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Compendium July 2009 (Vol 31, No 7)

Limited Fluid Volume Resuscitation

by Tara Hammond, DVM, DACVECC, Jennifer L. Holm, DVM, DACVECC

    CETEST This course is approved for 3.0 CE credits

    Start Test

    Abstract

    Volume replacement therapy is crucial to the treatment of hypovolemic shock. In patients with certain conditions, limiting the volume of fluid administered has many potential therapeutic benefits and technical advantages. Hypertonic saline and colloids have characteristics that allow effective treatment of hypovolemic shock using relatively smaller volumes than would be required for isotonic crystalloids alone. This article describes the theory and clinical application of limited fluid volume resuscitation in veterinary medicine.

    Hemorrhage is a common cause of hypovolemic shock in veterinary patients and can occur with trauma, coagulopathy, or rupture of a parenchymal mass. In general, affected animals were previously healthy and are acutely hypovolemic due to hemorrhage in the chest or abdomen or along a long bone fracture. The goal of rapid volume replacement in shock is to restore perfusion as quickly as possible by replacing intravascular losses and controlling further bleeding, thus limiting tissue injury that can result in organ dysfunction and death. However, the decision of whether fluid replacement or hemorrhage control takes priority and which fluid type to use is controversial.

    Research has shown improved hemodynamic parameters and outcomes with the use of limited fluid volume resuscitation (LFVR) in patients with hypovolemic shock.1-10 However, few clinical veterinary trials have been performed. This article reviews fluid distribution in the body, describes the use of LFVR in hypovolemic shock, and summarizes selected experimental and clinical studies of LFVR.

    Traditional versus Limited Fluid Volume Resuscitation

    The aim of any fluid therapy regimen for treating hypovolemic shock is resolution of shock, not simply administration of a specific volume of fluid. In addition, volume dosages for any type of fluid must be tailored to the individual patient, and recommended dosages are guidelines only. Treating hypovolemic shock is a dynamic process that requires frequent evaluation of the patient. The end points of optimal resuscitation include alleviation of peripheral vasoconstriction and decreased perfusion, as demonstrated by normalization of vital signs and other objective parameters (TABLE 1).

    Traditionally, isotonic crystalloids have been used at dosages based on estimated blood volume (90 mL/kg/hr in dogs, 40 to 60 mL/kg/hr in cats) to replace intravascular losses.11 Although they are essential in the treatment of dehydration, isotonic crystalloids can prolong the time needed to restore effective circulating volume, redistribute rapidly into the interstitial space, and contribute to hypothermia. Because <20% of an administered isotonic crystalloid remains in the intravascular space after 1 hour, large volumes of isotonic crystalloids may be required to maintain an effective circulating volume and may result in excessive interstitial edema.12

    Alternatives to isotonic crystalloids include colloids and hypertonic saline (HS). Colloids and HS are used at much smaller volumes than isotonic crystalloids, enabling faster administration and more rapid repletion of the intravascular space. Colloids are also retained in the intravascular space for longer periods of time than crystalloids and are therefore more effective at maintaining adequate circulating volume and reducing the risk of interstitial edema. Research has shown that HS may have additional benefits, such as restoration of cellular function following traumatic brain injury and immune system modulation.

    LFVR, also called small-volume resuscitation, differs from hypotensive resuscitation or delayed fluid resuscitation in that it is aimed at achieving normalization of clinical end points. With hypotensive or delayed fluid resuscitation, the patient is permitted to remain hypotensive or fluids are withheld, respectively, until bleeding is definitively controlled. During hypotensive resuscitation, the patient is resuscitated to a mean arterial blood pressure (MAP) of no greater than 60 mm Hg, which just maintains perfusion to vital organs while minimizing the risk of dislodging vascular clots.1,13,14 During delayed fluid resuscitation, no fluids are given until definitive control of hemorrhage is achieved. Once bleeding is controlled in either situation, aggressive fluid resuscitation is initiated.

