Critical Closing Pressure and PCO₂ Gap in Fluid Resuscitation for Septic Shock

This study is based on the following key hypotheses: PvaCO₂ (Venous-to-Arterial CO₂ Gap) Hypothesis: Changes in PvaCO₂ (ΔPvaCO₂) following fluid resuscitation correlate with changes in cardiac output (CO) and can serve as a reliable predictor of fluid responsiveness in septic shock patients. The relationship between PvaCO₂ and CO is influenced by metabolic factors (VCO₂), hemodynamic status, and the Haldane effect (oxygenation-dependent CO₂ binding to hemoglobin). Pcrit (Critical Closing Pressure) \& TPP (Tissue Perfusion Pressure) Hypothesis: Elevated Pcrit and reduced TPP (TPP = MAP - Pcrit) are associated with impaired microcirculatory flow and worse clinical outcomes (e.g., 28-day mortality, organ dysfunction). Restoring the "vascular waterfall" phenomenon (Pcrit \> mean systemic filling pressure) may improve macrocirculation-microcirculation coupling and tissue oxygenation. 2\. Study Design Overview 2.1 Study Type Prospective observational cohort with a retrospective validation arm using external databases (e.g., MIMIC-IV). Single-center study (Peking Union Medical College Hospital ICU). 2.2 Study Population Inclusion Criteria Adults (18-80 years) with septic shock (Sepsis-3 criteria + vasopressor-dependent hypotension + lactate \>2 mmol/L). Requires invasive hemodynamic monitoring (arterial line, central venous catheter). Undergoing fluid challenge (clinically indicated). Exclusion Criteria Pregnancy, non-septic shock (e.g., cardiogenic), mechanical circulatory support (ECMO/IABP). Severe COPD, intracardiac shunts, or conditions interfering with CO₂/VO₂ measurements. 2.3 Interventions \& Measurements Fluid Resuscitation Protocol 500 mL 4% gelatin infused over 15 min (pressure bag at 300 mmHg). Hemodynamic/metabolic data collected pre- and post-fluid challenge (within 10 min). Key Data Collected Hemodynamics: MAP, CVP, CO (PiCCO/ultrasound), Pcrit (derived from MAP-CO curve fitting). TPP = MAP - Pcrit. Metabolic Parameters: PvaCO₂, CvaCO₂, VCO₂