Comparison of ischemic and reperfusion injury in canine bowel viability assessment.

The purpose of these experiments was to evaluate two methods of bowel viability assessment in two distinct models of intestinal ischemia. Bowel viability was assessed in 32 dogs by means of three methods: (1) a probe that quantified the intestinal electromyographic (EMG) measurements in millivolts (mV), (2) Doppler ultrasonography, and (3) perfusion fluorometry, which quantified serosal blood flow in indexed dye fluorescence units (dfi). Ischemia was created using one of two methods: (1) a chronic model in which the blood supply to 40 cm of ileum was ligated and viability assessed 24 hours later, or (2) an acute model in which the main superior mesenteric artery was occluded for 3 1/2 hours and then released. Viability parameters were assessed every 5 minutes for 30 minutes after release. After viability assessment was completed, the ischemic bowel was resected and anastomosed at the site where the EMG measurements approximated 50% of the values obtained in normal bowel. In the chronic group 3 of 20 dogs died of necrosis in contrast to none of 12 dogs in the acute reperfusion group. In the acute model EMG values steadily increased after reperfusion, stabilizing by 15 minutes after release. Mean EMG values at 15 through 30 minutes after release were significantly greater than the 5- and 10-minute postrelease and prerelease values, suggesting that the electromyogram is affected by reperfusion. Conversely, postrelease fluorometry measurements rapidly increased to levels that exceeded measurements obtained in normal bowel. There was a significant difference in the number of audible Doppler signals in the marginal artery of survivors of the acute vs. the chronic model. Fluorometry measurements in survivors of the acute model (99+/-9 dfi) were significantly greater than measurements in the chronic model (54+/-4 dfi, P

Evaluation of quantitative approaches to assessment of bowel viability.

Predicting the survivability of intestine that has been made ischemic by impairment of blood flow is a major unsolved problem in gastrointestinal surgery. Currently, the surgeon must rely on qualitative, often subjective assessments that are known to have marginal reliability. This review describes various approaches to quantitatively assess the survivability of intestine compromised by ischemic disease. Much of the review centers on work done in the authors' laboratory to evaluate various approaches to predicting long-term survival and to develop new assessment parameters. Towards that end the authors have designed and developed techniques based on intestinal contractility and myoelectric activity (the intestinal EMG). Their evaluations of these and other methods of viability assessment utilize a highly representative canine model of intestinal ischemia that closely follows the development and treatment of ischemic intestinal disease in humans. Results to date suggest that the myoelectric measurements are more reliable than parameters based on blood flow or visual evaluation in terms of predicting bowel survival. However, improvements in instrumentation and technique are needed before this approach is suitable for clinical use.

Bacterial translocation after mesenteric ligation in dogs.

These experiments were designed to determine the relationship between translocation of Escherichia coli and viability of ischemic small bowel. Twenty beagles were gavaged with 14C-labeled E. coli at two time intervals (3 and 24 h) prior to ligation of the blood supply to a 40-cm segment of ileum. Mesenteric lymph node (MLN) biopsies and bacterial cultures of the peritoneal fluid, peripheral arterial blood, and splanchnic venous blood were taken immediately prior to ligation and 24 h later both before and after the ischemic bowel was resected and anastomosed. Biopsies of each resection margin were taken to measure translocation of E. coli into the bowel wall. Several hemodynamic hemodynamic parameters were also measured before and 24 h after ligation. Seven of the 20 dogs died of further bowel necrosis. In survivors A-alpha DO2 was significantly decreased 24 h after mesenteric ligation vs. preligation, whereas in dogs that died DO2 was significantly increased after ligation vs. preligation. The incidence of mesenteric venous cultures positive for E. coli was significantly higher 24 h after ligation vs. preligation. However, there was no correlation between survival and the incidence of positive E. coli cultures in the blood or peritoneal fluid. Mean MLN counts were significantly higher in dogs gavaged at 3 h vs. those gavaged 24 h prior to laparotomy. However, there was no correlation between survival and translocation into either the bowel wall or MLN at either time interval. Viability of ischemic small bowel in this canine model was not affected by translocation of E. coli. Hemodynamic parameters that are altered during the course of sepsis also did not correlate with survival.

Comparison of blood flow and myoelectric measurements in two chronic models of mesenteric ligation.

