Perioperative Predictors of Complications and Flap Loss in Microvascular Reconstructive Surgery: The Role of Fluid...

Background: Perioperative fluid therapy plays a critical role in the outcome of microvascular free-flap surgery. While both inadequate and excessive fluid administration may impair flap perfusion and systemic recovery, the impact of fluid balance and crystalloid volume-normalized to body weight and operative time-on postoperative complications remains underexplored. This study investigates the dose-dependent effects of intraoperative fluid and crystalloid administration on flap-related and systemic outcomes. Methods: This retrospective, single-centre cohort study included 495 adult patients who underwent microvascular free-flap transplantation between 2009 and 2020. Intraoperative fluid balance and crystalloid volumes were normalized to patient weight and operative duration (mL/kg/h) and stratified into pre-defined thresholds. The primary endpoint was the incidence of flap-related complications (partial/total flap loss, thrombosis, revision surgery). Secondary endpoints included flap loss, suture insufficiency, pneumonia, ICU length of stay (LOS-ICU), and in-hospital mortality. Results: Higher intraoperative fluid rates were significantly associated with higher complication rates. Flap-related complications occurred in 54.8% of patients receiving >10 mL/kg/h versus 37.1% in the ≤5 mL/kg/h group (p < 0.01) and reached 100% in patients receiving >20 mL/kg/h, although this category comprised only seven patients (p < 0.01). Suture insufficiency increased from 3.1% (≤5 mL/kg/h) to 57.1% (>20 mL/kg/h; p < 0.01). Pneumonia incidence rose from 8.8% (≤5 mL/kg/h) to 31.9% (>10 mL/kg/h; p < 0.01). A U-shaped trend was observed for flap loss, with the highest rate (24.6%) at >10 mL/kg/h. Crystalloid volume > 3000 mL was significantly associated with higher flap loss (20.2% vs. 0.2%; p < 0.01) and suture insufficiency (7.0% vs. 0.2%; p = 0.02). Red blood-cell (RBC) transfusions were associated with higher overall complication rates (45.6% vs. 34.2%; p < 0.01) and suture insufficiency (9.9% vs. 3.4%; p < 0.01). Gelatin-based colloids showed no negative impact. Operative time was the only strong independent predictor of total flap loss; each additional operative hour increased the odds of flap loss by 34% (p < 0.001). Intraoperative noradrenaline use and a history of neoadjuvant radiotherapy were not independently associated with flap-related complications or flap loss. Median LOS-ICU increased from 2 days to 10 days in patients receiving >20 mL/kg/h (p < 0.01). In-hospital mortality increased significantly with higher fluid volumes (0.3% for ≤10 mL/kg/h vs. 28.6% for > 20 mL/kg/h; p < 0.01). Conclusions: In 495 microvascular free-flap reconstructions, diagnosis, flap type, defect localization and operative time emerged as key determinants of postoperative outcomes, while defect type itself showed no predictive value. Intraoperative fluid overload-particularly crystalloid rates exceeding 10 mL/kg/h-is associated with a significantly higher risk of flap-related complications, pneumonia, prolonged ICU stay and mortality. These findings support the implementation of individualized or goal-directed fluid strategies in microvascular reconstructive surgery to optimize outcomes.