Mesenteric traction syndrome in pigs: A single‐blinded, randomized controlled trial

Abstract Background Mesenteric traction syndrome is commonly observed in patients undergoing upper abdominal surgery and is associated with severe postoperative complications. A triad of hypotension, tachycardia, and facial flushing seems provoked by prostacyclin (PGI2) release from the gut in response to mesenteric traction. The administration of nonsteroidal anti‐inflammatory drugs (NSAID) inhibits PGI2 release, stabilizing the hemodynamic response. Here, we examined the effect of mesenteric traction on splanchnic blood flow in pigs randomized to NSAID or placebo treatment. Materials and Methods Twenty pigs were allocated to either ketorolac or placebo treatment. Five minutes of manual mesenteric traction was applied. Plasma 6‐keto‐PGF1α, a stable metabolite of PGI2, hemodynamic variables, and regional blood flow (laser speckle contrast imaging) to the liver, stomach, small intestine, upper lip, and snout (laser Doppler flowmetry) were recorded prior to traction and 5 and 30 minutes thereafter. Results Both groups of pigs presented a decrease in systemic vascular resistance (P = .01), mean arterial blood pressure (P = .001), and blood flow in the gastric antrum (P = .002). Plasma 6‐keto‐PGF1α did not increase in either group (P = .195), and cardiac output, heart rate, central venous pressure, and blood flow to the liver, small intestine, upper lip, and snout remained unchanged. Conclusion Mesenteric traction resulted in cardiovascular depression, including reduced blood flow in the gastric antrum. Plasma 6‐keto‐PGF1α did not increase, and ketorolac administration did not alter the response to mesenteric traction. Furthers studies are needed to identify which substance is responsible for eliciting the cardiovascular response to mesenteric traction in pigs.


Mesenteric traction syndrome (MTS) is induced by traction and manipulation of the bowel and mesentery during open abdominal
surgery, 1,2 and an incidence of up to 80% is reported. [2][3][4] The syndrome is considered to be elicited by the vasodilator prostacyclin (PGI 2 ) released by endothelial cells upon stimulation, and a triad of hypotension, tachycardia, and facial flushing is provoked. The hemodynamic response is sudden, often presenting within 5 minutes of traction, and the severity of the PGI 2 -induced hypotension is usually moderate, 1,5 but severe and prolonged hypotension with little effect of vasopressor treatment has been described. 1,6 Also, severe postoperative complications after esophagectomy, gastrectomy, and pancreatic resection have been associated with MTS. 2,7 Splanchnic tissue blood flow is sensitive to hypotension, 8,9 and postoperative gastrointestinal tract complications are likely associated with inadequate blood flow to splanchnic organs during surgery. [10][11][12] Thus, hypotension in response to MTS could influence splanchnic tissue blood flow, but evaluation of this association is lacking. To attenuate the MTS response, treatment with nonsteroidal anti-inflammatory drugs (NSAID) is efficient, and both preoperative and intraoperative administrations of NSAID stabilize the hemodynamic response to MTS. 1,4,13 The anatomic organization and neurochemical and electrophysiological features of the autonomic nervous system in pigs are considered homologous to humans, 14 and, therefore, the study was conducted in pigs. We randomized pigs to ketorolac or placebo intervention for evaluation of MTS following mesenteric traction. We evaluated plasma PGI 2 , hemodynamic variables, and splanchnic and facial skin blood flow by laser speckle contrast imaging (LSCI) and hypothesized that ketorolac would inhibit the PGI 2 release in response to mesenteric traction and thus maintain cardiovascular integrity. Institute, University of Copenhagen, DK) under standardized room temperature, humidity, and light-dark cycles. No standardized feeding regime was applied, but the last feeding was provided >12 hours before anesthesia. The primary outcome was splanchnic blood flow assessed by LSCI, with hemodynamic variables and plasma PGI 2 as secondary outcomes.

| Randomization
Twenty pigs were allocated in a 1:1 ratio to the administration of either 10 mg intravenous ketorolac (ketorolac group) or saline (placebo group), and randomization (www.random.org) was concealed by numbered envelopes padded with nontransparent paper. A veterinarian was responsible for the envelopes and attested adherence to the randomization protocol by initiating the administration of ketorolac/placebo while the principal investigator was blinded during the study.