OBJECTIVE: To determine whether the predictive accuracy of intestinal motility and blood flow measurements is altered by the magnitude of ischemic damage. DESIGN: Inception cohort study (dogs). Motility was measured using a probe that quantifies both the electromyographic (EMG) measurements and the magnitude of evoked contractile response (ECR). Intestinal blood flow was assessed using Doppler ultrasonography in the marginal artery and perfusion fluorometry, which quantifies fluorescein in the bowel wall in dye fluorescence units. SETTING: Vivarium animal research facilities at a medical school. INTERVENTIONS: The blood supply of a 40-cm length of ileum was ligated in 102 dogs: 52 in which the marginal artery was ligated at two points 8 cm apart (severe model), and 50 in which the marginal artery was ligated only once (moderate model). Twenty-four hours after ligation, the motility and blood flow parameters were measured in normal bowel and at 2-cm intervals within the 40-cm ischemic segment. Resection and anastomosis of ischemic bowel was then performed using either EMG, ECR, or fluorometry to determine the site of resection. OUTCOME MEASURE: Anastomotic leak from progressive ischemia. RESULTS: There were 26 fatal anastomotic leaks, all due to necrosis at the anastomosis. Perfusion fluorometry and ECR measurements did not correlate with survival in either model. An audible Doppler pulse in the marginal artery correlated with survival in the moderate (P < or = .02) but not the severe model (P = .59). The EMG measurements were significantly greater in survivors vs nonsurvivors in both models. CONCLUSIONS: The EMG measurements may be useful in bowel viability assessment. Correlation of Doppler ultrasonographic findings with survival in the moderate model suggests that blood flow measurements may be more reliable in predicting viability in less ischemic bowel.

Determination of small bowel viability using quantitative myoelectric and color analysis.

Intraoperative assessment of viability in ischemic bowel remains a major unsolved problem in general surgery. Bowel viability was assessed in 31 dogs 24 hr after mesenteric arterial ligation in a 40-cm segment of ileum. The purpose of this study was to evaluate two new quantitative methods of viability assessment, electromyography (EMG) and reflection densitometry. These methods were compared with traditional methods of bowel viability assessment. EMG was measured using a specially designed probe which contains an electronic control unit for computer-assisted data acquisition. A computer algorithm quantified the EMG. Reflection densitometry quantified bowel color in the red and blue spectrum. Presence of visible peristalsis, Doppler ultrasound, and a visual color grading system were also used in viability assessment. Each parameter was measured at 2-cm intervals along the ischemic segment. Resection and anastomosis of ischemic bowel were then performed. There were nine deaths from anastomotic leak, all resulting from further bowel necrosis. EMG was the only viability assessment parameter that correlated with survival. EMG at the resection margin in survivors was 29 +/- 3% vs 23 +/- 4% at the resection site of nonsurvivors (P < or = 0.047 by unpaired Students t test). These results suggest that quantitative EMG measurements may be useful in assessment of viability in ischemic bowel.

Intraoperative assessment of bowel viability.

Viability of ischemic bowel was assessed in 30 dogs after mesenteric arterial ligation in a 40-cm length of ileum. Viability was evaluated using two gross features, color and peristalsis, and four objective methods including bowel wall surface oximetry (pO2), Doppler ultrasound, quantitative fluorescein fluorimetry, and myoelectric activity measured by a strain gauge probe. Each parameter was measured at 2-cm intervals within the 40-cm ischemic segment before resection and anastomosis was performed. There were seven fatal anastomotic leaks, all due to further bowel necrosis. Survival did not correlate with bowel color, presence of peristalsis, bowel wall pO2 Doppler ultrasound, or the myoelectric parameters. However, fluorescein fluorimetry was predictive of long-term viability. These results suggest that quantitative fluorescein fluorimetry may be a useful adjunct in intraoperative bowel viability assessment.

Comparison of isomolar vs hyperosmolar enteral diets in experimental ileus.

The diarrhea observed after infusing hypertonic enteral alimentation solutions may be due to the high osmolality. We compared Vivonex HN (810 mOsm) to Osmolite (300 mOsm) in two canine ileus models. After having bipolar electrodes implanted in the stomach, duodenum, jejunum, and colon, four dogs sequentially underwent operations to produce intestinal obstruction (SBO) or perforation/peritonitis (PER). The SBO was released and the perforation closed 24 hr later. GI myoelectric activity (MEA) was monitored during the first 4 postoperative days and again on the 10th day to determine steady-state MEA. Fasting MEA was recorded for 1 hr, followed by 1-hr recordings after intragastric cannula infusion of either Vivonex HN or Osmolite. There was no significant difference in MEA produced by Osmolite vs Vivonex at any recording site. The 24-hr postop gastric and small bowel MEA was significantly decreased (p less than 0.05) vs postoperative days 2 to 4 and 10 under both fasting and fed conditions. The ileus operations had no effect on colonic MEA. These data show that SBO and PER cause significant decreases in gastrointestinal MEA for 24 hr. There was no difference in MEA response of Osmolite vs Vivonex HN. These results suggest that infusion of hypertonic enteral alimentation solutions does not produce increases in GI myoelectric responsiveness vs isotonic solutions.

