Key Words:
vascular trauma , ischemia , reperfusion , damage control , resuscitation , combat , wartime , military , endothelial cell , plasma , red blood cells , platelets , TGF-β
Introduction
Acute limb ischemia in the setting of extremity vascular trauma is a common cause of morbidity in civilian and wartime settings. The traditional objective of surgeons in these settings has been expeditious restoration of blood flow to obtain the best limb-salvage rates. However, immediate extremity reperfusion at inexperienced civilian centers or at far-forward locations on the battlefield may not always be possible. Furthermore, the decision to restore extremity blood flow after prolonged ischemia may yield unpredictable outcomes due to deleterious metabolic effects in the tissue of the limb. Therefore the pathogenesis and mitigation of ischemia and reperfusion injury in the setting of vascular trauma remain topics of great interest. This chapter provides a basic review of the pathophysiology of extremity vascular trauma including a discussion on the impact of time to limb reperfusion, the role of hemorrhagic shock on functional limb salvage, and the optimal resuscitation fluids to mitigate ischemic reperfusion injury. Recognition of these elements of basic and translational science in vascular trauma is crucial for achieving the best probability of functional limb salvage and for advancing future clinical practice.
Pathogenesis of Ischemia and Reperfusion
Complete and partial ischemia occurs during the interruption of oxygen delivery and the accumulation of toxic metabolites. In the setting of hemorrhagic shock, reduction in blood flow further impairs the removal of these metabolic waste products. Energy depletion initiates both functional and structural cellular derangements that activate inflammatory responses. Reperfusion is the reestablishment of normal blood flow, and it is during this period that most of the injury is thought to occur. Reperfusion injury is largely due to neutrophil activation, infiltration into the ischemic tissue, and subsequent endothelial damage that leads to edema formation, microvascular thrombosis, and irreversible tissue necrosis. Ischemic reperfusion injury is based in part on the duration of ischemia. Prolonged ischemia results in primary membrane disruption due to depletion of energy reserves. Failure of the adenosine triphosphate (ATP)–dependent ionic pump disrupts osmotic gradients, resulting in cellular swelling and global failure of energy-dependent mechanisms. Ischemic endothelial cells play a major role as metabolic enzymes produce oxygen-derived free radicals upon reperfusion. The production of free radicals initiates a complex molecular interaction of various chemical mediators that are responsible for neutrophil activation.
Some of the key molecules in this process are complement, prostaglandins, cytokines, and platelet-activating factor (PAF). Activated neutrophils increase cell adhesion molecule activity that produces endothelial injury. Endothelial injury then leads to increased vascular permeability, cell swelling, edema, and changes in vasomotor tone from diminished release of nitric oxide. Neutrophils adhere in regions where luminal size is compromised by endothelial swelling, and increased vasomotor tone flow may stop entirely. This condition is known as the “no reflow” phenomenon. The amount of tissue injury is based on the degree of ischemia. A short period of ischemia does not cause primary injury or activation of a pathological inflammatory response. Prolonged ischemia results in widespread tissue injury secondary to energy depletion and the subsequent pathologic reperfusion injury.
Traditional teaching has been that irreversible cellular and mitochondrial damage, inability to regenerate ATP, and variable degrees of tissue necrosis occur at 6 hours of ischemia. However, recent translational research suggests that the neuromuscular ischemic threshold of the extremity likely is less than 5 hours of ischemia and that this threshold is even less (less than 3 hours) in the setting of hemorrhagic shock. The challenge on the battlefield is to understand what intermediate periods of ischemia in the setting of hemorrhagic shock can be tolerated without permanent tissue destruction and loss of nerve and skeletal-muscle function in an otherwise salvaged extremity with a normal pulse exam.