| Experimental protocol
Hemodynamic variables and plasma 6-keto-PGF 1α were obtained after induction of anesthesia. Then, a midline laparotomy was carried out, exposing the abdominal organs to mark ROIs on the organs.
After 15 minutes of stabilization, 10 mg intravenous ketorolac (ketorolac group) or saline (placebo group) was administered. After an additional 45 minutes of stabilization (60 minutes after laparotomy, "baseline"), to allow for the effect of the drug, regional blood flow, hemodynamic variables, and plasma 6-keto-PGF 1α were measured.
A 45-minute stabilization was chosen, as NSAID administration at 15, 90, or 120 minutes before mesenteric traction has proven prophylactic against MTS whereas administration just before surgery has not. 4 Following stabilization, continuous traction on the stomach and duodenum was applied for 5 minutes with 2 hands by the same individual as described by Brinkmann et al 1 Measurements of regional blood flow, hemodynamic variables, and plasma 6-keto-PGF 1α were subsequently collected 5 and 30 minutes after applied traction.
These intervals were chosen because the hemodynamic response to mesenteric traction is prompt, 1 and in prospective, randomized, placebo-controlled trials (NSAID versus placebo), the response was restored (or no longer statistically different) in both groups within 30 minutes. 1,18

| Statistics
The administration of NSAID has been shown to almost completely inhibit the PGI 2 release in response to bowel manipulation in humans. 4 Hence, assuming that MTS does not occur in pigs treated by ketorolac, 20 animals were considered to be required to detect a difference in plasma PGI To test for differences between groups at a single time point, a Mann-Whitney U test was applied. The endpoints were entered as the dependent variable in a linear mixed effect model, and Box-Cox transformed to reduce variance and to achieve assumptions required by the modeling approach. In the analyses, pig individual identification was entered as a random effect to correct for pseudoreplications. To control for false discovery rate, the Benjamini-Hochberg method was employed. 19 Data are presented as median with interquartile range, and a P-value <.05 was considered statistically significant.

| RE SULTS
Twenty pigs were included for analysis, and the randomization was balanced (ketorolac n = 10; placebo n = 10), and body weight was similar in the 2 groups (41.9 kg (SD ± 2.4); P = .684).
A significant interaction between time and placebo showed that placebo had an effect.

| Intentional traction
Blood flow measurements before and after abdominal traction are presented in Figure 1 and Table 1, and plasma 6-keto-PGF 1α , along with hemodynamic variables, is presented in Figure 2 and Table 1.
Values were stable at baseline apart from blood flow to the gastric corpus, which was elevated in the placebo group (P = .019). In both

