Near Infrared Spectroscopy (NIRS)
NIRS is a method that makes use of the near-infrared region of the electromagnetic spectrum. By measuring light scattered off and through a sample, NIRS reflectance spectra can be used to quickly determine a material’s properties. [31]
Unlike visible light, which has limited tissue penetration due to shorter wavelengths, near-infrared light (NIR) can penetrate several millimetres into tissues. In skin and muscle, the main absorbing chromophores are the haemoglobin (Hb), myoglobin (Mb), and cytochrome oxidase (cytox), with water and fat also exhibiting absorption peaks at longer wavelengths in the NIR region. Hemoglobin and myoglobin contain iron cores within heme groups, and their absorption of NIR light varies depending on oxygen binding and can be measured by the NIRS instrument. [32]
At the moment, more than 10 commercially available NIRS instruments are on the market. These instruments measure wavelengths ranging from 568 nm to 880 nm. [33] NIRS, therefore is an optical technique that measures changes in oxygen levels in tissues by employing camera-based tissue oximetry. [34] Threshold values for detecting vascular flap compromise varies among studies, with the majority adopting a criterion akin to Keller et al., involving a drop rate in tissue oxygen saturation (StO2) of 20% or more per hour, persisting for over 30 minutes. Akita et al. introduced a regional oxygen saturation index, with a threshold of 0.75 or lower indicating flap compromise. [33]
In reconstructive flap surgery this method was employed to monitor flap perfusion status, and also to differentiate between venous and arterial vascular compromise within the flaps. It could prove its effectiveness in breast reconstruction with deep inferior epigastric perforator (DIEP) flap, transverse upper gracilis flap, and latissimus dorsi flap with lipofilling. [34] The NIRS technology is highly accurate and has had no false-negative results in several studies. [35] It even can be used in buried flaps. [36] NIRS qualifies even in transcutaneous perfusion assessment, for example in kidney or liver allografts. [37]