To observe the effect of PEEP on CVP among patients with different respiratory compliance

30 patients with mechanical ventilation and inserted central venous line will be entered. After gathering demographic data, patients undergone 5,10 and 15cmH2O PEEPs and the respective CVPs of the mentioned points will be recorded. According to the change of CVP, the patients will be divided into high response group and low response group, then comparing the lung compliance and thorax elastic resistance differences between the two groups.

Inclusion Criteria:

1. Patients with mechanical ventilation and inserted central venous line

Exclusion Criteria:

1. The age is less than 18 years old or more than 80 years old；
2. Pregnant and lying-in woman；
3. Patients undergoing fluid resuscitation and the speed is more than 200ml/h；
4. Patients with norepinephrine dose are greater than 15 micro per minute；
5. Patients with pericarditis、pulmonary heart disease、right heart failure、atrial fibrillation；
6. Patients with COPD(endogenous PEEP is more than 2cmH2O)、tension pneumonthorax、acute episode asthma