| D ISCUSS I ON
The principal findings were significant reductions in SVR, MAP, and blood flow to the stomach in both the placebo and the ketorolac group after intentional mesenteric traction and manipulation of the stomach. However, traction did not alter blood flow to the small intestine or liver, nor did plasma 6-keto-PGF 1α change in either of the 2 groups.
In patients, traction on the abdominal content provokes MTS in 72%-100% of cases. 1,4,13 Data on the effects of mesenteric traction in pigs are limited, and to our knowledge, it has not been investigated previously. In humans, the release of vasodilating prostacyclin (PGI 2 ) by endothelial cells into the vascular bed upon traction is considered to be the primary causative agent of the syndrome. 1,5 Prostacyclin exerts vasodilatory effects also in pigs; the intravascular infusion of PGI 2 decreases splanchnic vascular resistance and thereby lowers blood pressure. 20 Hence, if mesenteric traction would cause PGI 2 release also in pigs, we would expect cardiovascular depression.
However, mesenteric traction did not increase the plasma concentration of 6-keto-PGF 1α (a stable metabolite of PGI 2 ) in either of the 2 groups of pigs; thus, the decreases in MAP, SVR, and blood flow to the stomach observed in both groups must presumably be driven by factors other than 6-keto-PGF 1α . As mentioned, PGI 2 is considered the mediator behind MTS. Yet, mast cell-derived vasoactive mediators could play a role. 21 Histamine release from mast cells of the small intestine in response to manipulation initiates the triad of hypotension, tachycardia, and facial flushing that characterizes MTS. 21 In a surgical setting, these symptoms were reduced by the administration of antihistamines. 21 In pigs, histamine decreases splanchnic vascular resistance, but it is unknown whether manual manipulations of the intestine promote its release. 22 Numerous vasodilatory substances could be involved in MTS, and their role could differ between humans and pigs. 21,23 For example, intravenous administration of substance P has little effect on splanchnic circulation in humans, whereas, in pigs, it is a potent vasodilator and increases splanchnic blood flow. 23 Perioperative administration of NSAID has a prophylactic effect against MTS, and patients who develop MTS present 7 to 20 times larger increases in plasma 6-keto-PGF 1α than patients treated with NSAID. 1,4,13 Ketorolac is a potent inhibitor of PGI 2 production and thereby hampers its vasodilatory effect and elicits potent analgesic, anti-inflammatory, and antipyretic actions in both pigs and humans. 24 However, administration of ketorolac did not affect the plasma concentration of 6-keto-PGF 1α after mesenteric traction. Indomethacin (NSAID) increases vascular resistance and blood pressure in pigs, attributed to the inhibition of PGI 2 production. 20 As such, NSAID has the same vasoactive effect in humans and pigs. 20 In humans, the recommended dosage of ketorolac is 10 mg every 6 hours, 25 and we therefore considered that the administered dose of ketorolac (10 mg) in this study should have been adequate for inhibiting the plasma PGI 2 response after mesenteric traction. However, pigs may require a higher dose of ketorolac than do humans, and hence the dose may have been too low to affect PGI 2 production. 24 Also, the plasma concentration was not determined.
In contrast to the reduction of blood flow to the stomach, blood flow to the small intestine and liver remained unchanged after mesenteric traction. The mesenteric vascular anatomy in pigs differs from that of humans and other animals. 26 Primarily the branching pattern of cranial mesenteric vessels gives rise to small arteries radiating towards the mesojejunum, which could cause high resistance to flow. However, it is not clear what effect this vascular organization has on the regulation of blood flow. In contrast, the arteries to F I G U R E 1 Plasma 6-keto-PGF 1α and hemodynamic variables in response to intentional traction. Different from baseline in the ketorolac (*) or the placebo ( † ) group, P < .05 the stomach and duodenum follow the arrangement in humans and other mammals. 26 This discrepancy in mesenteric vascular anatomy could perhaps explain why we did not observe a reduction in blood flow to the small intestine, but only to the stomach.
Blood flow to the gastric antrum was reduced in both groups, and interestingly, flow to the gastric corpus was only reduced in the placebo group (P = .002), perhaps indicating some protective effect of ketorolac. As presented in the results section, there was a significant difference in corpus blood flow at baseline (P = .019); however, this cannot explain the difference in blood flow over time as this is considered in the statistical model. Also, the coinciding reduction of blood flow to the antrum (P = .002) and stable levels of plasma 6-keto-PGF 1α in both groups argue against ketorolac being the only influencing factor. The sample size calculation of the study was, perhaps optimistically, based on the assumption that NSAID would reduce the level of plasma 6-keto-PGF 1α to the same extent in TA B L E 1 Plasma 6-keto-PGF 1α , hemodynamic variables, and blood flow after mesenteric traction presented as median (IQR) pigs as in patients. We did not experience any significant changes in plasma 6-keto-PGF 1α in either group, and thus, we could be underpowered with regards to other endpoints.
Other limitations should be considered. Mesenteric traction was conducted as described by Brinkman et al 1 However, the traction was not quantified, and the methodology of traction could have been standardized by using a pully with weights or similar techniques.
LSCI, rather than LDF, was chosen to evaluate splanchnic blood flow because the device does not require tissue contact, thus avoiding compression of capillaries in the region of interest and enabling the assessment of blood flow in a large tissue area. 15 Furthermore, LSCI has excellent reproducibility when evaluating microperfusion in the skin 27 and splanchnic organs. 15 Abdominal exploration and mesenteric traction reduced blood flow to the stomach, possibly because of both loss of SVR and a decrease in MAP. Nevertheless, plasma 6-keto-PGF 1α remained unchanged, and the administration of ketorolac did not influence the hemodynamic variables in response to mesenteric traction. Hence, hemodynamic instability upon mesenteric traction may be attributed to mediators other than plasma PGI 2, and further studies are needed to identify the mechanisms behind the hemodynamic response to mesenteric traction in pigs.

ACK N OWLED G M ENTS
We thank "Civilingeniør Johannes Elmqvist Ormstrup og Hustru Geret Omstrups fond" for providing the funding for this study. No grant number is provided by this fund. We also thank Professor Thomas Madsen (Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Waurn Ponds, Victoria 3216, Australia) for assistance with statistical analysis.

F I G U R E 2
Changes in blood flow in response to intentional traction. Different from baseline in the ketorolac (*) or the placebo ( † ) group, P < .05