Monitoring of Free Flaps Using Near-Infrared Spectroscopy: A Systematic Review of the Initial Trials

Abstract

Sir: Free tissue transfer is now a routine procedure in reconstructive surgery. Arterial and venous occlusions are common complications requiring revision surgery and flap salvage. Flap salvage rates remain approximately 50 percent,1 and salvage of the affected tissue depends largely on the time to reexploration. Assessing the viability of tissues by monitoring circulatory changes during harvesting and following transfer of free flaps is essential. Devices designed to augment clinical assessment (standard method) could lead to prompt surgical intervention and improved salvage rates. Popular adjuvant techniques include external and implantable Doppler monitoring and laser Doppler flowmetry.2 However, no method has proven to be more reliable than clinical assessment, and this supports the need to seek alternatives.1 Near-infrared spectroscopy exploits the relative transparency of biological tissue in the near-infrared wavelength range. Near-infrared light is capable of penetrating tissue and can still be detected on remission, following absorption by the primary light-absorbing molecules (chromophores) in biological tissues, oxyhemoglobin and deoxyhemoglobin. These chromophores have different absorption spectra in the near-infrared wavelength range (600 to 900 nm). Provided that at least two wavelengths are used, it is possible to resolve changes in oxyhemoglobin and deoxyhemoglobin, and their sum, total hemoglobin based on the amount of light absorption. Near-infrared spectroscopy is capable of noninvasively monitoring and differentiating between venous, arterial, and total occlusion of a skin flap.1 The parameters most commonly reported in the literature are tissue oxygen saturation, which represents oxygen delivery and consumption, and total hemoglobin, which reflects changes in blood tissue volume and tissue perfusion. We recently reviewed the literature systematically to determine whether near-infrared spectroscopy responses differ between successful and failing tissue flaps.1–8Figure 1 illustrates the difference between successful and failing tissue flaps in terms of longitudinal changes in tissue oxygen saturation measured using near-infrared spectroscopy. In successful flaps, an initial increase in tissue oxygen saturation is observed in the first 2 hours postoperatively, following which levels return to preoperative values. This can be explained by two mechanisms: (1) the autonomic denervation of cutaneous vessels during skin flap dissection and (2) the inflammatory reaction at the wound margin. Both result in local vasodilation.7 Several studies7,8 report an increase in total hemoglobin following successful free flap surgery, maintained up to 48 hours postoperatively, with subsequent recovery of levels toward preoperative values thereafter.Fig. 1.: Tissue hemoglobin oxygen concentration (StO2) in surviving and failing flaps. The data represent the mean preoperative and postoperative tissue oxygen saturation levels.It remains unclear whether increases or decreases in tissue oxygen saturation are more predictive of a failing flap. There appears to be little difference between preoperative and postoperative tissue oxygen saturation levels in flaps that ultimately fail. The early rise in tissue oxygen saturation levels observed in successful flaps is not observed in failing flaps. Postoperative increases in total hemoglobin occur in almost all breast flaps that ultimately fail.7 Failing flaps demonstrate a greater total hemoglobin increase than surviving flaps. The most likely explanation for excessive increases in total hemoglobin in failing flaps is associated venous congestion. Near-infrared spectroscopy is capable of providing useful information regarding tissue hemodynamics, especially with respect to the measure of tissue oxygen saturation in surviving flaps. However, there is limited current evidence that near-infrared spectroscopy is able to differentiate the specific cause for flap failure. However, near-infrared spectroscopy has been used successfully to guide early reoperation, at which point the source of flap failure can be determined. Further validation studies are necessary to determine sensitivity, specificity, and the differences in tissue hemodynamics between various tissue flaps, before the use of near-infrared spectroscopy can be fully endorsed as a clinical tool. Ali Najefi, B.Sc. Royal Wolfson Image Computing Laboratory and Department of Bio Surgery and Surgical Technology Imperial College London Daniel R. Leff, M.B.B.S. Royal Wolfson Image Computing Laboratory and Department of Bio Surgery and Surgical Technology Imperial College London Biomedical Optics Research Laboratory Department of Medical Physics and Bioengineering University College London Marios Nicolaou, Ph.D. Biomedical Optics Research Laboratory Department of Medical Physics and Bioengineering University College London Charles Nduka, M.A., M.D. Queen Victoria Hospital East Grinstead, West Sussex, United Kingdom Guang-Zhong Yang, Ph.D. Biomedical Optics Research Laboratory Department of Medical Physics and Bioengineering University College London Ara W. Darzi, F.R.C.S., F.A.C.S. Royal Wolfson Image Computing Laboratory and Department of Bio Surgery and Surgical Technology Imperial College London London, United Kingdom DISCLOSURE The authors have no financial interest to declare in relation to the content of this article.