Gregory B. Bulkley

PHARMACOLOGIC APPROACH TO TISSUE INJURY MEDIATED BY FREE-RADICALS AND OTHER REACTIVE OXYGEN METABOLITES

Highly toxic metabolites of oxygen are generated normally by aerobic metabolism in most cells, and this generation is often greatly increased in pathologic conditions. When this oxidant flux exceeds the capability of the multiple endogenous antioxidant mechanisms, tissue injury ensues. The pharmacologic modification of this injury process, with agents that scavenge these reactive oxygen metabolites, block their generation, or enhance the endogenous antioxidant capability, has shown great promise in animal models of common clinical conditions, and has already been successfully applied in controlled clinical trials. This approach represents an interruption of tissue injury at its most basic level.

The role of oxygen-derived free radicals in the pathogenesis of acute pancreatitis

Acute pancreatitis may be initiated in the ex vivo, perfused canine pancreas preparation by a variety of stimuli. These include 1) oleic acid infusion (FFA), 2) partial duct obstruction with secretin stimulation (POSS), and 3) a 2-hour period of ischemia (ISCH). In each model, pancreatitis is characterized by weight gain, edema, and hyperamylasemia. Oxygen-derived free radicals such as superoxide, hydrogen peroxide, and the hydroxyl radical are highly reactive toxic substances that are normally produced in small amounts during oxidative metabolism. Ordinarily, these substances are detoxified by endogenous intracellular enzymes called free radical scavengers (FRS), such as superoxide dismutase (SOD) and catalase (CAT). These studies were undertaken to evaluate the possible role of oxygen-derived free radicals in the initiation of acute pancreatitis in the isolated canine model. All preparations were perfused for 4 hours with autologous blood. Controls (N = 6): these glands remained normal in appearance, gained minimal weight (6 ± 1 g), and serum amylase remained normal (<1000 u/dl). FFA pancreatitis, FFA alone (N = 6): these glands became edematous, gained weight (113.5 ± 27.0 g), and developed hyperamylasemia (2087 ± 387 u/dl). FFA + FRS (N = 6), SOD (50 mg) and CAT (50 mg) were added to the perfusate at time zero: these glands became only minimally edematous, gained less weight (31.8 ± 10.1 g, p < 0.05), and amylase remained normal (p < 0.05). POSS pancreatitis, POSS alone (N = 8): these glands became edematous, gained weight (38.6 ± 4.6 g), and developed marked hyperamylasemia (9522 ± 3226 u/dl). POSS + FRS (N- 6): these glands did not develop edema, gained less weight (15.1 ± 2.6 g, p < 0.05), and serum amylase only increased to 1815 ±343 u/dl, (p < 0.05). ISCH pancreatitis, ISCH alone (N = 6): these glands became edematous, gained weight (75.8 ± 25 g), and developed hyperamylasemia (1679 ± 439 u/dl). ISCH + FRS (N = 6): these glands did not develop edema, gained only 18.3 ± 9.0 g (p < 0.005), and serum amylase remained normal (p < 0.05). These studies demonstrate that, in this canine preparation, acute pancreatitis is significantly ameliorated by oxygen-free radical scavengers. Since this was true whether the pancreatitis was produced by FFA infusion, POSS, or ischemia, it suggests that oxygen-derived free radicals may mediate a common essential step in the pathogenesis of all forms of pancreatitis.

Preoperative recognition of intestinal strangulation obstruction: Prospective evaluation of diagnostic capability

Early recognition of intestinal strangulation in patients with small bowel obstruction is essential to allow safe nonoperative management of selected patients. We prospectively evaluated preoperative diagnostic parameters as well as the preoperative judgement of the senior attending surgeon for the determination of the presence or absence of intestinal strangulation in 51 consecutive patients who were about to undergo laparotomy for complete mechanical small bowel obstruction. Strangulation was present in 21 (42 percent) of the 51 patients. No preoperative clinical parameter, including the presence of continuous abdominal pain, fever, peritoneal signs, leukocytosis, or acidosis, or a combination thereof proved to be sensitive, specific, and predictive for strangulation. Moreover, the senior surgeons experienced clinical judgement detected strangulation in only 10 of 21 patients with strangulation preoperatively (sensitivity, 48 percent). Indeed, only 1 of these 10 patients had an early, reversible lesion, whereas 9 had advanced, irreversible infarction. Only 25 of 36 preoperative assessments of simple obstruction proved correct (predictive value of an assessment of no strangulation, 69 percent). Overall, the preoperative assessment was correct in only 35 of the 51 patients (efficiency, 70 percent). These data show that in patients with complete mechanical small bowel obstruction, the preoperative diagnosis of strangulation cannot be made or excluded reliably by any known clinical parameter, combination of parameters, or by experienced clinical judgement. Nonoperative management of complete intestinal obstruction is therefore undertaken at a calculated risk (31 +/- 51 percent in the present series) of delaying definitive treatment of intestinal ischemia.

The role of oxygen-free radicals in ischemic tissue injury in island skin flaps.