Comparison of five methods of assessment of intestinal viability.

A strain gauge device called the electronic contractility meter (ECM) was designed to measure quantitatively intestinal ischemic damage by delivering electrical stimuli to the intestine. Threshold stimulus level (TSL) was the minimum stimulus in milliamps necessary to produce a smooth muscle contractile response. TSL scale ranged from zero to 100 milliamps. Resection and anastomosis in ischemic intestinal segments was carried out in 30 dogs to compare TSL, intestinal color, peristalsis, Doppler ultrasound and resection margin histology with survival. There were five fatal anastomotic leaks, all resulting from intestinal necrosis. Doppler pulse in the marginal artery (MA) was absent at four of the five anastomoses that leaked versus eight of the 25 that healed (p less than or equal to 0.01). Mean TSL at the resection site was 38 +/- 7 milliamps in surviving dogs versus 51 +/- 4 milliamps in nonsurvivors (p less than or equal to 0.001). Mean TSL of normal intestine was 22 +/- 2 milliamps. Both TSL and Doppler ultrasound also correlated with resection margin histology (p less than or equal to 0.02 and p less than or equal to 0.005, respectively). Presence of peristalsis did not correlate with histologic grade or survival rate. Intestinal color correlated with resection margin histology (p less than or equal to 0.001) but not survival. The ECM and Doppler ultrasound were both superior to gross visual assessment in predicting intestinal survival. The ECM quantitatively measures tissue damage, a potential advantage over Doppler ultrasound, which provides only an estimate of local arterial blood flow.

Myoelectric assessment of bowel viability.

A device capable of quantitative measurement of intestinal ischemic damage has been designed. The device is called the electrical contractility meter and employs a clip-on strain gauge transducer that delivers a precisely controlled electrical stimulus to the bowel. Threshold stimulus level (TSL) is the stimulus current in milliamperes (mA) that is necessary to produce a clearly defined smooth-muscle contraction. In 30 dogs, TSL was used to establish viability boundaries in 40 cm ischemic bowel segments. Bowel viability was assessed with the use of TSL in comparison with gross features (color, peristalsis) and blood flow measured by means of Doppler ultrasound at 2 cm intervals in the ischemic small-bowel segments. The TSL scale ranges to 100 mA and varied from a low of 22 +/- 2 mA in normal bowel outside the ischemic segment to 97 +/- 4 mA in grossly gangrenous bowel. There was a consistent correlation between rising TSL and worsening bowel color, disappearance of visible peristalsis, and progressive disappearance of audible Doppler signals. Resection and anastomosis were performed in three groups of 10 dogs at TSL measurements of 30 mA, 40 mA, and 50 mA, respectively. Dogs were killed and anastomoses were inspected on the tenth postoperative day. There were no leaks at TSL = 30 mA, one leak at TSL = 40 mA, and four leaks at TSL = 50 mA. The number of leaks at TSL = 50 mA was significantly greater than at TSL = 30 mA (p less than 0.04, Fisher's exact test). These results show that the quantitative myoelectric parameters established by this device provide a reliable in vivo assessment of bowel viability. The electrical contractility meter is easy to use and may have clinical applicability.

Quantitative myoelectric determination of bowel viability.

