Dr. Mohamed E. MustafaDept. of Surgery

SMC

Ischemic colitis is a rare but possibly fatal complication of aortic reconstruction.

Its diagnosis is often delayed, resulting in significant patient morbidity and mortality.

To avoid this complication, the vascular surgeon must:

1. Be aware of the preoperative, intra operative, and postoperative risk factors.

2. Be knowledgeable of the vascular anatomy and the many vascular collaterals that may protect the colon when blood flow is reduced.

The reported incidence of colonic ischemia in patients undergoing aortic surgery ranges from 0.2 to 10%.

Colonic ischemia has been detected in 6% of patients undergoing aortic reconstruction who had routine post operative colonscopy.

60% of patients routinely studied following repair of a ruptured AAA will have evidence of ischemia.

Pre Operative: 1. Age 2. Ruptured AAA 3. Renal Disease 4. Coagulopathy 5. Loss of collaterals from a previous colectomy/

GI surgery. 6. Occlusive disease of the internal iliac arteries and superior mesenteric arteries. 7. Prior pelvic radiation therapy

Intra Operative: 1. Intraoperative embolization of the

inferior mesenteric artery from manipulation of the aortic aneurysm.

2. Insertion of an aortobifemoral graft 3. Increasing the length of cross-clamp

and operative time. 4. ligation of one or both of the IIA.

Post Operative:

1. Hypoxemia 2. Hypotension/Hypovolemia 3. Use of vasoactive drugs.

Anatomy

Colon receives arterial flow from 3 main vascular beds:

1. Superior Mesenteric Artery2. Inferior mesenteric Artery3. Internal Iliac Arteries.

These are usually interconnected by collaterals.

Cecum and Ascending colon: Terminal branches of SMA, mainly the ileocolic artery.

Transverse colon: supplied by 2nd branch of the SMA, the middle colic artery (There is a significant anatomical variation in MCA; up to 20% of individuals may not have an MCA.

Left colon, Sigmiod colon and superior part of the rectum: Inferior mesenteric artery.

Collaterals: Collateral arteries may be an important source

of blood to the colon if one of the major arteries is occluded.

Marginal Artery: runs parallel to the mesenteric edge of the colon. There are 2 parts of the marginal artery , the Arc of Riolan and the marginal artery of drummond, which refer to the parts of the artery that run next to the distal transverse and splenic flexure.

Meandering mesenteric: This is a more centrally located artery

connecting the MCA and the IMA. It can become quite large and provide important collateral flow.

Based on pre op aortograms, the meandering artery has been found in 35% of pts with aortoiliac occlusive disease and 27% of pts with aortoiliac aneurysm. Its presence should alert the surgeon that there is an abnormality in the mesenteric circulation.

Methods of Intra Operative Assessment of Bowel

Viability during Aortic Repair

1. Clinically 2. Doppler Ultrasound 3. IV Fluroscien 4. IMA Stump Pressure 5. Sigmoid Intramural pH 6. Photoplethysmography ( Pulse Oximetry) 7. Laser Doppler Flowmetry

Loss of arterial pulsation of the supplying mesenteric artery.

Blue/black color of bowel Loss of bowel sheen Los of peristaltic activity Temperature Studies have shown that using clinical

judgment has an Overall accuracy of 87% Predictive value (69%) Predictive value in first laparotomy (58%)

In 1978, Ernst et al. proposed measuring IMA stump pressures in pts undergoing AAA repair.

Pressures were measured pre- & post-aneurysm repair with an 18 gauge angiocatheter inserted either directly into the IMA or by threading the catheter into the orifice of the IMA from within the aneurysm sac.

The study included 52 patients undergoing AAA repair.

39 of the 52 patients had IMA stump pressures measured. All had postoperative colonoscopy. One of the 39 patients on colonoscopy had evidence of mild mucosal ischemic colitis. This patient had an IMA stump pressure of

37mmHg, whereas all the other patients had IMA stump pressures greater than 40mmHg.

Expansion of this study was done in 1983 to include 64 pts. Of the 64 patients, pelvic blood flow could not be restored in 3.

Two of the patients had stump pressures greater than 40mmHg, had their IMA ligated, and did not develop colonic ischemia.

One patient had an IMA stump pressure of less than 40mmHg, did not have IMA reconstruction, and developed ischemic colitis.

Two other patients were found to have an IMA stump pressure less than 40mmHg.

One patient underwent IMA reimplantation and did not develop ischemic colitis, and the other patient had the IMA ligated and developed colonic ischemia.