The contribution of free radical-mediated reperfusion injury to the ischemic damage caused by total venous occlusion of island skin flaps was investigated in a standardized rat model. Control flaps subjected to 8 hours of total venous occlusion showed complete, full thickness necrosis when followed for 7 days following release of the vascular occlusion. Treatment with superoxide dismutase, a scavenger of superoxide radicals, prior to and immediately following the onset of reperfusion, significantly enhanced island flap survival from 0/11 (0%) to 8/15 (53%), p < 0.005, and from 0/9 (0%) to 6/12 (50%), p < 0.02, respectively. These findings are consistent with the hypothesis that oxygen free radicals generated at the time of reperfusion following a period of ischemia contribute significantly to the ultimate damage caused by ischemic injury. Such findings are consistent with similar reported observations on other tissues and suggest a means by which ischemic tissue injury might be therapeutically modified, even after the period of ischemia.

Vasoactive mediators and splanchnic perfusion

ObjectiveTo provide an overview of the splanchnic hemodynamic response to circulatory shock. Data SourcesPrevious studies performed in our own laboratory, as well as a computerassisted search of the English language literature (MEDLINE, 1966 to 1991), followed by a selective review of pertinent articles. Study SelectionStudies were selected that demonstrated relevance to the splanchnic hemodynamic response to circulatory shock, either by investigating the pathophysiology or documenting the sequelae. Article selection included clinical studies as well as studies in appropriate animal models. Data ExtractionPertinent data were abstracted from the cited articles. Results of Data SynthesisThe splanchnic hemodynamic response to circulatory shock is characterized by a selective vasoconstriction of the mesenteric vasculature mediated largely by the renin-angiotensin axis. This vasospasm, while providing a natural selective advantage to the organism in mild-to-moderate shock (preserving relative perfusion of the heart, kidneys, and brain), may, in more severe shock, cause consequent loss of the gut epithelial barrier, or even hemorrhagic gastritis, ischemic colitis, or ischemic hepatitis. From a physiologic standpoint, nonpulsatile cardiopulmonary bypass, a controlled form of circulatory shock, has been found experimentally to significantly increase circulating levels of angiotensin II, the hormone responsible for this selective splanchnic vasoconstriction. ConclusionsWhile angiotensin II has been viewed primarily as the mediator responsible for the increased total vascular resistance seen during (and after) cardiopulmonary bypass, it may also cause the disproportionate decrease in mesenteric perfusion, as measured in human subjects by intraluminal gastric tonometry and galactose clearance by the liver, as well as the consequent development of the multiple organ failure syndrome seen in 1% to 5% of patients after cardiac surgery. (Crit Care Med 1993; 21:S55-S68)

Free radicals and other reactive oxygen metabolites in inflammatory bowel disease: cause, consequence or epiphenomenon?

Oxygen-derived free radicals and other reactive oxygen metabolites have emerged as a common pathway of tissue injury in a wide variety of otherwise disparate disease processes. This has given rise to the hope that efforts directed towards the pharmacologic control of free radical-mediated tissue injury (Reilly, P.M., Schiller, H. J. and Bulkley, G. B. (1991) Pharmacologic approach to tissue injury mediated by free radicals and other reactive oxygen metabolites. Am. J. Surg. 161: 488-503) may have particular application to patients suffering from Crohns disease and/or ulcerative colitis. However, because tissue injury by any mechanism, even direct mechanical trauma, can elicit an inflammatory response which entails the secondary generation of toxic oxidants by neutrophils and tissue macrophages, it is important that the evidence for this association be examined critically, so as to discriminate the possibility of an etiologic role for these toxic compounds from their presence as a reflection of injury caused primarily by other agents. Similarly, in considering the therapeutic potential of free radical ablation for the treatment of patients with IBD it is important to distinguish between interventions that might specifically block the fundamental injury mechanism from those which would act in a more nonspecific, anti-inflammatory role.

The pathogenesis of acute pancreatitis: the source and role of oxygen-derived free radicals in three different experimental models

Recent experimental work has suggested that oxygen-derived free radicals may play an important role in initiating the early capillary injury in acute pancreatitis. Data from models of ischemic injury in other organs have suggested the enzyme xanthine oxidase is important in generating oxygen-derived free radicals. The present study was performed to determine whether xanthine oxidase is the source of free radical production in experimental pancreatitis. Utilizing the isolated, perfused, ex vivo canine pancreas preparation, three models of pancreatitis were initiated with (1) free fatty acid infusion (FFA), (2) partial duct obstruction and secretin stimulation (POSS), and (3) ischemia (ISCH). In each model, during a 4-hour perfusion, edema developed, weight gain occurred (FFA 120.6 +/- 21.1 gm; POSS 44.5 +/- 6.9 gm; ISCH 63.3 +/- 14.0 gm), and the serum amylase became elevated (FFA 1827 +/- 397 u/dl; POSS 10,171 +/- 1487 u/dl; ISCH 1860 +/- 365 u/dl). When the xanthine oxidase enzyme inhibitor allopurinol was added to the perfusate prior to the 4-hour perfusion, edema formation was absent or minimal, weight gain was significantly less (FFA 15.2 +/- 2.5 gm p less than 0.05; POSS 8.8 +/- 2.7 gm p less than 0.001; ISCH 12.3 +/- 2.8 gm p less than 0.01), and the amylase remained normal or the elevation was significantly decreased (FFA 996 +/- 189 u/dl p less than 0.05; POSS 3021 +/- 1074 u/dl p less than 0.001; ISCH 993 +/- 214 u/dl p less than 0.002). These data confirm that oxygen-derived free radicals play an important role in the pathogenesis of experimental acute pancreatitis, and suggest that the enzyme xanthine oxidase may well be the source of their production.

