16 Indocyanine Green/SPY Imaging in Perforator Flap Breast Reconstruction
Michael R. Zenn
Fluorescent angiography is a simple and effective tool for measurement of tissue perfusion both in and out of the operating room. This technology has been used clinically since the 1970s and is based on indocyanine green (ICG) dye that absorbs light in the near-infrared spectrum.1–4 Since 2007, the technology has been used by plastic surgeons to assess the perfusion of tissues in real time in the operating room.5–25 The technology can also be used multiple times throughout the same procedure as needs change. This ability of the surgeon to directly control the imaging of perforators and tissue perfusion in real time has been a game changer for the burgeoning field of perforator flap surgery.
The ICG dye is a water-soluble tricarbocyanine dye that was originally developed by Eastman Kodak for use in infrared photography. It was introduced into clinical use when, as a token of appreciation for his treatment at the Mayo Clinic, an executive at Eastman Kodak sent a variety of dyes to a clinician who was looking for suitable dyes that could be safely administered to patients and easily measured in blood. The clinically useful formulation of the dye, a stable lyophilized powder, was developed. For a time ICG was referred to as Fox Green after the cardiologist, Dr. Irwin Fox, who was responsible for its introduction into clinical practice. Initially, ICG was used to assess cardiac output by means of dye dilution technique.26 The near-infrared absorption and emission of light by ICG makes it particularly well suited to visualizing small blood vessels. In the early 1970s, Flower1 developed techniques for acquisition of fluorescence angiograms of the choroid utilizing ICG. Since that time, the basic technique has changed little other than improvement in near-infrared-sensitive cameras and light sources.
One of the biggest advantages of ICG over other dyes such as fluorescein is its rapid clearance from the tissues. Following intravascular administration, ICG is rapidly and extensively bound to plasma proteins, with α-lipoproteins being the major carrier in humans. The ICG is thus confined to the intravascular compartment with little leakage into the interstitium, making it an ideal tissue perfusion contrast agent compared with dyes such as fluorescein that remain in the interstitium, are not rapidly cleared, and therefore can only be used once.27 Fluorescein, therefore, provides only a static picture of perfusion. The plasma half-life of ICG is very short, 3 to 5 minutes in humans, with the dye being taken up by the liver and being excreted into the bile without any further metabolism. There is no renal excretion of ICG, and so there is no contraindication for use in patients with renal insufficiency. Since its introduction into clinical practice, ICG has shown an excellent safety profile, with a low incidence of adverse events of 1 in 42,000 patients. Anaphylactic reactions are rare. ICG contains iodide, so patients sensitive to iodides should avoid this technology. Most reactions are usually mild in nature and entail a feeling of warmth or a sore throat.
A typical intraoperative imaging system comprises an imaging camera that houses an 806-nm diode laser to provide near-infrared illumination. This low-level laser does not require the use of protective goggles by the surgeon or the operating room staff. Optics within the camera can even provide near-infrared illumination over fields as large as 20 x 20 cm or as small as 1 cm in an operating microscope. Cameras have also been adopted for use in robotic surgery (e.g., the DaVinci, Intuitive Surgical, Sunnyvale, CA) and endoscopic surgery (e.g., Firefly, Intuitive Surgical). Image sequences are usually captured at 30 frames/second, and the image sequences are displayed on a monitor in real time for the surgeon to assess. Most systems have a computer for further analysis, review, and archiving of the data.
The SPY system (Novadaq/Lifecell, Bonita Springs, FL) is the most commonly used system in plastic and reconstructive surgery ( Fig. 16.1 ), although there are others.28 It was originally developed for use in coronary artery bypass graft (CABG) surgery to confirm the patency of bypass grafts,26 and was cleared for this indication in Canada, Europe, and Japan in 2001. The SPY system received regulatory clearance in 2005 from the U.S. Food and Drug Administration (FDA) for use in CABG procedures, and subsequently clearance in 2007 for use in plastic and reconstructive and microsurgical procedures.
Technique
Once the surgeon is ready to perform the imaging, the camera is positioned and the anesthesiologist administers a 5-cc bolus containing 2.5 mg of ICG. No toxicity has been seen with total doses of 5 mg/kg (400 mg in an 80-kg patient). Typical acquisition times are about 1 minute. If one is attempting to visualize and localize perforating blood vessels, the camera is turned on immediately to record the first blush. If perfusion is the goal, the image is captured from the start of ICG injection to 1 to 3 minutes for full tissue evaluation. If a second scan is desired, the surgeon should wait 10 minutes to allow washout of the ICG for optimal scanning. Current software, such as the SPY-Q (Novadaq/Lifecell) enables the data to be seen with colormetric analysis for easier interpretation ( Fig. 16.2 ). This software can analyze data and plot the areas of maximal intensity that correlate with the best perfusion or perforator location, and can subtract any background ICG still present from a previous injection. This software has been used successfully to predict mastectomy skin and nipple necrosis in multiple studies.29–32
Applications
Indocyanine green imaging remains a superficial evaluation of tissue blood flow, which can limit its applicability. Despite this, the technology is highly useful when there is a need for perfusion assessment a few millimeters deep, and it can help the surgeon make clinical decisions. This is especially useful in perforator breast reconstruction where (1) evaluation of mastectomy skin flaps is critical, (2) localizing of perforators is desirable, and (3) definition of the angiosome/perforasome supplied by selected perforators can be visualized and marked directly. Postoperative monitoring with ICG is currently impractical but may have future applications.
Evaluation of Mastectomy Skin Flaps
As the mastectomy field has evolved to more nipplesparing and skin-sparing mastectomies, the incidence of nipple and skin necrosis has increased, being as high as 40% in some studies.27,29–32 Often, this is a result of overdissection of the skin flaps and loss of axial blood flow within the flaps. These “random” skin flaps can be effectively evaluated by ICG ( Fig. 16.3 ). Areas within mastectomy skin or nipples that do not fluoresce are relatively ischemic and have an associated higher risk of necrosis. Software analysis can further define the at-risk areas. When confronted by such ischemia, often in the face of a normal clinical exam, the reconstructive surgeon should pay heed to it, and further surgical planning should account for it. In some cases this may mean skin resection and replacement with flap skin. In other cases, this may mean skin flap delay or delay of the entire surgical procedure until the skin declares itself ( Fig. 16.4 ).
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