Investigate effects of PEEP on pulmonary regional ventilation and perfusion assessed by EIT

1. When the research team was available, adult patients within 7days who were sequentially admitted to the Department of Critical Care Medicine and required central venous catheters and mechanically ventilation were eligible for the study.
2. Written informed consent was obtained from all patients or next of kin before data were included in the study.
3. Information collected at enrollment included demographic characteristics, such as age, sex.
4. The blood pressure（BP）,heart rate（HR）, pulse O2（SpO2）,tidal volume and lung regional ventilation and perfusion were collected at PEEP 0 centimeter H2O column and PEEP 12-15 centimeter H2O column.
5. Indicator-based EIT method: rapidly inject 10ml 10% Sodium chloride（NaCL） through central venous catheter during a 8s respiratory hold, and the curve of impedence change was collected by EIT machine

Inclusion Criteria:

1. have been placed central venous catheter based on patient's condition
2. Under mechanically ventilation
3. Under sedation and control ventilation mode
4. Year>18

Exclusion Criteria:

1. Pneumothorax
2. pregnancy
3. Severe chest wall wound
4. Severe hypernatremia

adult patients within 7 days, who were sequentially admitted to the Department of Critical Care Medicine and required central venous catheters and mechanically ventilation were eligible for the study