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Cevallos et al. Plast Aesthet Res 2023;10:30 https://dx.doi.org/10.20517/2347-9264.2023.01 Page 11 of 17

Biochemical markers
Using a femoral vessel anastomosis rodent model, Su et al. reported in 1982 that both venous and arterial
occlusion decreased tissue glucose content and increased lactate content. Venous occlusion was found to
[115]
have a more detrimental effect on flap survival . The theory behind this observation suggests that vessel
occlusion impairs perfusion, resulting in decreased tissue oxygenation and glucose delivery, leading to a
shift from aerobic to anaerobic metabolism [116-118] .

Since then, tissue biomarkers such as glucose with or without lactate have been successfully used for flap
monitoring in the clinical setting [119-122] . These biomarkers have been utilized at different threshold values
and rates of change to indicate venous occlusion, showing variable sensitivity and specificity. Measurement
of flap tissue glucose levels is a simple, inexpensive, and rapid option for post-operative flap monitoring
[13]
compared to other techniques . However, the efficacy of glucose measurements in detecting arterial
occlusion and in diabetic patients is limited, and this technique lacks applicability in buried flaps [123-125] .

Microdialysis
Microdialysis has been employed as a method for microinvasive monitoring of flap ischemia in various flap
types, including myocutaneous, buried flaps and TRAM flaps [126-129] . This technique involves a double-lumen
catheter with a semipermeable membrane at the end, which draws fluid from the flap through the
membrane into the catheter. By continuously assessing small molecules in the dialysate, such as glucose,
lactate, pyruvate, and glycerol, microdialysis provides a measure of the flap’s metabolic activity. These
values can be compared values to a baseline value or ratios, non-flap tissue away from the operative site, or
reference values. The analysis of the interstitial fluid contents allows for flap examination without the need
for direct clinical examination and can potentially detect ischemia one to two hours before clinical evidence
[130]
of flap failure . This technique is limited by its high cost and relatively high false-positive rate.
Additionally, widespread adoption is restricted by the requirement for a skilled nursing team and clinical
familiarity with microdialysis.

Technetium-99m sestamibi scintigraphy
Technetium-99m (99mTc) sestamibi, a metabolically inactive radionuclide, is injected and measured by
scintigraphy to visualize areas of tissue hypoperfusion and evaluate microvascular anastomotic patency. It
has been employed to assess free muscle flap viability, particularly in detecting a delayed secondary
thrombosis caused by microemboli, which may not be evident using other techniques relying on more
obvious signs [131-133] . Additionally, this technique permits visualization of partial necrosis, providing
[131]
important information of surgical significance in the case of future surgical debridement .

Perfusion-weighted MRI
Several reports have described the use of contrast-enhanced magnetic resonance imaging (MRI) as a
method for analyzing flap perfusion, with parameters optimized for vascular contrast uptake [134,135] .
Compromised flaps exhibit significantly reduced signal intensity on MRI [134,135] . Perfusion-weighted MRI can
assess the overall perfusion of the entire free flap. The drawbacks of this technique are mostly resource-
related: the financial and time-based costs of MRIs are high. Additionally, perfusion-weighted MRI does not
directly evaluate vascular pathology and necessitates specialized support staff throughout the process.

Future directions
In the past 40 years, post-operative flap monitoring technologies have advanced to provide continuous,
remote, highly sensitive, and non-invasive options for surgeons. Lee et al. developed a new dual-camera flap
monitoring system that combines mask region-based convolutional neural networks in the visible-light