Choosing Fluids in Traumatic Hypovolemic Shock:The Role of Crystalloids, Colloids, and Hypertonic Saline

Traumatic injuries causing the eventual development of hypovolemic shock are common in small animals, particularly dogs. Adequate fluid therapy to reverse hypovolemia and to maintain perfusion to vital organs is essential for successful case management. It is important for the veterinary clinician to be familiar with all of the available options for fluid resuscitation.Injuries can be classified as either blunt or penetrating. Common blunt force injuries include hit-by-car (HBC) or kicks, while penetrating injuries include bite wounds or gunshot injuries. Knowledge of the mechanism of injury is helpful in predicting the extent of the injuries and determining the type and frequency of patient monitoring. For example, in a small dog with bite wounds over the chest, the astute clinician may worry about intrathoracic injuries such as pneumothorax, hemorrhage, or lung laceration.Shock is defined as inadequate oxygen delivery to the tissues to meet the metabolic needs for the individual animal.1 Commonly, the causes of shock are considered to be cardiogenic, septic, or hypovolemic. Each type of shock eventually results in inadequate oxygen delivery to tissues, although through different mechanisms. In trauma, the most common cause is hypovolemic shock resulting from hemorrhage. Significant bleeding most often occurs in the abdomen, chest, or a fracture site (particularly long bones or pelvis).The initial assessment of the patient with trauma should include a rapid primary survey to assess for presence or absence of signs of hypovolemia. Typical physical examination findings of a dog with hypovolemic shock include dull or quiet mentation, tachypnea, tachycardia, pale mucous membranes, and weak pulse quality. Shock or suspected shock should be treated promptly. A large-bore (i.e., 16 to 18 gauge) intravenous (IV) catheter may be placed in a peripheral vein, and pretreatment blood samples may be collected for assessment with a minimum database including an in-house determination of the hematocrit, total protein, blood glucose, and estimate of blood urea nitrogen.a Fluid resuscitation should begin immediately. Continuous electrocardiographic monitoring, blood pressure monitoring, and supplemental oxygen are beneficial if available. It is essential to recall that the endpoint of fluid therapy for a patient in shock is the normalization of the vital signs rather than administration of a specific volume of fluids.There are many options available for fluid therapy, including crystalloids, colloids, hypertonic saline, and blood components. Each product has its own specific advantages and disadvantages. The crystalloid versus colloid debate has been ongoing in both human and veterinary medicine for many years.2–6 No controlled studies have shown a significant survival benefit with either type of fluid; rather, outcome is linked to severity of injury.Crystalloids are solutions containing small molecules that may easily pass through blood vessels. Commonly used crystalloids include lactated Ringer’s solution, normal (0.9%) saline, and Plasma-lyte A. Crystalloids are found in virtually every small animal practice in North America. Specific advantages are that they are readily available, inexpensive, and have a long shelf life. Crystalloids are essential when treating a patient for dehydration in order to replace interstitial fluid deficits.In treatment of a patient with hypovolemic shock due to trauma, crystalloids are the most commonly used fluid in small animal practice. The recommended starting dose of crystalloids for stabilizing a patient in shock is 90 mL/kg per hour (dogs) and 60 mL/kg per hour (cats). This dose is extrapolated from the blood volume of dogs and cats. For example, to treat a large dog weighing 100 pounds (45 kg) would potentially require 4,050 mL per hour or 65 to 70 mL per minute of crystalloids; this may require the placement of two or more IV catheters and result in additional demands on technician or support team time. Commonly, approximately half of the calculated shock dose is administered over 15 to 30 minutes, and the patient is carefully reassessed for changes in vital signs. The cost of 1 L of lactated Ringer’s solution is approximately $2.00 US.b Potential disadvantages of crystalloid solutions include hypothermia if room temperature fluids are administered, dilutional effects on coagulation factors and albumin, and eventually the potential for the development of peripheral edema due to the loss of intravascular oncotic forces. The duration of action of infused crystalloids is short, with only about 10% of the infused crystalloid remaining in the intravascular space at 1 hour.5The use of hypertonic saline and colloids has been recommended to counteract some of the potentially harmful effects of crystalloids and to promote more rapid restoration of intravascular circulating volume and subsequent reversal of shock. Additionally, hypertonic saline and colloids may decrease the technician time required to treat an injured