A new device designed to quantitatively measure viability in ischemic bowel was studied in 20 dogs. This strain gauge device is clipped on the serosal surface and can rapidly deliver an electrical stimulus. The threshold stimulus level (TSL) was the stimulus necessary to produce a clearly defined smooth muscle contraction. TSL was compared with blood flow measured by Doppler ultrasound at 2-cm intervals in 30-cm ischemic segments as a determinant of bowel viability. Doppler readings were taken from the bowel wall (BW), peripheral arteriolar branches of the marginal artery (PA), and marginal artery (MA), itself, at each 2-cm interval. Bowel color and peristalsis were correlated with Doppler and TSL data. TSL scale ranged from 0 to 100 mA and varied from 21 +/- 2 mA in normal bowel to 98 +/- 2 mA in gangrenous bowel. At the locus of the last audible Doppler signal in the BW, PA, and MA, mean TSL readings were 27 +/- 5, 34 +/- 6, and 48 +/- 16 mA, respectively. These readings were each significantly greater than the TSL of normal bowel at P less than or equal to 0.0001 level. Portions of each ischemic segment were resected and reanastomosed. Absence of Doppler signals in the BW and PA did not preclude healing of ischemic bowel. These data suggest that TSL measurement is more sensitive than either Doppler ultrasound or gross visual evaluation in assessment of bowel viability.

Gastrointestinal myoelectric activity in mechanical intestinal obstruction.

Experiments were designed to study the effect of duration of small bowel obstruction (SBO) on rate of recovery of fasting and nonfasting GI myoelectric activity (MEA) in 10 dogs. Two weeks after implantation of a gastric cannula and bipolar silver electrodes in the antrum, duodenum, proximal and midjejunum, terminal ileum, right and left colon, complete SBO was created in the distal ileum. Duration of SBO was 24 hr in group I (five dogs) and 48 hr in group 2 (five dogs). MEA was recorded on 5 consecutive postop days after creating SBO. Water (500 cc) was given via cannula 90 min after release of SBO and on subsequent days after 90 min of fasting MEA recording. Control data were pooled from MEA recordings obtained on 10th and 12th days after electrode implant, each dog serving as its own control. On the day of SBO release there were no significant differences in MEA between groups 1 and 2. Twenty-four hours after SBO release, group 2 fasting and nonfasting jejunal and ileal MEA was significantly decreased vs group 1 and control levels. These significantly decreased jejunal and ileal MEA levels persisted for 48 hr after SBO release. After water by cannula, group 2 antral MEA was significantly less than group 1 and control levels and remained at these low levels for 72 hr after SBO release. Colonic MEA in group 1 and group 2 was not significantly decreased vs control levels. These data suggest that the colon does not play a regulatory role in ileus caused by SBO. These results also imply that prolonged ileus can be avoided by early operation for mechanical SBO.

Gastrointestinal myoelectric activity in fasting and nonfasting canine models of ileus.

We studied postoperative gastrointestinal (GI) tract myoelectric activity (MEA) in three fasting and nonfasting models of ileus. After implantation of a gastric cannula and bipolar electrodes in the antrum, duodenum, proximal jejunum, midjejunum, terminal ileum, and ascending and descending colon, five dogs underwent ileus-producing laparotomies, including handling, in which the entire GI tract was manually crushed, obstruction, in which a distal ileal obstruction was created and released 24 hours later, and peritonitis, in which an ileal perforation was created and closed 24 hours later. The fasting 24-hour postoperative colonic MEA in the handling and obstruction models was significantly less than control levels but returned to normal by 48 hours. The nonfasting 24-hour postoperative duodenal and jejunal MEA in the obstruction model was significantly less than control levels. These significantly decreased MEA levels persisted for 72 hours. Nonfasting 24-hour postoperative ileal and colonic MEA in the peritonitis model was significantly greater than control levels and remained significantly elevated for 48 to 72 hours before returning to normal.

Mechanisms of protein activation of the intestinal phase of gastric secretion.

To determine if the secretory response to protein in the gut is due to direct effects of absorbed amino acids on the parietal cells, to hormonal effects, or to a combination of both, Heidenhain pouch secretion and plasma amino nitrogen levels were measured during enteral infusions of casein or liver extract, as well as during intravenous infusions of amino acids, in three dogs with and three dogs without portacaval transposition. In the dogs without portacaval transposition, gastric secretory responses were slight and did not differ significantly; secretory responses were seen only during infusions in which elevations of plasma amino nitrogen values occurred. In dogs with portacaval transposition, secretory responses to both casein and liver extract were significantly higher (P less than 0.02) than was the response to intravenous amino acids, while plasma amino nitrogen levels rose to virtually identical levels during all three kinds of infusion. These data support the hypothesis that absorbed amino acids contribute to the intestinal phase of gastric secretion. The finding that casein and liver extract in the gut evoked greater gastric secretion responses than did intravenous amino acids, even though increases in plasma amino nitrogen levels were similar, suggests that the intestinal phase results form the combined effects of absorbed amino acids and hormonal influences rather than from the effects of amino acids alone.