Clinical Practice on the Battlefield
Military operations in Afghanistan and Iraq resulted in over 40,000 extremity injuries and nearly 2500 amputations. This burden of injury represents approximately 75% of all Afghanistan and Iraq war-related injuries, and the rate of extremity vascular injury is now fivefold higher than reported in previous conflicts. The widespread use of tourniquets has resulted in improvements in survival in those with compressible extremity hemorrhage. The success of tourniquets and improved survival of extremity injury eventually shifted much attention to casualties with ischemic limbs in need of expedited reperfusion in order to save both life and limb. In the beginning of these wars, the accepted clinical paradigm, which touted the ability to salvage an extremity following as many as 6 hours of ischemia, was challenged. As the concept of outcomes evolved from statistical limb salvage to functional or quality limb salvage, investigators examined the potential for restoring extremity blood flow earlier and farther forward on the battlefield with the use of temporary vascular shunts. Dawson and colleagues at Lackland Air Force Base, in San Antonio, Texas, set the stage for the use of temporary shunts to maintain limb perfusion after arterial injury based on the findings of an animal study that demonstrated excellent patency and decreased lactic-acid production. Early reports from the war in Iraq demonstrated the feasibility and general benefit of this surgical adjunct, and the use of temporary vascular shunts subsequently expanded to nearly a quarter of all extremity vascular injuries. Although shunt patency often exceeded 90% for over 5 hours, the effectiveness of shunts in protecting against ischemic injury was not definitively established until Rasmussen and colleagues performed several randomized large animal studies at Lackland Air Force Base a decade after the original studies by Dawson.
The Ischemic Threshold
In an attempt to evaluate the physiologic benefit of temporary vascular shunts, a porcine model of extremity hind-limb ischemia showed that early restoration of perfusion using shunts protected from further ischemic insult and reduced circulating markers of tissue injury. During reperfusion, specimens were collected to assess circulating markers of muscle injury and inflammation and included lactate, myoglobin, potassium, creatine phosphokinase, aspartate aminotransferase, and lactate dehydrogenase. The values were used to compute an ischemia index score. This was the first investigation to demonstrate that early restoration of flow with a temporary vascular shunt before 3 hours of ischemia was associated with reduced tissue and circulating markers of muscle injury. This study was in contrast to previous animal research that focused only on small animals lacking translational wartime applicability. Gifford and colleagues also reported that the presence of the shunt did not increase the ischemic injury and that patency was maintained in the absence of systemic anticoagulation (i.e., heparin). This report refuted the surgical doctrine that a 6-hour time window was adequate or acceptable and validated the use of temporary shunts in forward maneuver units. Although the lack of tissue studies limited the ability to translate these markers to actual damage, Gifford and colleagues paved the way for a subsequent analysis of neuromuscular recovery.
Although several small animal studies suggest an ischemic interval after which irreversible neuromuscular injury occurs, this has been incompletely and variably defined. Burkhardt and colleagues used a porcine survival model to define an ischemic threshold beyond which surgical restoration may not be beneficial. The group randomized swine with iliac artery occlusion for various time intervals followed by ligation or vessel repair and 14 days of recovery. In this series of studies, a physiologic measure of recovery served as the functional outcome endpoint and included the Tarlov gait score, and electromyography (EMG) measures the results of which were coupled with muscle and nerve histology. In this study, surgical and therapeutic adjuncts to restore extremity perfusion early (1 to 3 hours) after extremity vascular injury were found to provide improved neuromuscular outcome compared to delayed restoration of flow or ligation. Interestingly, the group also reported that ligation instead of repair after 6 hours of ischemia was associated with improved neuromuscular recovery and determined that this benefit intercepts the slope of physiologic recovery at 4.7 hours of ischemia. The authors suggested that knowledge of the ischemic threshold may stimulate the development of adjuncts (shunts, fasciotomy, pharmacologic agents) that continue to shift the threshold in a more favorable direction.
