Canine Fluid Therapy

Recognition and management of disturbances in fluid, electrolyte, and acid-base balance are critical to successful management of the surgical dog. Pre-existing disturbances in water, electrolyte, and acid-base balance may cause life-threatening consequences unless they are identified and managed. No time period can be isolated in management of these disturbances; an integrated approach initiated during the preoperative period and carried through the postoperative phase is necessary for greatest success. Although acute resuscitation of water, electrolyte and acid-base balance is necessary in specific dogs, continued support is needed following surgery to ensure success. To rationally plan and manage support, a working knowledge of critical issues associated with fluid therapy and acid-base regulation in dogs must be understood to ensure appropriate management.

Fluid therapy in dogs - General Principles

Body water

Water requirements vary with body weight, age, environmental factors, pregnancy, and pre-existing disease states that affect ingestion, transport, or regulation of water in dogs. Conditions that demand additional fluid support in dogs include trauma, infection, peritonitis, edema, shock and hemorrhage.

Body water is divided into two functional compartments - the extracellular and intracellular spaces. The extracellular space may be further subdivided into the intravascular and interstitial spaces. In dogs, the intracellular compartment contributes 30 to 40 percent of total body water, with the extracellular compartment contributing 20 to 30 percent. The intravascular compartment contributes approximately 5% of total body water.

Changes in extracellular sodium and chloride levels can produce altered water balance between fluid compartments. Reduction in salt concentration in the extracellular space causes redistribution of free water to to the intracellular compartments. As water shifts between compartments, the intracellular osmotic pressure falls until a new equilibrium is established. Conversely, presence of increased salt concentration in the extracellular space facilitates water redistribution from the intracellular to the extracellular compartment. Lymphatic injury or water redistribution in excess of lymphatic pump capacity can also result in interstitial water accumulation in dogs.