Perioperative Predictors of Complications and Flap Loss in Microvascular Reconstructive Surgery: The Role of Fluid Balance, Crystalloid Administration and Operative Time

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.