This study tests whether giving a low-dose norepinephrine infusion just before induction of anesthesia can reduce low blood pressure during emergency surgery for peritonitis. Adults with secondary peritonitis who are scheduled for emergency surgery and have stable blood pressure after initial resuscitation will be randomly assigned to receive either norepinephrine or placebo (normal saline) during induction of anesthesia. The main outcome is the cumulative time spent with mean arterial pressure (MAP) below 65 mmHg from the start of propofol administration to 10 minutes after endotracheal intubation. The study will also evaluate rescue vasopressor use, heart rate changes, drug-related adverse events, and selected early postoperative outcomes.

The study is a single-center, randomized, double-blind, placebo-controlled interventional trial conducted at the Center for Anesthesia and Surgical Intensive Care, Bach Mai Hospital, Hanoi, Vietnam.

Adults undergoing emergency surgery for secondary peritonitis often have marked hemodynamic vulnerability during induction of general anesthesia because of systemic inflammation, vasodilation, capillary leak, and relative hypovolemia. Propofol-based induction may further reduce arterial pressure and organ perfusion in this setting.

Participants who meet eligibility criteria and have a mean arterial pressure (MAP) of at least 65 mmHg after initial resuscitation, without ongoing vasopressor support, will be enrolled and randomized 1:1 to a norepinephrine infusion group or a placebo group.

In the intervention group, norepinephrine will be infused at 0.05 micrograms/kg/min starting 3 minutes before fentanyl. In the placebo group, 0.9% sodium chloride will be infused with the same presentation, timing, and pump settings.

All participants will undergo a standardized induction protocol with fentanyl, titrated propofol, rocuronium, arterial line monitoring before induction, and protocolized hemodynamic rescue treatment.

The primary endpoint is the cumulative duration of MAP below 65 mmHg during the induction-intubation window, defined from the start of propofol administration to 10 minutes after endotracheal intubation. Secondary endpoints include the incidence of MAP below 65 and below 55 mmHg, the largest decrease in MAP from baseline, heart-rate changes, rescue vasopressor requirements, atropine use, and predefined early postoperative outcomes.

Inclusion Criteria:

• Adults aged 18 years or older.
• Secondary peritonitis requiring emergency surgery under general anesthesia with endotracheal intubation.
• Planned induction with fentanyl, propofol, and rocuronium according to the study protocol.
• Hemodynamically stabilized after initial resuscitation, with MAP at least 65 mmHg and no ongoing vasopressor infusion before induction.
• ASA physical status I to III.
• Written informed consent from the participant or a legally authorized representative.

Exclusion Criteria:

• Allergy or contraindication to norepinephrine, propofol, fentanyl, or rocuronium.
• Severe cardiovascular disease likely to confound hemodynamic assessment, including decompensated heart failure, acute coronary syndrome, severe arrhythmia, severe valvular disease, or major cardiomyopathy.
• Ongoing vasopressor therapy before induction.
• Shock before surgery, including hypovolemic, septic, or cardiogenic shock.
• Severe hypertension before induction (systolic blood pressure >180 mmHg or mean arterial pressure >110 mmHg).
• Severe peripheral vascular disease or clinically significant limb ischemia.
• Current monoamine oxidase inhibitor use.
• Pregnancy or breastfeeding.
• Contraindication to arterial catheter placement.
• Refusal of consent.

The investigators have not yet determined whether de-identified individual participant data will be shared. Any future data sharing decision will depend on institutional policy, ethics approval requirements, and participant privacy considerations.

Study drug syringes will be prepared by personnel not involved in anesthesia management, postoperative assessment, or statistical analysis. Participants, anesthesia care providers, investigators, outcome assessors, and statisticians will remain blinded to treatment allocation until database lock unless unblinding is required for participant safety.