Efficacy and Safety of Methylene Blue in the Treatment of Severe Septic Shock

The goal of this clinical trial is to investigate whether methylene blue injection can safely and effectively improve the survival rate of patients with severe septic shock, shorten the duration of norepinephrine use, reduce the dosage of vasopressors, promptly correct hemodynamics, and improve tissue perfusion and organ function impairment.

Researchers will compare methylene blue to a placebo (a look-alike substance that contains no drug) to see if methylene blue works to treat severe septic shock. Participants will: Administer a loading dose of 2.5 mg/kg via micro-pump over 15 minutes, followed by a maintenance infusion at a rate of 0.25 mg/kg/hour for 12 hours or until norepinephrine has been continuously discontinued for 4 hours, whichever comes first. If norepinephrine needs to be increased to 0.25 μg/kg/min before the 12-hour period ends, continue using methylene blue until the 12-hour infusion is completed. Primary Study Endpoint \- 28-day all-cause mortality rate starting from the diagnosis of septic shock. Secondary Study Endpoints * Time from the start to the discontinuation of norepinephrine after the diagnosis of septic shock. * Total dose of norepinephrine used (from enrollment to 72 hours). * Number of days without norepinephrine within 28 days. * Duration of mechanical ventilation. * Rate of CRRT (Continuous Renal Replacement Therapy). * Length of ICU stay. * Total hospital stay. Record the following indicators before methylene blue intervention and at 24 hours, 72 hours, and 5 days after the intervention: * Highest serum lactate level. * Lowest mean arterial pressure. * Alanine aminotransferase (ALT). * Total bilirubin. * Creatinine. * Oxygenation index (P/F ratio). * Creatine kinase-MB (CK-MB). * Troponin I (TNI). * Systemic vascular resistance index (SVRI).