Unveiling the dynamics of postoperative edema in free flaps: A hyperspectral insight through linear mixed models

Aims of the study: This prospective cohort study investigates postoperative fluid accumulation in free flaps using hyperspectral imaging, analyzed through linear mixed models. Effects of edema on flap viability and microcirculatory changes during the critical postoperative phase should be detected.

Materials and methods: Patients who underwent free tissue transfer were monitored for six postoperative days (POD) using hyperspectral imaging to quantify tissue water index (TWI) for viable and failing flaps and the recipient site. Secondarily, oxygen saturation (StO2), near-infrared perfusion index (NPI) and tissue hemoglobin index (THI) were assessed. Microvascular blood flow was recorded with laser Doppler flowmetry. Linear mixed models generated prototypical TWI growth curves for estimation of TWI dynamics. Independent predictors of TWI were examined. Simple regression analysis explored correlations between flap TWI and microcirculation parameters.

Results: 41 free flaps were monitored, with 4 requiring revision surgery. TWI of viable flaps (b=48.70); p=<.001) increased linearly (b=.76; p=<.001), reaching +9.4 % on POD 6. In contrast, revision flaps, while similar at baseline (b=-.95; p=.854), demonstrated a negative trend (b=-3.34; p=.005), ending at -33.6 % on POD 6. Male sex was associated with higher TWI at the recipient site (b=-3.34; p=.005). Notably, TWI showed no significant correlation with StO2 (p=>.999), NPI (p=.110), blood flow (p=.120) or THI (p=.140).

Conclusion: Hyperspectral imaging is feasible for bedside flap monitoring, enabling quantification of tissue edema progression. Although the sample size of failing flaps was limited, these findings highlight the potential of low TWI as a warning sign and may offer insights into mechanisms involved in ischemia-reperfusion injury.