Fluid Therapy for Burn (Thermal) Injury in Horses (Box 44.9)
K. Gary Magdesian
Horses with severe burn injuries develop hypovolemic shock and require large volumes of balanced electrolyte replacement fluids.1 Hypertonic saline and colloids (primarily plasma) may also be used but should be followed with isotonic crystalloids.
In humans with burn injuries, the guidelines for rate of administration of isotonic fluids is 2 to 4 mL/kg for each percentage of surface area burned (Parkland formula).2 A retrospective study found that significantly larger volumes of fluid (i.e., 5.58 mL/kg per percent total body surface area affected) were administered to patients during the first 24 hours of hospitalization than was predicted by the Parkland formula.3If significant smoke inhalation has occurred, the affected horse is at risk for pulmonary edema. In this case, fluid deficits should be corrected carefully so as not to overhydrate the
■ BOX 44.9
Fluid Guidelines for Burn Injury Patients
1. Hypertonic saline can be used in the resuscitative phase of burn patients but should be followed with isotonic crystalloids.
2. Isotonic crystalloids are often necessary because of tremendous sodium losses through burn wounds.
3. Plasma is an important component of fluid therapy in the severely burned patient because of the potential for protein loss.
4. Administration of excessive volumes of fluids should be avoided because edema is a common complication of burn injuries.
■ BOX 44.10
Fluid Guidelines for Acute Respiratory Distress Syndrome pulmonary capillaries, affected horses and foals often have significant pulmonary edema, with protein-rich fluid. Pulmonary capillary hydrostatic pressure becomes the main determinant of edema in these patients due to the increased permeability of pulmonary capillaries. Despite this propensity toward edema formation, horses and foals with ARDS should not be allowed to develop dehydration.
Maintenance of normal blood volume and hydration status is critical to tissue oxygenation, including the diseased pulmonary tissue. The goal in therapy of ARDS patients is to reduce extravascular lung water while still maintaining hemodynamic stability and perfusion. In a human clinical trial, conservative fluid management in patients with acute lung injury (ALI) improved lung function and shortened duration of mechanical ventilation and intensive care as compared with patients managed with a liberal fluid strategy, although there was no difference in mortality. The conservative management did not increase nonpulmonary organ failures.3 It should be noted that the patients in this study were volume replete and relatively stable hemodynamically. Conservative fluid therapy therefore applies to the postresuscitation phase of these patients, when administration in excess of physiologic needs can be detrimental.Large swings in pulmonary vascular pressures should be avoided to minimize increases in pulmonary hydrostatic pressure. To avoid these wide fluctuations, intravenous fluids should be provided as a continuous infusion rather than as intermittent boluses in horses with respiratory distress.4 The ideal type of fluid for volume replacement at the time of initial therapy of ARDS patients is unknown; however, colloids should be used judiciously due to the theoretical risk of potentiating pulmonary edema through extravasation of colloids across the leaky pulmonary vasculature. This concern warrants further study; one study demonstrated no increase in transmicrovascular flux of radiolabeled colloids when COP was raised with albumin administration in human patients with pulmonary edema.5 Isotonic crystalloids, administered at a modest rate and only to effect in normoproteinemic patients, and replacement of plasma proteins in hypoproteinemic patients with the use of plasma are probably the safest fluid guidelines for these patients with the current level of understanding.
Patients with ALI or ARDS should therefore be monitored closely for hypoproteinemia. Correction of hypoproteinemia in human patients with ALI benefits from concurrent administration of albumin.6 Hypoproteinemic patients receiving both furosemide (as a CRI) and albumin had improved oxygenation, increased net fluid loss, and better maintenance of hemodynamic stability compared with those receiving only furosemide.6 In addition, 50% of patients in the treatment group achieved resolution of the ALI or ARDS, compared with only 11% of controls. Consideration should be given to correction of hypoproteinemia in foals and horses with ARDS or ALI. An additional benefit of albumin is its antioxidant properties. Albumin can also reduce microvascular permeability and endothelial cell apoptosis.7-9
Monitoring of fluid therapy in these patients is especially important in order to avoid marked increases in pulmonary hydrostatic pressure. This can be accomplished through measurement of pulmonary capillary wedge pressure; however, this requires placement of an intracardiac catheter. Alternatively, CVP can be measured. This is easily performed through central lines—those placed in the cranial vena cava. In a study comparing the use of pulmonary artery catheters with that of central venous catheters in guiding treatment of ALI in humans, it was found that pulmonary catheter-guided therapy did not improve survival or organ function and was associated with more complications than central venous catheter-guided therapy.10 The predominant catheter-related complication was development of arrhythmia. It was concluded that pulmonary catheters should not be routinely used for the management of ALI.10
■ BOX 44.11
Fluid Guidelines for Metabolic Acidosis
1. The two primary forms of metabolic acidosis in horses are organic acidosis, most often resulting from hyperlactatemia, and inorganic acidosis, often resulting from relative or absolute hyperchloremia.
2. Treatment of hyperlactatemia consists of addressing the underlying cause. In many cases, lactic acidosis is caused by hypovolemia.
3. Commercial acetated fluids, such as Normosol-R or Plasma- Lyte 148, and lactated Ringer's solution are good fluid choices for hypovolemia, although volume rather than type of fluid is the most important component of treatment.
4. The use of sodium bicarbonate is controversial in lactic acidosis and may be contraindicated.
5. Sodium bicarbonate is the treatment of choice for inorganic acidosis (hyperchloremia).
6. Sodium bicarbonate can be administered as a fluid in the form of an isotonic solution.