    The goal of LFVR is to use the smallest volume of fluid possible to restore the intravascular compartment and resolve shock while minimizing fluid extravasation into the brain or lungs and the risk of disrupting an incipient blood clot.11 LFVR should be considered in cases with hemoperitoneum, traumatic pulmonary contusions, traumatic brain injury, and other forms of active hemorrhage (BOX 1). In patients with active, noncompressible hemorrhage, achieving an MAP of 70 mm Hg or a systolic arterial pressure (SAP) of 90 mm Hg with improvement of clinical signs during LFVR is acceptable until hemorrhage is definitively controlled.15,16 Once acceptable clinical parameters have been achieved, the dosage and type of fluid can be adjusted to meet the current and anticipated needs of the patient.

    Fluid Distribution and Dynamics

    Total body water accounts for approximately 60% to 70% of body weight.17 Of this water, 66% is located in the intracellular space (including red blood cell mass) and 33% in the extracellular space. The extracellular space is subdivided into intravascular (25%) and interstitial (75%) compartments, which are separated by endothelial cells and a basement membrane.11,17 The membrane is permeable to water but not to most solutes, creating differences in osmotic pressure between the compartments. Osmotic pressure is determined by the number of nonpermeable particles in solution.11 A solution that has more, fewer, or an equal number of osmotically active particles per unit of volume or weight compared with intracellular fluid is said to be hyperosmolar, hyposmolar, or isosmolar, respectively.

    Water moves across the basement membrane from areas of low solute concentration to areas of high solute concentration via osmosis. Sodium is the most abundant cation in the extracellular fluid and accounts for most of the osmotically active particles in this space. The volume of extracellular fluid is determined by total body sodium content, whereas the osmolality and sodium concentration of extracellular fluid are determined by water balance. Regulation of sodium and water balance by the kidneys18 maintains the volume and osmolarity of body fluids within a narrow normal range. In response to the renin-angiotensin-aldosterone system, brain atrial natriuretic hormones, and the posterior pituitary-vasopressin-renal axis, the kidneys maintain extracellular fluid sodium levels and tonicity and contribute to the maintenance of blood pressure.19

    Normally, fluid movement between the intravascular and interstitial compartments depends on membrane pore size and differences between osmotic, hydrostatic, and colloid osmotic pressure (COP), according to Starling's law of capillary hemodynamics (BOX 2 and FIGURE 1).17 The major source of intravascular COP is albumin. As the COP increases, fluid movement out of the intravascular space decreases and the amount of plasma fluid retained increases, causing the intravascular hydrostatic pressure to rise. Fluid movement out of the intravascular space occurs in three situations: (1) when the intravascular hydrostatic pressure is greater than the COP, (2) when capillary permeability increases, or (3) when intravascular COP falls below the interstitial COP12 (BOX 3). Any water-containing fluid (e.g., crystalloids) given intravenously can move out of the vasculature and into the interstitium via any of these mechanisms, and the process may be exaggerated after injury.

    In many conditions (e.g., acute pancreatitis, trauma, bite or burn wounds), fluid is lost from the intravascular space via more than one of these mechanisms. Vasculitis causes increased systemic vascular permeability, and plasma proteins and fluid are sequestered into the interstitial space, resulting in interstitial edema.19 Interstitial edema can also occur as a result of lymphatic obstruction or leakage or when interstitial fluid accumulation exceeds lymphatic drainage.12

    Fluids Used in Limited Fluid Volume Resuscitation

    TABLE 2 summarizes the advantages and disadvantages of the various types of fluids used in resuscitation of hemorrhaging patients, including isotonic crystalloids. Dosing recommendations for the various types of fluids used in LFVR are summarized in TABLE 3 .