The mesenteric hemodynamic response to circulatory shock: an overview.

ABSTRACT— The mesenteric hemodynamic response to circulatory shock is characteristic and profound; this vasoconstrictive response disproportionately affects both the mesenteric organs and the organism as a whole. Vasoconstriction of post‐capillary mesenteric venules and veins, mediated largely by the &agr;‐adrenergic receptors of the sympathetic nervous system, can effect an “autotransfusion” of up to 30% of the total circulating blood volume, supporting cardiac filling pressures (“preload”), and thereby sustaining cardiac output at virtually no cost in nutrient flow to the mesenteric organs. Under conditions of decreased cardiac output caused by cardiogenic or hypovolemic shock, selective vasoconstriction of the afferent mesenteric arterioles serves to sustain total systemic vascular resistance (“afterload”), thereby maintaining systemic arterial pressure and sustaining the perfusion of non‐mesenteric organs at the expense of mesenteric organ perfusion (Cannons “flight or fight” response). This markedly disproportionate response of the mesenteric resistance vessels is largely independent of the sympathetic nervous system and variably related to vasopressin, but mediated primarily by the renin‐angiotensin axis. The extreme of this response can lead to gastric stress erosions, nonocclusive mesenteric ischemia, ischemic colitis, ischemic hepatitis, ischemic cholecystitis, and/or ischemic pancreatitis. Septic shock can produce decreased or increased mesenteric perfusion, but is characterized by an increased oxygen consumption that exceeds the capacity of mesenteric oxygen delivery, resulting in net ischemia and consequent tissue injury. Mesenteric organ injury from ischemia/reperfusion due to any form of shock can lead to a triggering of systemic inflammatory response syndrome, and ultimately to multiple organ dysfunction syndrome. The mesenteric vasculature is therefore a major target and a primary determinant of the systemic response to circulatory shock.

Interhospital differences in severity of illness. Problems for prospective payment based on diagnosis-related groups (DRGs).

We evaluated the ability of the diagnosis-related-group (DRG) classification system to account adequately for severity of illness and, by implication, for the costs of medical care. Hospital inpatients on medicine, surgery, obstetrics/gynecology, and pediatrics services in six hospitals were evaluated to provide a spectrum of patient and hospital characteristics. This evaluation was based on data from a generic index of severity of illness obtained by trained personnel from a review of hospital charts after patient discharge. Within each DRG, substantial differences were found in the distribution of severity of illness in different hospitals. Some hospitals treated larger proportions of severely ill patients and had a wide range of severity within each DRG, but these differences did not always agree with the teaching classification or the Health Care Financing Administrations case-mix index. These findings suggest that patient classification by means of unadjusted DRGs does not adequately reflect severity of illness, and they indicate that prospective payment programs based on DRGs alone may unfairly and adversely discriminate against certain hospitals.

The physiology of endothelial xanthine oxidase: from urate catabolism to reperfusion injury to inflammatory signal transduction.

Xanthine oxidoreductase (XOR) is a ubiquitous metalloflavoprotein that appears in two interconvertible yet functionally distinct forms: xanthine dehydrogenase (XD), which is constitutively expressed in vivo; and xanthine oxidase (XO), which is generated by the posttranslational modification of XD, either through the reversible, incremental thiol oxidation of sulfhydryl residues on XD or the irreversible proteolytic cleavage of a segment of XD, which occurs at low oxygen tension and in the presence of several proinflammatory mediators. Functionally, both XD and XO catalyze the oxidation of purines to urate. However, whereas XD requires NAD+ as an electron acceptor for these redox reactions, thereby generating the stable product NADH, XO is unable to use NAD+ as an electron acceptor, requiring instead the reduction of molecular oxygen for this purine oxidation and generating the highly reactive superoxide free radical. Nearly 100 years of study has documented the physiologic role of XD in urate catabolism. However, the rapid, posttranslational conversion of XD to the oxidantgenerating form XO provides a possible physiologic mechanism for rapid, posttranslational, oxidant‐mediated signaling. XO‐generated reactive oxygen species (ROS) have been implicated in various clinicopathologic entities, including ischemia/reperfusion injury and multisystem organ failure. More recently, the concept of physiologic signal transduction mediated by ROS has been proposed, and the possibility of XD to XO conversion, with subsequent ROS generation, serving as the trigger of the microvascular inflammatory response in vivo has been hypothesized. This review presents the evidence and basis for this hypothesis.