The investigators preliminary study showed that dexmedetomidine might improve microcirculation in septic shock patients. To test this effect, the investigators will conduct double-blinded study.

Dexmedetomidine is a highly selective α2-adrenoreceptor agonist for sedation of adult critically ill patients which exhibits sedative and analgesic effects. Recent studies suggest that dexmedetomidine to restore adrenergic vasoconstrictor responsiveness in septic shock, and prevents alterations of pain rat model intestinal microcirculation Induced by surgical stress，even it can reduce the mortality of endotoxemic rats and patients with severe sepsis. The investigators preliminary study showed that dexmedetomidine might improve microcirculation in septic shock patients. To test this effect, the investigators will conduct double-blinded study.

Inclusion Criteria:

• Septic shock patients despite early goal directed therapy
• Agree to participate this study

Exclusion Criteria:

• Age< 18
• Pregnancy
• Bradycardia (HR<55bpm)
• Systolic Blood Pressure < 80 mmHg / Mean arterial pressure < 50 mmHg on maximal support
• Death imminent
• Unlikely to survive 90 days
• Acute liver failure
• Dementia
• High-grade block in the absence of a functioning pacemaker.