Inclusion Criteria:  Septic shock, receive 0.5 ug/kg/min NA to maintain blood pressure for more than 2 hours after fluid resuscitation.  Exclusion Criteria:  Pregnant women Without internal jugular vein or subclavian vein catheterization Child-Pugh grade C with liver cirrhosis Severe chronic obstructive pulmonary disease (severe acidosis caused by type II respiratory failure, PaCO2 ≥60mmHg and PH<7.2 ) End-stage of malignant tumors Glucose-6-phosphate dehydrogenase (G6PD enzyme) deficiency Positive end-respiratory pressure (PEEP) >10mmHg, the oxygenation index <150mmHg under mechanical ventilation Definitive pulmonary hypertension or chronic pulmonary heart disease. Oral use of 5-hydroxytryptamine in recent 3 months