ventilating patients can also change dynamic parameters and the magnitude of the change of stroke volume variation, pulse pressure variation can predict fluid responsiveness. The investigator applied PEEP 10 mmHg in one-lung ventilating patients and observed the dynamic parameters including stroke volume variation, pulse pressure variation and then infused 6ml/kg of crystalloid for fluid challenge and diagnosed fluid responsiveness as 10% increase of stroke volume after fluid challenge.

Inclusion Criteria:

• patients undergoing lung surgery under one-lung ventilation

Exclusion Criteria:

• arrhythmia
• moderate to severe valvular heart disease moderate to severe pericardial effusion left ventricular ejection fraction < 40%
• moderate to severe chronic obstructive lung disease
• unable to insert oesophageal Doppler Monitor (ODM) probe ( esophageal stent, esophageal cancer, previous esophageal surgery, esophageal stricture, esophageal varices, pharyngeal pouch and severe coagulopathy )
• patient refusal
• cannot understand the protocol
• less than 50kg or over 100kg in weight

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