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61 JANUARY/FEBRUARY 2018 ● TVPJOURNAL.COM FEATURES Fluid Therapy in Hospitalized Patients: Patient Assessment and Fluid Choices FLUID THERAPY: PART 1 Bridget M. Lyons, VMD Lori S. Waddell, DVM, DACVECC University of Pennsylvania Fluid therapy is an essential component of the treatment plan in many hospitalized small animal patients. Choice of fluid type and dose depends on available resources as well as the patient's interstitial hydration status, hemodynamic stability, and electrolyte balance. This article reviews fluid distribution, hydration assessment, types of body fluid losses, types of fluids available, and calculation of fluid needs. A second article will focus on calculating free water deficit, monitoring fluid therapy, when to discontinue fluids, and the approach to fluid therapy in patients with electrolyte abnormalities and specific fluid balance states. DISTRIBUTION OF BODY WATER Total body water (TBW) is commonly described in terms of individual fluid compartments. Although fluid balance in the body is a dynamic process—water is constantly being lost through elimination and metabolic processes and gained from food and water intake—it is useful to use compartments to understand how water is distributed in the body and thus how fluid therapy affects overall hydration. TBW makes up approximately 60% of total body weight in adults ( FIGURE 1 ) and 80% in pediatric animals. 1 This percentage is lower in overweight patients because fat contains less water than other body tissues. 1 Intracellular fluid accounts for two-thirds of TBW. 1 Of the remaining third, approximately 75% is in the interstitial space and 25% is in the intravascular space 1 ; a small amount is contained in the transcellular space (eg, cerebrospinal fluid, synovial fluid). Separating the intracellular and extracellular compartments are cell membranes, through which water can move freely by osmosis. 1 The intravascular space is partitioned off from other compartments by the vascular endothelium. Within the capillaries is the endothelial glycocalyx, a web of membrane- bound glycoproteins and proteoglycans that plays an important role in fluid dynamics. 2 shutterstock.com/NEstudio GO WITH THE FLOW 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.