Fluid therapy is an essential therapeutic component in small animal practice. Normal cellular function can be impaired without water and potentially lead to patient death.1 Intravenous fluids may be prescribed to hospitalized patients to treat hypovolemia, dehydration, electrolyte imbalance, and acid-base abnormalities and to ensure that adequate cellular maintenance requirements are met.2
Understanding the concept of fluid compartments can help the clinician determine the location of the fluid deficit and appropriate treatment. The body weight of nonobese cats and dogs is composed of approximately 60% water.2
Puppies and kittens have higher total body water amount (ie, up to 80% of body weight), as total body water decreases with age.3
In addition, fat has a lower water content; thus, the fluid prescription should be based on estimated lean body weight.4
In adult nonobese cats and dogs, approximately two-thirds of total body water (ie, 66% of total body water or ≈40% of body weight) is in the intracellular space. The remaining one-third (ie, 33% of total body water or ≈20% of body weight) is in the extracellular space; of this extracellular body water, 75% (≈15% of body weight) is in the interstitial space and 25% (≈5% of body weight) is in the intravascular space (Fig 1). Intracellular fluid loss is generally not appreciated on physical examination and typically manifests as hypernatremia. Treatment of intracellular fluid deficit is beyond the scope of this article.
Distribution of total body water in an adult nonobese cat or dog
Intravascular fluid deficit (ie, hypovolemia) leads to inadequate oxygen delivery to the cells (ie, poor perfusion or shock). Untreated intravascular fluid deficit can be life-threatening, as oxygen is important for minute-to-minute cellular function maintenance. Inadequate oxygen delivery can lead to hyperlactatemia through anaerobic glycolysis, cell membrane disruption, cell death, and organ death.5
Physical examination findings of hypovolemia include tachycardia in dogs, bradycardia in cats (and in the terminal stages of shock in dogs), prolonged capillary refill time, pale mucous membranes, weak peripheral pulses, cold extremities, and altered mental state. Patients exhibiting these signs require emergent treatment to rapidly restore oxygen delivery. Common clinical conditions that lead to intravascular fluid loss include hemorrhage secondary to trauma, coagulopathy, neoplasia, gastroenteritis, pancreatitis, and peritonitis.
Review the essentials of fluid types, resuscitation, rehydration, and maintenance as well as potential complications of administration of fluids here
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