In this study aimed to compare bioelectrical impedance analysis (BIA)-guided fluid resuscitation with conventional fluid management strategies in patients admitted to the intensive care unit (ICU) following major surgery. The primary objective is to evaluate whether BIA-guided fluid therapy reduces 28-day mortality by optimizing fluid balance and preventing volume-related complications. Secondary outcomes include cumulative fluid balance, ICU and hospital length of stay, duration of mechanical ventilation, and need for vasopressor or inotropic support. This study is expected to provide evidence for the clinical utility and applicability of BIA in guiding postoperative fluid therapy in critically ill patients.

Fluid resuscitation plays a critical role in the perioperative care of patients undergoing major surgery.

Traditionally, fluid therapy decisions in the intensive care unit (ICU) have relied on static clinical parameters, which may not always reflect the patient's actual volume status. Bioelectrical impedance analysis (BIA) offers a noninvasive and dynamic assessment of hydration status, enabling more precise fluid management tailored to individual needs. This prospective, randomized controlled study aims to compare BIA-guided fluid resuscitation with conventional fluid management strategies in postoperative ICU patients. A total of 80 adult patients who meet the inclusion criteria will be randomized into two groups. Group 1 will receive standard fluid therapy based on clinical evaluation, including heart rate, blood pressure, urine output, passive leg raising test, and vena cava ultrasound assessment. Group 2 will undergo BIA measurements at 0, 12, 24, and 48 hours post-ICU admission to guide fluid management according to hydration status (dehydrated, euvolemic, or hypervolemic). The primary outcome is 28-day all-cause mortality. Secondary outcomes include cumulative fluid balance, ICU and hospital length of stay, duration of mechanical ventilation, and need for vasopressors or inotropic agents. By evaluating clinical outcomes between the two approaches, this study seeks to determine whether BIA-guided fluid management offers a clinically meaningful advantage in postoperative critical care.

Inclusion Criteria:

• Undergoing major surgery under general anesthesia(Major surgery defined as: vascular clamping or organ ischemia, intraoperative blood loss >1000 mL, need for >10 mcg/min norepinephrine infusion, surgery duration >4 hours, or requirement for perioperative blood transfusion)
• Admission to the intensive care unit (CU) after surgery
• Informed consent obtained

Exclusion Criteria:

• Refusal to participate or failure to provide informed consent
• Undergoing laparoscopic or emergency surgery
• Severe major organ dysfunction:Acute kidney injury stage 2 or 3 (KDIGO 2012) • Acute or chronic liver failure (ALT >3x or Child A-C cirrhosis
• Conditions preventing accurate BIA measurement: • Limb amputation • Metallic cardiac or joint prostheses • Pacemakers or intracardiac stents
• Body mass index >35 kg/m or < 18 kg/m
• Contraindications to lactated Ringer's solution (e.g. hypercalcemia, increased intracranial pressure)

Individual participant data (IPD) will not be shared due to ethical restrictions and the absence of prior informed consent for data sharing. Additionally, the study does not include a data sharing provision in the approved protocol or institutional ethics approval.

To preserve the integrity of the study results, group assignments (BIA-guided fluid resuscitation vs. conventional fluid resuscitation) were concealed from the clinical staff involved in patient care and data collection. In addition, the investigators responsible for evaluating the primary and secondary outcomes (outcome assessors) were blinded to group allocation. Prior to participation, all patients or their legal representatives were thoroughly informed about the study's purpose, the nature of the two fluid management strategies, the randomization process, and the blinding of group information. Informed consent was obtained after ensuring full understanding of the methodology, including the confidentiality of group assignments.