Effect of Obesity Phenotype on Intraabdominal Pressure and Bleeding in Major Lumbar Spinal Surgery

Major lumbar spinal surgery, including lumbar canal stenosis decompression and instrumented fusion, is associated with substantial intraoperative blood loss and notable perioperative morbidity. Obesity is highly prevalent in this patient population due to its role in lumbar degenerative disease pathogenesis. Prone positioning during surgery significantly increases intraabdominal pressure, which in turn may impair venous drainage from the spinal epidural plexus, thereby augmenting operative blood loss. While BMI has been studied as a predictor of spinal surgical outcomes, the differential impact of fat distribution pattern (central vs. peripheral obesity) on IAP dynamics and bleeding has not been adequately characterized. Study Design: Prospective, single-center, observational cohort study conducted at Marmara University Pendik Training and Research Hospital and Asaf Ataseven Hospital, Istanbul, Turkey. No interventions beyond routine clinical anesthesia practice will be performed. Patient Groups: Patients will be categorized into three groups based on BMI and waist-to-hip ratio (WHR): * Group NO (Non-obese): BMI \<30 kg/m² (n=30) * Group PO (Peripheral obese): BMI ≥30 kg/m² and WHR \<0.85 (n=30) * Group CO (Central obese): BMI ≥30 kg/m² and WHR ≥0.85 (n=30) Anesthetic Protocol: All patients will receive standard intraoperative monitoring (ECG, non-invasive blood pressure, pulse oximetry, end-tidal CO₂, BIS). Anesthesia will be induced with propofol 2 mg/kg, fentanyl 2 µg/kg, and rocuronium 0.6 mg/kg, and maintained with total intravenous anesthesia (TIVA) using propofol and remifentanil infusion, consistent with the institutional protocol for neuromonitoring-guided spinal surgery. Adjusted body weight will be used for drug dosing in obese patients. Intraabdominal Pressure Measurement: IAP will be measured via urinary bladder catheter (standard intravesical technique) at three time points: (1) supine position after intubation, (2) prone position, and (3) supine position at the end of surgery. Data Collection: Preoperative: age, sex, BMI, weight, height, waist and hip circumferences, WHR, ASA classification, comorbidities, baseline hemogram, BUN, creatinine, INR, aPTT, anticoagulant/antiplatelet use. Intraoperative: surgical procedure type and level, revision status, anesthesia and surgery duration, neuromonitoring use, hemodynamic parameters at standardized time points, estimated blood loss, fluid and blood product administration, vasopressor/inotrope use, urine output, cell saver use. Postoperative: hemoglobin and hematocrit drop, BUN, creatinine at 24 hours, Aldrete score, VAS score, ICU admission and indication, postoperative complications (surgical and medical), transfusion requirements, and total hospital length of stay. Statistical Analysis: SPSS 21.0 will be used. Continuous variables will be reported as mean ± SD or median (IQR) depending on normality (Kolmogorov-Smirnov/Shapiro-Wilk). Categorical variables will be reported as n (%). Between-group comparisons will employ one-way ANOVA or Kruskal-Wallis test for continuous variables and chi-square or Fisher's exact test for categorical variables. Post-hoc pairwise comparisons will be performed with appropriate correction. Significance threshold: p\<0.05. Sample Size: Based on published BMI-stratified spinal surgery data, a minimum of 90 patients (30 per group) is required to achieve 80% power at α=0.05 (G\*Power 3.1).