Chapter 48 Fluid Therapy
Dogs and cats with gastrointestinal (GI) disease commonly need fluid therapy. The most basic reasons for fluid therapy in patients with GI disease are replenishment or maintenance of intravascular fluid volumes and correction of specific metabolic deficits, such as electrolytes, serum protein concentrations, and others, that arise secondary to GI disease. As such, there is no “one-size-fits-all” approach to fluid therapy in patients with GI disease; rather, fluid therapy should be tailored to the needs unique to the individual patient. Provision of appropriate fluid therapy requires an inventory of a variety of fluid types, access to laboratory and other technologies for patient monitoring, and a staff trained in the monitoring and care of patients receiving fluid therapy.
The need for fluid therapy in patients with GI disease arises from a number of pathophysiologic conditions. Because of malaise and anorexia, dogs and cats with GI disease may have fluid intake reduced to a level unable to offset sensible and insensible fluid losses. Patients with GI disease are also susceptible to increased fluid losses as a result of vomiting, diarrhea, and, in some cases, polyuric states. Lastly, some types of GI disease put patients at risk for fluid redistribution into third spaces, such as edema or effusion caused by hypoalbuminemia and peritonitis. These pathophysiologic processes are not mutually exclusive, and all three mechanisms could be operative in a given patient. This chapter is a broad overview of fluid therapy in patients with GI disease. Readers interested in more in-depth information regarding fluid therapy are referred to other references.1,2
Goals of Fluid Therapy
Some important goals of fluid therapy, and the general types of fluids commonly used to meet these goals, include the following:1,2
• Restoration and maintenance of circulating volume for tissue perfusion: crystalloid and colloidal fluids
• Correction of abnormalities of electrolytes, glucose: crystalloids, electrolyte, and glucose supplements
• Provision of oncotic support: synthetic and natural colloids
• Restoration of oxygen-carrying capacity: whole blood and blood products
• Provision of nutritional support: enteral and parenteral nutritional solutions
Crystalloids and colloids are the fluid types most commonly used in the treatment of dogs and cats with GI disease. Crystalloids are compositions of fluid and electrolytes in varying proportions that are divided generally into replacement fluids and maintenance fluids. Box 48-1 provides examples of each of these types of fluids. Replacement fluids, as the name suggests, are designed to replace water and electrolytes lost as a consequence of GI (or other) disease, and are characterized by higher concentrations of sodium than maintenance fluids, which have proportionally more water than replacement solutions. Colloids can be synthetic or natural. Box 48-1 also outlines examples of synthetic and natural colloids.
Fluids Used in the Treatment of Gastrointestinal Disease of Dogs and Cats
Rational Use in the Diagnosed Patient
Indications for Fluid Therapy
Determined by History and Physical Exam
The decision to administer fluids to a patient with GI disease should take into account historical elements (e.g., is the patient voluntarily eating and drinking?), physical examination findings, results of laboratory or other diagnostic tests, and the underlying disease process. Patients that are unwilling, or unable, to drink emerge as more likely candidates for fluid therapy than those patients that can and will drink. Dogs or cats that are dehydrated, as suggested by the physical examination and/or laboratory results, also become candidates for fluid therapy. The physical examination assessment of hydration status has potential errors in interpretation. Animals that have lost considerable body weight commonly have reduced skin turgor, prolonged skin retraction, and ocular recession even if normally hydrated. Heart rates can be elevated for reasons other than dehydration. Nausea may contribute to a degree of mucous membrane moistness in the dehydrated patient, and mucous membranes may be overly dry in the patient that has been panting. Capillary refill times may be prolonged as a result of marked sympathetic stimulation and constriction of peripheral vascular beds, or may be very rapid in patients with peripheral vasodilation or hyperdynamic states of cardiovascular shock (as occurs, e.g., with sepsis).