    Hypertonic Crystalloids

    The most commonly used hypertonic crystalloid is HS containing 7.2% to 7.5% sodium chloride (depending on the manufacturer), although solutions containing 3% to 23.4% sodium chloride are available. HS is best used in previously healthy animals with acute hypovolemia. The high sodium content of HS rapidly pulls fluid from the extravascular space via the osmotic gradient to expand intravascular volume, resulting in rapid resolution of shock at low infusion volumes. When HS is used alone, this effect typically lasts less than 30 minutes. However, when HS is used in combination with colloids, the effect on volume expansion (and therefore cardiac output) is sustained for 2 to 3 hours. HS and colloids can be mixed for simultaneous administration, and the rate of administration should not exceed 1 mL/kg/min. Using HS is advantageous because it more rapidly restores intravascular circulating volume at smaller infusion volumes compared with isotonic crystalloids alone. Shock resolves faster with less personnel time, and the period of hypoperfusion is minimized. It is an ideal solution for resuscitation in transit (e.g., ambulance, battlefield, search and rescue), when there are volume restrictions, or in large animals with acute hypovolemia. HS should never be used as the only fluid therapy because it dehydrates the interstitium. It should be followed by isotonic crystalloids with or without colloids.

    HS has many properties that other fluids do not. It has beneficial effects on the heart, including increased inotropy, chronotropy, coronary blood flow, and venous return. Increases in renal, splanchnic, and coronary blood flow have been attributed to a decrease in peripheral vascular resistance and redistribution of cardiac output.4 HS has also been shown to decrease leukocyte adhesion and migration, which may help to blunt the inflammatory response and activation of the coagulation cascade.20-22 It has been shown to reduce albumin leakage, neutrophil counts in bronchoalveolar lavage fluid, and histopathologic injury compared with resuscitation using lactated Ringer's solution.23 HS use in hemorrhagic shock also prevents immunosuppression after injury by decreasing plasma levels of interleukin-4 and certain prostaglandins and by limiting neutrophil activation, bacterial translocation, and pulmonary lesions.24-26

    Hemoperitoneum

    Hemoperitoneum is one of the most common emergencies in veterinary medicine. Cases may be due to trauma or may be spontaneous. Most spontaneous cases are caused by rupture of a neoplasm. Most veterinarians agree that the main treatment goals are to reestablish or maintain effective circulating volume, maintain oxygen-carrying capacity, and arrest ongoing hemorrhage.27 However, which goal takes precedence and which fluid resuscitation protocol to use are controversial. LFVR to achieve specific end points (i.e., SAP ≤”90 mm Hg, MAP ≤”70 mm Hg, normalization of clinical signs) should be considered in these cases. Until hemorrhage is arrested, the use of small boluses of HS and colloids help to resolve shock and perfuse vital organs. At MAPs of 60 to 70 mm Hg, cerebral and renal blood flow are preserved.27 LFVR also helps minimize the risk of clot disruption caused by sudden increases in intravascular hydrostatic pressure, thereby reducing the risk of rebleeding during resuscitation.

    Traumatic Brain Injury

    HS is particularly useful in treating patients with traumatic brain injury, cerebral edema, and increased intracranial pressure (ICP). It has been shown to decrease ICP and increase survival following head injury.3,28,29 HS is effective at reducing brain volume because it does not cross the blood-brain barrier and mobilizes interstitial fluid from the brain into the intravascular space. Mannitol works in a similar fashion with a longer duration of action, but it also induces an osmotic diuresis, leading to further volume loss.30-32

    It is critical to maintain a cerebral perfusion pressure (CPP) above 70 mm Hg in dogs and cats. Given that CPP = MAP - ICP and that normal ICP in dogs and cats is approximately 5 to 10 mm Hg, MAP must be maintained above 80 mm Hg.33 Hypotension is one of the most important extracranial contributors to secondary neurologic damage with traumatic brain injury.