As an extension of these translational large animal studies, Hancock and colleagues noted that extremity vascular injury typically occurs in association with hemorrhage and that the potential deleterious effect of shock on the ischemic threshold was unknown. The researchers extended the original Burkhardt study by characterizing the impact of hemorrhagic shock on neuromuscular recovery in the setting of defined hind-limb ischemia (1, 3, and 6 hours). This study found that animals with less than 1 hour of ischemia had clinically normal limb function by the end of the 14-day recovery period with minimal histologic alterations of muscle and nerve tissue. However, in the presence of Class III hemorrhagic shock (35% blood volume reduction) only 3 hours of ischemia resulted in impaired functional recovery with moderate to severe degeneration of extremity muscle and nerve tissue. Interestingly, Class III shock was associated with a decrement in neuromuscular recovery across all groups but was greatest in groups who experienced more than 3 hours of ischemia. The researchers concluded that hemorrhagic shock reduces the ischemic interval of the limb and that, in this constellation of injury (extremity ischemia and shock), revascularization within 1 hour is necessary for best neuromuscular recovery and functional limb salvage. These findings underscored the importance of recognizing and treating shock with optimal resuscitation fluids designed to maximize hemoglobin concentration and oxygen delivery to prevent further neuromuscular impairment.
Damage Control Resuscitation
Damage control resuscitation (DCR) has evolved as an effective strategy to treat hemorrhagic shock. This includes earlier and increased use of packed red blood cells (PRBCs), thawed plasma, and platelets, while limiting (4 L/24 hours) crystalloid fluids. Several studies have associated improved survival with the early, aggressive use of this plasma-based resuscitation strategy. DCR has changed the current practices in military and civilian trauma centers. Recent evidence suggests that DCR also modulates the ischemic reperfusion response to vascular injury as the treatment of hemorrhagic shock extends the ischemic threshold. Fresh red blood cells (RBCs) have greater oxygen delivery, and freshly thawed plasma products may stabilize cell membranes, which reduces capillary edema and endothelial permeability that is known to exacerbate reperfusion injury and the no reflow phenomenon observed with microvascular thrombosis. Studies on RBCs have led to the concept of the “storage lesion” that has been associated with proinflammatory changes and deleterious effects. These studies have suggested deleterious effects associated with aged RBCs. In both patients and in vitro studies, the storage age of RBCs has been associated with increased inflammatory gene expression, infection, and decreased survival. Moreover, recent work has shown that storage of platelets increases a variety of growth factors, including transforming growth factor-β, that have the potential to destabilize the vasculature contributing to undesirable outcomes.
Because many trauma centers now place thawed plasma directly in the emergency department, the optimal age of the transfused plasma is being questioned. According to the American Association of Blood Banks, thawed plasma may be stored at 1° C to 6° C for up to 5 days before transfusion. Although this may reduce waste, experiments have shown decreased hemostatic potential and clotting factors in stored plasma compared to freshly thawed plasma. Letourneau and colleagues demonstrated that aged (5 days old) plasma transfusion increases mortality in a rat model of uncontrolled hemorrhage. In addition, aged plasma has diminished endothelial repair activity. Further studies investigating the interaction between endothelial biology and resuscitative fluids continue. For example, Pati and colleagues demonstrated that increased endothelial permeability is associated with aged plasma when compared to freshly thawed plasma. Their group hypothesized that in addition to reversing coagulopathy, fresh frozen plasma has protective and stabilizing effects on the endothelium that translate into diminished endothelial cell permeability. Endothelial permeability was induced by hypoxia and this group studied the passage of 70-k-Da Dextran between monolayers. Pati and colleagues noted that thawed plasma inhibits permeability in vitro and that those effects of plasma on vascular endothelium diminish over 5 days of standard storage. Endothelial cell stability is crucial for vascular integrity. Tight junctions are important to structural support and, when this fails, endothelial cells become fragile and water and other molecules begin to invade the interstitial space ( Fig. 4-1 ). This may be an important mechanism demonstrated in the massive extremity edema commonly associated with reperfusion of an acutely ischemic limb necessitating fasciotomy to avoid compartment syndrome and limb loss. Additionally, exposure of the subendothelium can lead to unwanted activation of the coagulation cascade and propagate microvascular thrombosis. In concert with the luminal contraction by expansive edema, the no reflow phenomenon can be either prevented or exacerbated depending on the resuscitation.