Thus, it was recommended that if the IMA stump pressure following aneurysm repair is greater than 40mmHg or if the IMA was chronically occluded, then the IMA could be ligated without ischemic colitis occurring.

However, Schiedler et al., found 5 (21%) patients among 24 who had stump pressures lower than 40 mmHg who did not develop ischemic colitis.

Thus, the reliability of this technique has been questioned.

This technique cannot be applied to many patients undergoing aortic reconstruction for occlusive disease.

In addition, IMA stump pressure measurements may not prevent the postoperative development of ischemic colitis in patients with marginal or normal IMA stump pressures who become hypotensive in

the perioperative period.

Hobson et al., using a doppler US probe against the antimesenteric border of the colon suggested that if the arterial signals were present during temp. occlusion of the IMA, then there is adequate collateral flow to prevent colonic ischemia.

The significance of lost or reduced arterial signals is not known as no controls were used in this study.

The weakness of this method is that it is a qualitative method, and may be dependent on where the probe is placed.

Also, Doppler signals may be present despite less than adequate perfusion to prevent colonic ischemia.

This can be measured using a tonometer, which is a silicone balloon permeable to co2, which is filled with saline and introduced transanally into the colon.

Once the co2 in the saline filled balloon equlibriates with the co2 of the mucosa, the Pco2 in the balloon and the arterial blood HCo3 can be entered into the Henderson-Hasselbalch equation to get the intamural pH.

Fiddian-Green’s study showed that there was a sensitivity of 100% and specificity of 87% in predicting ischemic colitis when the intramural pH was < 6.86.

This method has the advantage that it can be used not only intraoperatively, but also during the post operative period to detect ischemic colitis.

However, its accuracy is dependent on several factors:

the balloon being in the proper position in the sigmoid/descending colon.

The co2 in the balloon accurately reflecting mucosal co2

Good apposition of the balloon to the mucosal surface

Using IV fluorescein and a UV light (woods light), tissue perfusion can be assessed by the uptake of fluorescein by the examined tissues.

In a retrospective study, IV fluorescein was used in 186 pts.

3 fluorescein patterns were identified: Normal (169 pts) Patchy uptake (11 pts) Absent (4 pts)

None of the pts with normal or patchy uptake developed clinical evidence of colonic ischemia.

The recommendations of this study was that pts with an absent or patchy fluorescein pattern should undergo IMA reconstruction.

Disadvantages of this method include: 1. It provides a qualitative, not quantitative

result. 2. It assesses mainly serosal, and not mucosal

blood flow. 3. Difficulties in reusing fluorescein accurately

in the same pt.

Pulse oximetry detects alteration in reflected infrared light as a function of Hb absorption and has been a widely accepted reflection of o2 saturation.

Ouriel et al. did a study on pts undergoing aortic reconstruction which showed:

colonic ischemia did not occur in 28 pts in which the pulsality did not change from the baseline.

2 pts had loss of pulsality & developed ischemic mucosal changes based on post op colonoscopy.

The disadvantages of this method are: 1. when pulsatility is lost, there is no longer the

ability to accurately measure transcolonic o2 saturation.

2. Pulse oximetry is unable to differentiate potentially recoverable mucosal ischemia from complete irreversible ischemia.

LDF uses monochromatic light that is reflected from moving RBCs and is conducted back to a photodiode. The signal is then processed & expressed in millivolts, which is directly proportional to the velocity of blood.

Ahn et al. found that colonic LDF values in 62 pts varied between 5 & 42ml/min per 100g.

LDF has been shown to be more sensitive than doppler US in detecting bowel ischemia.

Lynch et al. compared Doppler US, laser Doppler and fluorometry in a dog model of small bowel ischemia.

It was found that Laser doppler Index (= Experimental segment velocity/reference segment velocity x 100) had the greatest sensitivity in detecting ischemic bowel at 94% sensitivity.

The earlier Ischemic colitis is identified in the post op period, the lower the mortality.

Because there is no 100% accurate method of intraoperatively predicting ischemia, one must look for post op signs suggesting Ischemic colitis

1. Diarrhea – occurs in 40% of post pts developing ischemic colitis

2. Bloody Diarrhea : highly suggestive of Ischemic colitis, but only occurs in 25% of pts.

3. Peritonitis or severe abdominal pain occurs only in 12% of pts.

4. Acidosis, raised LDH, Leucocytosis

Late Signs: Low Cardiac output Coagulopathy Multi organ failure

Flexible sigmoidoscopy/colonoscopy is the investigation of choice.