    Fluid therapy should never be withheld in patients with traumatic brain injury, as dehydration only minimally decreases ICP and fluid restriction can result in hypovolemia, compromising CPP. In these patients, the goal is to balance fluid therapy and treat shock (thereby ensuring adequate oxygen delivery) without exacerbating cerebral edema or bleeding. With head injury, adenosine triphosphate depletion leads to retention of sodium in the intracellular space, which results in intracellular accumulation of water and calcium and extracellular liberation of excitatory amino acids (e.g., glutamine). HS helps to increase extracellular sodium concentrations and restore gradients so that calcium is returned to the extracellular space and glutamate to the intracellular space, limiting secondary injury and neuronal death.20,33 It also works to minimize vasospasm, encourage local vasodilation, and limit endothelial cell swelling and permeability by promoting microcirculatory blood flow and improving local oxygen delivery.

    Pulmonary Contusions

    The management of pulmonary contusions is controversial, with clinical studies yielding inconsistent results. However, pulmonary contusions, which have been reported in up to 50% of animals with thoracic trauma, are another potential indication for LFVR.16 With pulmonary contusions, the alveolar capillary membranes sustain damage and the alveoli flood with blood, impairing ventilation. HS can be beneficial in these patients, as administration of large amounts of isotonic crystalloids during resuscitation may exacerbate pulmonary parenchymal compromise and further impair ventilation, especially during the first 24 hours after resuscitation. It is recommended that fluid therapy be given to restore and maintain cardiac output and tissue perfusion while avoiding excessive interstitial edema. Although controversial, HS and colloids should be considered the treatment of choice (with judicious use of isotonic crystalloids) until further research suggests otherwise.34

    Adverse Effects and Contraindications

    Adverse effects of HS administration can include transient hypernatremia, occasional premature ventricular contractions, bradyarrhythmias, temporary hypotension, and bronchoconstriction.35 However, with appropriate patient selection and administration (no faster than 1 mL/kg/min), these effects are rare. In addition to rapid increases in cardiac output and blood pressure, the sodium load of HS may elevate the risk of congestive heart failure in some patients. Also, HS may promote blood loss at the site of vascular injury due to breakdown of a blood clot (i.e., rebleeding). Clinically significant rebleeding has not been observed in studies when current dosing recommendations were followed.28,36-38 In fact, the MAP and SAP in experimental swine that experienced rebleeding were found to be 64 ±2 mm Hg and 94 ±3 mm Hg, respectively, during treatment, arguably supporting the use of LFVR so that the SAP does not exceed 90 mm Hg.36

    Contraindications to HS use include dehydration, preexisting hypernatremia, cardiac failure, hyperosmolar conditions (e.g., diabetic ketoacidosis), renal failure, and intravascular volume overload.39 It is essential to monitor electrolyte levels in patients receiving HS. As with any sudden increase in sodium, there is a potential for neurologic signs due to cellular dehydration and acute neuronal shrinkage. HS can cause an increase in sodium and chloride concentrations and a decrease in potassium and bicarbonate levels, but these are of minimal clinical importance unless the animal has preexisting electrolyte abnormalities. HS can also increase serum osmolarity by 20 to 30 mOsm in hydrated patients, but this generally normalizes within a few hours.15

    Synthetic Colloids

    Colloids exert a prolonged effect on intravascular volume compared with isotonic crystalloids, achieving better tissue perfusion and restoration of blood pressure at lower infusion volumes. In addition to LFVR for acute hypovolemia, colloids may be indicated in animals with hypoalbuminemia, decreased COP, or increased capillary permeability. Colloids can be biologic (e.g., blood, albumin, plasma) or synthetic (hetastarch, pentastarch, tetrastarch, dextran, gelatin, hemoglobin-based oxygen carrier [HBOC]). Biologic colloids are indicated for a variety of conditions but are rarely used as sole fluid support in shock. Synthetic colloids have molecular weights of 69,000 Da (the weight of albumin) or greater. In healthy patients, they cannot pass through the basement membrane of the intravascular space; therefore, they contribute to increases in the intravascular COP. Practitioners should be familiar with the concentrations and molecule sizes of synthetic colloids, which determine the initial amount and duration of intravascular volume expansion, respectively. The properties of synthetic colloids are summarized in TABLE 4 .34,40,41

