Methods of tissue viability assessment should be classified into those that measure blood flow and those that monitor tissue metabolism. The problem with measuring blood flow is that it can be misleading due to the phenomenon of arteriovenous shunting. Near infrared (NIR) spectroscopy is capable of identifying certain molecules in the tissues. In this study, using the reverse McFarlane rat skin flap as a model, oxygen delivery to the tissue along the flap was demonstrated in the form of a spectrum. This was achieved by using the differential absorption of oxy- and deoxyhemoglobin between wavelengths of 650 nm and 900 nm. NIR imaging works on a similar principle, but an oxygen saturation image is obtained, with the darkest area indicating the most deoxygenated area of the flap and vice versa. There were two types of studies done. In the chronic study (n=10), NIR spectroscopy was done on the intact skin preoperatively for three days and then after elevation of the flap at various sites for three days. Preoperative measurements showed excellent reproducibility, and postoperative measurements showed progressive deoxygenation toward the distal aspect of the flap. NIR imaging at 1 h after flap elevation showed a zone of demarcation that corresponded with that noted clinically at 72 h. In the acute study (n=3), NIR spectroscopy, imaging and laser Doppler flowmetry were acquired before, immediately following and 1 h after raising the flap, and then measurements were taken after applying a vascular clamp across the base of the flap; reperfusion was evaluated after clamp release. Spectroscopy immediately following flap elevation indicated deoxygenation of the most distant part of the flap. Clamping the base of the flap caused deoxygenation of the whole flap; this was immediately evident on both spectroscopy and imaging. These changes recovered after releasing the clamp in the areas that were expected to survive. Laser Doppler flowmetry results generally correlated well with the NIR spectroscopy and imaging results. However, the method was very sensitive to fine movements during monitoring. The main advantage of NIR monitoring is that it looks directly at oxygen consumption rather than measuring blood flow. In addition, the NIR imaging gives a global picture of the eventual fate of the flap. These properties make these devices much more practical when the flap's well-being is concerned.
Comparison Between Near-infrared Spectroscopy and Laser Doppler Flowmetry in Free Flap Adjunct Monitoring