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]