    The use of a bolus of colloids at 5 mL/kg to achieve an SAP of no greater than 80 to 90 mm Hg may be warranted in patients with active hemorrhage during LFVR. It is important to follow colloids with crystalloids because colloids produce a state of relative dehydration in the interstitial space. Colloids can reduce the requirement for maintenance crystalloids by 40% to 60%.42

    The disadvantages of using synthetic colloids include cost, increased risk of volume overload compared with crystalloids, potential for anaphylaxis, potential exacerbation of coagulopathies, and interference with crossmatching. However, these side effects are rarely seen clinically in veterinary patients receiving recommended doses.34,35 Although often recommended in the treatment of patients with increased capillary permeability, colloids may contribute to the development of edema in animals with vasculitis if the large colloid molecules leak into the interstitial space.11 This is a controversial issue and the topic of ongoing studies.

    The effect of colloids on coagulation is likewise controversial, and studies have yielded inconsistent results.43-45 The hemostatic effects of hydroxyethyl starches are dose dependent and more common with products that have higher molecular weights, a greater degree of substitution (more hydroxyl groups per glucose unit in the molecule), and higher C2/C6 ratios (more hydroxyl groups on the glucose molecule at carbon position 2 compared with position 6).46 Studies have shown decreases in circulating von Willebrand's factor and factor VIII, increases in activated partial thromboplastin time (APTT), and hypocoagulable changes on a thromboelastogram with the use of higher doses of these products.46,47 Thrombocytopathia has also been reported with the use of hydroxyethyl starches due to blocking of the glycoprotein IIb/IIIa receptor, which is required for platelet activation.48 Overall, when used at recommended doses, synthetic colloids have not been associated with clinical signs of bleeding or clinically significant alterations in platelet counts, prothrombin time, APTT, or buccal mucosal bleeding time. Clinical bleeding appears to be rare if doses do not exceed 20 mL/kg/day. However, if massive amounts of synthetic colloids are required for stabilization, concomitant administration of blood products may be warranted to reduce the risk of dilutional coagulopathies.34

    Hydroxyethyl Starches

    Hydroxyethyl starches are highly branched polysaccharides primarily consisting of amylopectin (e.g., hetastarch [HES], pentastarch, tetrastarch). HES is available as a 6% solution in 0.9% sodium chloride (Hespan, B. Braun Medical, Inc.) or as a 6% solution in a lactated electrolyte solution (Hextend, Hospira, Inc.), which is intended to mimic the principal ionic constituents of normal plasma. Compared with isotonic crystalloids, smaller volumes of HES can normalize and maintain SAP without lowering COP. Rapid administration of boluses may cause nausea in cats, and some experts recommend giving HES at a slower rate in this species.11 The smaller particles in HES are almost immediately excreted in urine, whereas the larger particles are absorbed by the liver and spleen and slowly returned to the circulation for up to 36 hours. The larger molecules are degraded by the reticuloendothelial system and excreted by the liver and kidneys several days later.34 HES can also reverse changes in microvascular permeability caused by oxygen free radicals during reperfusion injury, decrease leukocyte-endothelial cell adhesion, and improve microcirculation.49

    Pentastarch is available in a 10% solution in 0.9% sodium chloride (Pentaspan, B. Braun Medical, Inc). It is used as a volume-expanding and hemodiluting agent in leukopheresis. In theory, its branched shape may help to plug gaps between separated endothelial cells. Recommended doses of pentastarch are similar to those for HES.34

    Tetrastarch is available in a 6% solution in 0.9% sodium chloride (Voluven, Hospira, Inc.). It has a lower molecular weight and degree of substitution than HES and, therefore, causes fewer alterations in coagulation. However, its half-life is shorter, possibly necessitating more frequent dosing. Recommended dosing in people is up to 50 mL/kg/day, although amounts up to 70 mL/kg/day have been used in a research setting without adverse effects.46,47