patient.Colloids are solutions containing large molecules that cannot easily move out of vessels. The most commonly used colloidal solution in the United States is hetastarch, although dextran is also used to some extent. Pentastarchc is popular in Canada. Plasma is also considered a colloid. In recent years, the use of colloids has increased substantially. Potential advantages of colloids include the ability to restore intravascular volume with smaller volumes of fluids (which may limit the number of IV catheters, additional support personnel time, or both), prolonged duration of action, and less hemodilution. Colloids may be used either as a maintenance IV infusion in a patient with hypoproteinemia, or in patients with shock as a tool to more rapidly increase intravascular volume. Colloids exert their effect on plasma volume by acutely increasing colloid osmotic pressure within the vasculature. The duration of the effect on volume expansion is dependent upon the underlying condition. In a healthy animal, some volume expansion may remain for up to 24 hours following the initial dose. However, in animals with significant vascular leak or ongoing hemorrhage, the duration of effect may be much shorter.7 Following the use of artificial colloids, measurement of plasma protein levels with a refractometer is an inaccurate reflection of the oncotic force within the plasma. Colloids are considered to have approximately three times the potency on a per mL basis as crystalloids for intravascular volume support. When colloids are used in hypovolemic shock, the recommended starting dose is 10 to 20 mL/kg per hour (dog) and 10 to 15 mL/kg per hour (cat). Therefore, for the previously used large dog (100 lbs; 45 kg) treated with 4 L of crystalloids, hetastarch (675 to 900 mL per hour or 11 to 15 mL per minute) would be recommended. The cost of hetastarchd is approximately $37.00 per 500 mL or $56.00 for the equivalent of 4 L of crystalloids ($8.00 US). Potential disadvantages of colloids include increased costs, potential for volume overload, potential for exacerbation of coagulopathies, and potential for magnifying the development of edema in cases of vasculitides. Colloidal therapy is considered particularly indicated in patients with preexisting hypoproteinemia. Combination therapy with colloids and crystalloids may be useful in patients with severe shock. In such patients, it is essential that the clinician consider other interventions in addition to colloidal therapy, such as abdominal wrapping or exploratory surgery for a patient with a hemoabdomen.Hypertonic saline (3% to 7.5%) is a crystalloid solution that acts to rapidly pull fluid from the interstitium to expand the intravascular space by taking advantage of the osmotic gradients created by its infusion. The duration of action is brief, but it may be prolonged if combined with colloids.8 Clinical use in dogs with naturally occurring traumatic shock has been described.9 The use of hypertonic saline requires preexisting normal hydration and therefore is primarily useful in dogs or cats with sudden development of hypovolemia (i.e., intravascular volume depletion) rather than hypovolemia from untreated dehydration (i.e., intravascular and interstitial volume depletion). Hypertonic saline is given as a rapid IV bolus (1 mL/kg per minute) at a dose of 4 to 6 mL/kg. In the large dog described earlier (100 lbs; 45 kg), a volume of 225 mL versus 4 L of crystalloids would be required. Hypertonic saline is available in several concentrations. Practically, when purchased as a 23.4% solution,e it may be diluted with a colloid to prolong the duration of the resuscitative effects. For mixing ease, 17 mL of 23.4% hypertonic saline may be combined with 43 mL of hetastarch in a 60-cc syringe. This solution can be given as a 5 mL/kg bolus over 5 minutes. Advantages of hypertonic saline include rapid infusion times, which are particularly helpful with small-bore catheters, and a rapid restoration of circulating volume. Side effects of hypertonic saline include transient hypernatremia and occasional ventricular premature contractions. Hypertonic saline is very useful in large dogs with acute hypovolemia. In a busy practice, it is often cost-effective and time-saving to add hypertonic saline to shock and stabilization protocols. Contraindications to use of hypertonic saline include dehydration, preexisting hypernatremia, or intravascular volume overload.Treatment of traumatic shock requires rapid restoration of circulating volume to limit the potential complications of long-standing tissue hypoxia. Crystalloids are commonly used for this purpose, but the addition of colloids, hypertonic saline, or both as part of the treatment options can improve both case management and hospital efficiency.aAzostix; Bayer Corporation, Elkhart, INbLactated Ringer’s solution; Baxter, Deerfield, ILcPentaspan; DuPont Pharma, Mississauga, Ontario, CanadadHetastarch; Abbott Laboratories, North Chicago, ILeHypertonic saline; American Pharmaceutical Partners, Inc., Los Angeles, CA

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