Background： The mean arterial pressure（ MAP） is a key pressure index to improve tissue perfusion. At present, there are no surprising results of large-scale clinical studies on sublingual microcirculation. The changes of sublingual microcirculation were more severe in septic shock non-survivors than survivors. Purpose： This study is mean to increase the MAP in septic shock patients whether with chronic hypertension or not, so as to observe the change of the microcirculation and prognosis. Method： This is a single-center, randomized, prospective cohort study. Eligible patients will be allocated into chronic hypertension or denying chronic hypertension group. These patients will be treat with vasopressors to maintain MAP at 90±5 mmHg and 70±5 mmHg. Outcome： The 28-day all-cause mortality, the 90-day all-cause mortality, the 28-day without organ dysfunction days, the Changes of sublingual microcirculation, SOFA， APACHE-Ⅱ score Will be recorded.

Adult patients (≥18 years) admitted to The Critical Medicine Departments Three of Fujian Provincial Hospital will be considered eligible，who are presence of infection or suspected infection，requiring mechanical ventilation. At the same time, without the vasopressors the MAP would be less than 65mmHg, and the blood lactic acid level is higher than 2mmol/l.