    Dextrans

    Dextrans are macromolecular polysaccharides produced by bacterial fermentation of sucrose. Dextran 70 (6% in 0.9% sodium chloride) is beneficial in reperfusion injury because it decreases neutrophil-endothelial interactions. Dosing of dextran 70 is similar to that of HES, and life-threatening allergic reactions to dextran 70 in dogs are extremely rare.42 Dextran 70 can elevate blood glucose concentrations due to metabolism of the dextran. It has been associated with decreased platelet adhesion and reduction of factor VIII complex activity when very large volumes are infused rapidly for acute plasma volume expansion.43

    Oxypolygelatin

    A gelatin colloid plasma expander produced from bovine bone marrow (oxypolygelatin) is available, but information about its clinical use is limited. The volume of expansion is double the administered amount, and the clinical effect lasts for 2 to 4 hours.11 Histamine- and complement-mediated allergic reactions and increases in APTT have been reported with its use, but signs of clinical bleeding have not. In one study, oxypolygelatin caused fewer hemostatic abnormalities compared with dextran 70 when administered to healthy dogs.45 Oxypolygelatin can significantly lower serum calcium levels and is currently marketed with supplemental calcium. The manufacturers recommend extreme caution with oxypolygelatin use in animals with coagulation defects, hypoproteinemia, cardiac or pulmonary insufficiency, or renal disease. Due to its small particle size, side effects, and short duration of action, this product is unlikely to gain widespread use versus other synthetic colloids.

    Hemoglobin-Based Oxygen Carriers

    Hemoglobin binds pulmonary oxygen and transports it to tissues, where it is offloaded to the cells. HBOCs improve oxygen delivery to tissues by increasing hemoglobin levels and circulating volume via colloidal pull. On a gram-for-gram basis, HBOC hemoglobin is more potent than erythrocyte hemoglobin at delivering oxygen to tissues because the molecules are smaller than erythrocytes and are able to pass through the microcirculation more readily. It is thought that HBOCs scavenge nitric oxide in the blood, causing vasoconstriction and increasing systemic vascular resistance.50 They allow administration of an effective colloid and oxygen carrier without blood typing or crossmatching and can be stored at room temperature for long periods of time. In veterinary medicine, HBOCs have been investigated mostly for use during canine hypovolemic shock, hemorrhagic shock, and isovolemic hemodilution. They may also be clinically indicated in the treatment of acute, severe anemia when compatible blood is not readily available or in septic patients to increase oxygen delivery. In animal models of severe, uncontrolled hemorrhage, HBOC infusions resulted in rapid resuscitation and survival with superior cardiac output and oxygen delivery compared with isotonic crystalloids.51

    Oxyglobin (Biopure) is an ultrapurified, polymerized hemoglobin solution of bovine origin in a modified lactated Ringer's solution that is used in veterinary patients. It is approved for one-time use in dogs but should be avoided in animals that are predisposed to volume overload.50 A retrospective study on the use of Oxyglobin in cats revealed a high incidence of pleural effusion and pulmonary edema, likely due to acute circulatory overload; therefore, use in cats remains off-label.52 Oxyglobin has been suggested for preoperative use in animals with hemoperitoneum to maximize perfusion as part of an LFVR protocol. Approximately 2 to 4 mL/kg is administered with lower doses of crystalloids to raise the SAP just to 80 to 90 mm Hg while minimizing the risk of dislodging clots.53 Side effects of Oxyglobin can include discoloration of mucous membranes, sclera, and urine; mild gastrointestinal upset; and an increase in central venous pressure. Because Oxyglobin has such a strong colloidal pull, concurrent use of synthetic colloids is not recommended. In addition, some clinical laboratory values are temporarily immeasurable after its use, depending on the type of analyzer used. However, Oxyglobin does not interfere with complete blood cell counts, blood typing, co-oximetry, or blood gas analysis.50

    Research in Limited Fluid Volume Resuscitation

    In 1980, it was found that HS resuscitated 100% of dogs in hemorrhagic shock with rapid normalization of MAP, acid-base status, and cardiac output.4 Since then, multiple studies have investigated the use of LFVR in hemorrhagic shock. Many studies have shown that HS with or without a colloid is superior to the use of crystalloids alone for resuscitation,1-10 but others have shown no benefit.54-56 Despite many encouraging findings, strong experimental evidence is still lacking due to conflicting results, and there are few prospective veterinary clinical studies.

