Bioelectrical Impedance Vector Analysis for Assessing Metabolic Phenotype and Fluid Status in Septic Shock: A Prospective Observational Study

BACKGROUND: Fluid resuscitation in septic shock lacks bedside tools that simultaneously evaluate fluid distribution and cellular function, often leading to fluid overload and increased mortality. Bioelectrical Impedance Vector Analysis (BIVA) is a non-invasive bedside technique for comprehensive assessment of body composition and hydration status, but its prognostic value and application criteria in septic shock remain undefined. METHODS: A prospective observational study was conducted in an intensive care unit (ICU) of a university-affiliated hospital. Consecutive adult patients meeting Sepsis-3.0 criteria for septic shock were enrolled. BIVA parameters (whole-body and segmental PhA, ECW/ICW ratio, tolerance ellipses), hemodynamic indices, and serum lactate were dynamically monitored at ICU admission (within 24h of diagnosis), 6 hours post-resuscitation (T1), 24 hours (T2), and then daily until ICU discharge or death. Receiver operating characteristic (ROC) curve analysis determined prognostic cutoffs. Spearman's correlation and linear mixed models analyzed BIVA-lactate relationships. Passive leg raising (PLR) test served as the reference for fluid responsiveness, with predictive performance analyzed for mean arterial pressure (MAP) gradient (ΔMAP) between bilateral upper arms at different lateral decubitus positions combined with BIVA parameters. RESULTS: Among 128 enrolled patients, multivariate Cox regression identified initial PhA ≤ 3.10° (Hazard Ratio [HR] = 3.85, 95% Confidence Interval [CI]: 2.12-6.99) and BIVA-derived hydration fraction >74% (HR = 2.41, 95% CI: 1.32-4.39) as independent predictors of 30-day mortality. The Extracellular Water/Intracellular Water ECW/ICW ratio strongly correlated with lactate levels (r = 0.71, p < 0.001), and their combination predicted acute kidney injury more effectively (Area Under the Curve [AUC] = 0.92) than either parameter alone. In 95 patients undergoing positional testing, ΔMAP≥8.00 mmHg at 40°lateral decubitus position predicted fluid responsiveness with 85.7% sensitivity and 84.2% specificity; combining this with upper-arm BIVA parameters further enhanced predictive value (AUC = 0.93). CONCLUSION: BIVA provides an effective individualized assessment tool for metabolic and fluid management in septic shock. PhA and hydration status are robust prognostic indicators; the ECW/ICW-lactate correlation reveals the pathophysiological link between tissue edema and hypoperfusion; and positional BIVA testing with upper-arm pressure gradient offers an innovative method for non-invasive assessment of fluid responsiveness.