1. COLORECTAL VASCULARIZATION
The colon derives embryologically from the midgut and hindgut. It is divided into the cecum, ascending, transverse, descending and sigmoid colon and rectum.
- Blood supply to the midgut portion (cecum to splenic flexure) derives from the superior mesenteric artery, namely ileocolic artery, right colic artery (inconsistent) and middle colic artery, which further divides into right and left branch.
- The blood supply to the hindgut portion (splenic flexure to rectum) derives from the inferior mesenteric artery, namely left colic artery, sigmoid branches and superior rectal artery.
- Superior mesenteric artery and inferior mesenteric artery connect at the splenic flexure via the marginal artery (of Drummond), thus enabling collateral supply.
- The distal part of the rectum derives additional supply from the internal iliac artery via the pudendal artery which gives rise to the inferior and middle rectal arteries.
Arteries, veins and lymphatic drainage are located in the mesocolon, which during the oncological surgical resections of total mesorectal excision and complete mesocolic excision is removed in its entirety depending on the part of the colon to be removed.
2. VASCULARIZATION IN COLORECTAL SURGERY
The most common indication for surgical resection of the colon is colorectal malignancy, and the resection should be carried out following oncological principles, which include removing the vasculature and lymphatic drainage at the level of origin of the primary feeding vessel.
The key to adequate resection is the blood supply to the colon. Resection for benign disease need not be as extensive, but resection for malignancy should aim to resect the colonic vessels supplying the cancer-bearing portion of the colon as close to their origin as possible in order to yield an adequate number of lymph nodes in the colonic mesentery (> 12). The bowel proximal and distal to the resection must be mobilized to allow for a tension-free anastomosis, and the anastomosis should have a good blood supply.
The colon is protected from ischemia by a collateral blood supply via the marginal artery of Drummond, and a system of arcades connecting the major arteries. To have correct vascularization, we must consider patient factors during surgery (functional. Such as blood pressure and inotropes; organic, such as peripheral vascular disease, calcified vessels, and thrombosis, plaques), other colonic pathology (diverticulosis, proximal dilatation in obstructed cases) and freedom from tension (adequate length of the bowel, adequate length of the mesentery). It is also important to perform a careful surgical technique avoiding damage to the vessels that are not included in the oncological resection (marginal artery, arcade system) so as to ensure a correct vascularization at the level of the colon or the bowel and consequently at the level of the anastomosis.
3. INDOCYANINE GREEN PERFUSION ASSESSMENT
Traditionally, tissue perfusion and subsequent gastrointestinal anastomotic viability have been assessed intraoperatively based on pulsation in the mesentery, coloration of tissue, and bleeding from resection lines. These clinical tools are, however, limited when minimally invasive techniques are used. The current standard for intraoperative testing of anastomotic integrity involves “air-leak” testing and assessment of completeness of anastomotic “donuts”. Air-leak testing is easy and cheap and has been shown to more than halve the radiological anastomotic leak rate. In some centers, this is combined with intraoperative endoscopic assessment of the anastomosis. Alternative strategies include intraoperative assessment of anastomotic tissue oxygenation and white light spectroscopy.
Recently, intraoperative fluorescence angiography (IFA) has been introduced to evaluate anastomotic blood supply, with promising early results. Proof of concept for IFA has been established, representing an 8–9 fold reduction in the 12% documented leak rate following anterior resection.
The technique involves intravenous administration of Indocyanine Green (ICG). ICG is approved for human use by the United States Food and Drug Administration (FDA). It is a sterile tricarbocyanin compound, soluble in water, which can be administered intravenously or intraarterially. It absorbs NIR light at 800 nm and emits fluorescence (light) at a slightly longer wavelength of 830 nm. Indocyanine green binds quickly and extensively to plasma proteins and is limited to the intravascular compartment, with minimal leakage to the interstitium. It clears from the liver to the bile in 3 – 5 minutes with no known metabolites.
ICG contains no more than 5% sodium iodide and should be used with caution in patients who have a history of allergy to iodide or iodinated contrast. The most serious but rare risk of ICG when administered intravenously in humans, according to the product label for ICGREEN (by Akorn), is anaphylactic death, which has been reported after administration of IC-GREEN during a cardiac catheterization.
When irradiated with near-infrared light through an operating laparoscope, ICG fluorescence can be visualized on a standard visual display unit providing an image of tissue perfusion. Figure 5 illustrates the use of ICG-NIR angiography in selecting well-perfused bowel for anastomotic construction, and clearly demonstrates the potential advantage over white light assessment.
4. INTRAOPERATIVE FLUORESCENCE ANGIOGRAPHY
IFA assesses intestinal perfusion in two critical steps of the operation: just before the proximal colonic transection, and after performing the anastomosis. For the initial evaluation, the planned transection colonic line must be marked by the surgeon, after bowel mobilization, the section of the corresponding vessels, and the section of the mesocolon. Following the manufacturer’s instructions, a 0.1 mg/kg ICG bolus must be administered intravenously by the anesthesiology team. The fluorescence intensity in the bowel or the colon is evaluated as “fluorescent” or “non-fluorescent” thus ensuring the correct vascularization of the colon and marking this point as the transection point, both distally and proximally, depending on the colorectal surgery that we are performing. The anastomosis is then performed and a second 0.1 mg/kg bolus is administered if the surgeon finds it necessary to check whether perfusion is correct (for example, after an additional surgical maneuver such as mobilization of the splenic flexure).
5. QUANTITATIVE PERFUSION ASSESSMENT
Currently, new ICG systems are being put into practice that allow for quantitative assessment of tissue perfusion, ensuring even more that the best perfused area to perform a safe anastomosis is identified.
Laparoscopic fluorescence imaging is applied to colorectal cancer patients and the fluorescence intensity of colonic flow is measured sequentially, producing perfusion graphs using a video analysis and modeling tool. Colon perfusion patterns are categorized as fast, moderate, or slow based on their fluorescence slope. Clinical factors and quantitative perfusion factors are analyzed to identify predictors for anastomotic complications. With these quantitative analyses of ICG, perfusion patterns can be applied to detect segments with poor perfusion, thereby reducing anastomotic complications during laparoscopic colorectal surgery.