Today's Veterinary Practice

JAN-FEB 2018

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PEER REVIEWED 70 FLUID THERAPY cellular oxygenation. Cardiogenic shock should be ruled out before the administration of crystalloid and colloid fluids because of the risk for exacerbation of pulmonary vasculature fluid overload. The "shock bolus" of fluids is based on the estimated blood volume of the patient: 60 to 90 mL/kg in dogs and 40 to 60 mL/kg in cats. This amount of fluid is typically not administered because most patients have not lost their entire blood volume and respond to lower volumes of fluids. In dogs, recommendations are to start by administering 10-20ml/kg boluses of isotonic crystalloids over 15 to 30 minutes and reassessing the patient's intravascular volume status after each bolus. Subsequent boluses can be administered according to whether perfusion parameters (eg, heart rate, blood pressure, capillary refill time, mucous membrane color) are improved. Cats are typically less tolerant of fluid boluses than dogs, so slightly lower volumes (10 to 15 mL/kg) are recommended. If a synthetic colloid is used, 5 mL/kg in dogs and 3 mL/kg in cats should be administered over 15 to 30 minutes. Daily Fluid Therapy In stable dehydrated patients, the goal of fluid therapy is to replace interstitial losses while simultaneously taking into account the patient's baseline fluid requirements and any excessive losses. Thus, daily crystalloid fluid needs are calculated on the basis of 3 values: ■ Dehydration ■ Maintenance needs ■ Ongoing losses The volume of crystalloid needed to correct dehydration can be calculated using the following formula: Body weight (kg) × [% dehydration/100] = fluid deficit (L) Dehydration can be corrected over 24 to 48 hours. Once the deficit has been determined, the total volume can be divided by the number of hours over which the dehydration is to be corrected to obtain a mL/h rate. Maintenance fluid needs are based on both sensible and insensible losses. These can be estimated as 40-60 ml/kg per day or 2-3 ml/kg per hour in adults (pediatric patients require slightly higher rates). Because of alterations in lean body mass, these estimates may not be as accurate for animals that weigh <2 or >40 kg. 1 Because daily water requirements parallel resting energy requirements (RERs), the formula to determine RER can be used: Body weight (kg) 0.75 × 70 = RER ≈ daily fluid requirement Ongoing losses include excessive urine output in polyuric animals, vomiting, diarrhea, blood loss, and third spacing. These losses vary by patient and may be difficult to calculate. A general rule is to estimate the amount of ongoing loss in a patient in mL/h and then reassess the patient in 4 to 6 hours to determine whether this amount needs to be adjusted. As an example, the daily fluid needs of a 5-kg patient that is estimated to be 6% dehydrated (to be corrected over 24 hours) and has estimated ongoing losses of 1 mL/kg per hour can be calculated as follows: ■ Dehydration deficit: 5 kg × 0.06 = 0.3 L × 1000 = 300 mL to be corrected over 24 hours (300 mL/24 h) = 12.5 mL/h ■ Maintenance rate: 2 mL/kg/h × 5 kg = 10 mL/h ■ Ongoing losses: 1 mL/kg/h × 5 kg = 5 mL/h ■ Total rate (dehydration + maintenance + losses): 12.5 + 10 + 5 = 27.5 mL/h SUMMARY Fluid therapy is a mainstay of care in the hospitalized small animal patient. Assessment of a patient's fluid deficits and ongoing needs will help determine what variety of fluid and rate to use. References 1. Rudloff E. Assessment of hydration. In: Silverstein D, Hopper K, eds. Small Animal Critical Care Medicine. 2 nd ed. St. Louis. MO: Elsevier; 2015:302-311. 2. Woodcock TE, Woodcock TM. Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy. Br J Anaesthes 2012;108(3):384-394. 3. Liu DT, Silverstein DC. Crystalloids, colloids, and hemoglobin-based oxygen carrying solutions. In: Silverstein D, Hopper L, eds. Small Animal Critical Care Medicine. 2 nd ed. St. Louis, MO: Elsevier; 2015:311- 315. 4. Cazolli D, Prittie J. The crystalloid-colloid debate: consequences of resuscitation fluid selection in veterinary critical care. J Vet Emerg Crit Care 2015;25(1):6-19. 5. Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, ed. Current Veterinary Therapy VIII. Philadelphia: WB Saunders; 1983:33. 6. Silverstein DC, Aldrich J, Haskins SC, et al. Assessment of changes in blood volume in response to resuscitative fluid administration in dogs. J Vet Emerg Crit Care 2005;15(3):185-192.

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