Obesity is prevalent among patients with lumbar degenerative disease and is a known risk factor for perioperative complications in spinal surgery. However, most existing evidence relies solely on body mass index (BMI) without distinguishing between central and peripheral obesity phenotypes. Central and peripheral fat distribution may have distinct biomechanical and hemodynamic consequences, particularly with respect to intraabdominal pressure (IAP) changes during prone positioning and intraoperative blood loss.

This prospective single-center observational study aims to evaluate the effect of obesity phenotype - classified as non-obese, centrally obese, or peripherally obese based on BMI and waist-to-hip ratio - on intraabdominal pressure, intraoperative blood loss, and other perioperative outcomes in adult patients undergoing major lumbar spinal surgery (decompression and/or instrumentation)

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).

Inclusion Criteria:

• Age 18-80 years
• ASA Physical Status Classification I-III
• Scheduled for elective major lumbar spinal surgery (canal stenosis decompression and/or instrumented fusion)
• No prior lumbar spinal surgery
• Written informed consent obtained

Exclusion Criteria:

• - Pediatric patients (<18 years)
• Pregnancy
• Emergency surgical procedures
• Pre-existing renal failure
• Coagulation disorder
• Pre-existing anemia
• Known drug allergy relevant to study medications
• Inability to obtain written informed consent

Adult patients aged 18-80 years with ASA physical status I-III scheduled for elective major lumbar spinal surgery (decompression and/or instrumentation) at Marmara University Pendik Training and Research Hospital and Asaf Ataseven Hospital, Istanbul, Turkey. Patients are classified by obesity phenotype based on BMI and waist-to-hip ratio. Patients with renal failure, coagulation disorders, anemia, pregnancy, prior lumbar surgery, or emergency procedures are excluded.