Transcatheter Embolotherapy in the Lower GI Tract

If stable, a patient with lower GI bleeding should ideally undergo colonoscopy. As mentioned earlier, an nuclear medicine study may prove useful if colonoscopy is negative or indeterminate. The 99mTc tagged RBC study may help localize the bleeding site (ie, SMA versus IMA vascular territory) (Bentley and Richardson, 1991). The IMA injection is generally performed first during arteriography of the lower GI tract, because as contrast is excreted by the kidneys, the bladder becomes opacified. This opacification may obscure a sigmoid colon or rectal bleeding site. If the tagged RBC study indicates that the patient has a left colon/rectal bleed, an inferior mesenteric arteriogram is performed, generally using a Simmons I (ie, "shepherd's crook") catheter. Once formed, the catheter is used to selectively catheterize the proximal aspect of the IMA trunk. This catheterization technique requires forming the Simmons catheter into a hook configuration prior to engaging the vessel (ie, over the aortic bifurcation or in the aortic arch/descending thoracic aorta).

If the bleeding site is identified in the descending colon, super-selective catheterization is attempted (ie, advancing a microcatheter through the diagnostic catheter), followed by embolization of the bleeding site with microcolis or tiny particles (Nicholson et al, 1998; Cynamon et al, 2003; Bandi et al, 2001). Vasopressin may also be used and is generally infused directly into the IMA trunk (see Table 102-1 for preparation and infusions rates of vasopressin). The use of vasopressin does not require super-selective catheteriza-tion to deliver the drug. Instead, the Simmons I catheter may be directed so the catheter tip is engaged in the proximal aspect of the main trunk of the IMA. The technique is similar for the SMA (ie, vasopressin is infused into the main trunk of the SMA, rather than through a super-selective catheter). Vasopressin is then infused according to the scheduled outlined in Table 102-1 (Darcy, 2003).

Lower GI bleeding from the rectum has been treated with transcatheter Gelfoam embolization via super-selective catheterization of the distal most branches of the superior hemorrhoidal artery arising from the IMA. Reports using larger particles or Gelfoam delivered super-selectively to stop small bowel and colonic bleeding have appeared in the literature. This has been shown to be safe and effective in experienced hands.

Vasopressin is not without risk (see Complications). Once vasopressin therapy has been initiated, the drug is tapered slowly. It is important that a 20-minute follow-up arteriogram be performed after initiation of vasopressin therapy. If there is excessive "pruning" of SMA or IMA vessels, bowel infarction may occur. Therefore, the vasopressin dosage must be reduced initially by half the amount or discontinued for a period of time, then restarted at a lower dosage. Patients will normally complain of mild abdominal cramping at the start of vasopressin therapy. However, unrelenting and continuous severe cramping necessitates a decrease in the vasopressin dosage because of the risk of bowel ischemia/infarction.

To reemphasize, in the SMA or IMA vascular distributions, transcatheter embolotherapy may be performed if the site of bleeding can be reached using super-selective catheterization techniques. Unlike the upper GI tract, embolization in the lower GI tract is "less forgiving." The microcatheter tip must be positioned as close as possible to the site of hemorrhage (eg, to the level of a single arterial branch at the level of the vasa recta). If possible, the marginal artery of Drummond should be preserved. If super-selective catheterization is not possible, alternative therapies to stop bleeding using vasopression, endoscopy, or surgery must be considered.

As an alternative to the use of iodinated contrast (eg, in patients with renal dysfunction), limited use of Gadolinium may prove helpful. Such magnetic resonance imaging contrast agents are injected intra-arterially and are only useful if digital subtraction arteriography is used (ie, computer-based imaging). In the authors' experience, the image quality and anatomic detail is not as good with Gadolinium as compared to the use of iodinated contrast agents. Intra-arterial injection of Gadolinium is considered "off-label" (ie, not FDA approved). If the cause of GI bleeding (upper or lower) is obscure, arteriography may identify a structural abnormality and should be performed as part of the comprehensive evaluation when other diagnostic tests (endoscopy, nuclear medicine, etc, are negative) (Rollins et al, 1991).

Provocative Testing

In the patient who is intermittently bleeding and in whom endoscopic and transcatheter techniques have failed to localize the site, the use of "provocative" measures to induce bleeding is controversial. The use of heparin, pharmacologic vasodilators and thrombolytic agents is not without considerable risk. Though described in the literature (Ryan et al, 2001), the authors of this chapter reserve this technique as a "last resort" (eg, in the patient who is "transfusion depen dent" despite multiple extensive endoscopic and radiographic evaluations).

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