    Many of the LFVR studies conducted to date were retrospective and nonstandardized and used animal models of controlled hemorrhage. These studies measured values such as volume expansion or intraabdominal blood loss, not clinically important values such as end points of resuscitation or overall survival. More recent studies1,57 have used more realistic animal models with uncontrolled hemorrhagic shock. However, it is challenging to find a humane and clinically realistic long-term method of analgesia that does not interfere with cardiovascular responses.1,57

    It is hard to apply human clinical and experimental evidence to veterinary patients. For example, in some human studies of LFVR, the elapsed time from a traumatic event to arrival in the operating room was less than 30 minutes, which often is not feasible in veterinary situations. In veterinary patients with nontraumatic hemorrhage, such as a ruptured hemangiosarcoma, surgery is often delayed for much longer than 30 minutes, and employing a hypotensive or delayed fluid resuscitation technique would prolong hypotension and promote multiple organ dysfunction. Furthermore, some veterinary patients with intraabdominal hemorrhage achieve spontaneous hemostasis and do not require surgery.

    Multiple veterinary studies of hemorrhagic shock have shown that resuscitation with HS and 6% dextran (HS/D) returned hemodynamic parameters to baseline more rapidly than large volumes of crystalloids.4,9 In addition, the HS/D groups consistently maintained better MAP, cardiac output, and oxygen delivery with superior recovery of splanchnic hemodynamics versus animals receiving crystalloids alone.4,9 In studies of dogs with gastric dilatation-volvulus,7-9 the HS/D groups maintained higher cardiac output and oxygen delivery than the isotonic crystalloid groups. In addition, treatment time was significantly reduced, limiting the period of shock, gastric ischemia, and technical staff involvement. However, there was no significant difference in overall survival between the two groups. In a study comparing HS/D to 0.9% sodium chloride and dextran in swine with severe endotoxic shock, the cardiac output, portal blood flow, intestinal mucosal blood flow, and overall survival were significantly higher in the HS/D group.6

    Conversely, studies using animal models of controlled hemorrhage comparing LFVR with resuscitation using crystalloids alone have found no hemodynamic benefit, and some report no difference in blood loss or blood pressure between the groups.54-56 However, increased intraabdominal bleeding and shortened survival time were reported with the use of increasing volumes of lactated Ringer's solution, and significant improvement in oxygen delivery was reported with the use of LFVR.54-56 A review of studies in animal models found that LFVR appeared to reduce the risk of death in animal models of severe hemorrhage but increased the risk of death in animals with less severe hemorrhage. However, it also found that hypotensive resuscitation reduced the risk of death in all the trials in which it was used.10 Overall, some of the results are promising, but more research is needed in the veterinary community.

    Conclusion

    Clinically, LFVR using HS and colloids is a safe and highly effective alternative to use of crystalloid fluid alone for resuscitation in hypovolemic shock. Previously healthy animals that sustain catastrophic hemorrhage, pulmonary contusions, or traumatic brain injury may benefit most. The use of HS with colloids is superior to the use of isotonic crystalloids alone for vascular volume expansion and may expedite stabilization of hemodynamic parameters while reducing the risk of rebleeding and edema in the lungs and brain. HS may also have immunomodulating and other beneficial properties that alternative fluids do not have. It is important to note that these recommendations apply only to initial shock resuscitation. Treatment must be individualized, and many animals will benefit from combination therapy. Continued research in LFVR is needed in veterinary medicine.

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