“INVOS” WE TRUST. Tissue Oximetry for Free Flap Monitoring in Lower Limb Reconstruction

Introduction

Free flaps on the lower limb could make the difference between salvage and amputation. Regional tissue oximetry (rSO₂) measured by near-infrared light is a tool that is not yet widely described or recognized, as most published studies focus on its use in breast flap monitoring. However, in the context of lower limb reconstruction, it offers an objective and real-time evaluation of flap tissue perfusion, enabling faster responses for salvage compared to traditional clinical monitoring.

Material and Methods

We conducted a retrospective study comparing lower limb free flap monitoring using two techniques. Group A (June 2016–January 2020) used local real-time rSO₂ monitoring with the INVOS-TM 5100C Somatic Oximeter (Medtronic Inc., Minneapolis, MN); each patient had two sensors, one over the flap, another (control) over a nearby non-flap area. Group B (February 2013–May 2016) relied on traditional clinical examination.

Results

A total of 148 free flaps were included (74 in each group). There was a small, non-significant difference in overall flap survival (Group A: 94.6% vs. Group B: 90.5%, p = 0.344). The flap salvage rate, when reoperated within the first 72 h, was higher but not significantly so (66% vs. 43%, p = 0.483) and significantly faster (121 vs. 181 min, p = < 0.001) in Group A. According to our study, INVOS demonstrated 100% sensitivity and negative predictive value (NPV), with 90% specificity.

Conclusions

Regional tissue oximetry monitoring of lower limb free flaps is a real-time, objective, non-invasive, and reliable method for early detection of complications. This study allows us to affirm that the revisions of the flaps are statistically significantly faster. It also provides valuable information about anastomotic failure, clearly differentiating between arterial and venous issues, as well as identifying local or systemic issues.