Fluid Therapy: an Overview

Fluid therapy can be a major component to the management of the emergent or critically ill patient.

Determining Fluid Requirements

A patient history along with a complete physical examination is the foundation for the development of a fluid therapy plan. It is important to determine the degree of dehydration and the perfusion status of the patient prior to beginning fluid therapy. There are several clinical and laboratory methods which may be used to determine the hydration status of the patient.

History

The owner should be asked questions about food and water intake. Is the animal eating? If not, when did it last eat? Is the animal drinking water, if so, increased, or decreased amounts? Is the animal suffering any abnormal losses such as vomiting, diarrhea, or polyuria? What is the duration of theses abnormal losses?

Physical Findings

Skin turgor or skin elasticity is a crude way of determining the interstitial compartment volume (hydration) status. When assessing skin turgor its best to use the same location for consistency in technique. The lateral thorax or between the shoulder blades are good locations to assess skin turgor. With 5% dehydration the skin, when lifted, will return to its normal position quickly but slightly slower than normal. With 8% dehydration the skin returns to its normal position slower than 5% dehydration but faster than 10% dehydration. When the patient is 10% dehydrated the skin will remain tented and not return to its normal position. Elasticity of the skin is affected by cachexia and obesity. It is possible to have a normally hydrated patient that has reduced skin elasticity due to cachexia; or a dehydrated patient that has normal skin elasticity because of being fat. Other signs consistent with dehydration include dry skin and mucus membranes, oliguria, and signs of compensatory peripheral vasoconstriction. Perfusion is assessed by evaluating 6 parameters known as the perfusion parameters. They include mentation, mucous membrane color, capillary refill time, heart rate, pulse quality, and extremity temperature. In poor perfusion states it is not uncommon to see decreased mention, pale mm color, prolonged capillary refill, increased heart rate (in the dog), cool extremities, and poor pulse quality.

Laboratory Analysis

Packed cell volume (PCV) and total protein (TP) are simple tests that can be used to evaluate hydration. PCV and TP are often elevated with dehydration. In an animal with both anemia and dehydration, the PCV may appear to be normal, but this is due only to hemoconcentration. A urine specific gravity greater than 1.030 usually indicates that the kidneys are responding to the dehydration in an appropriate manner. Electrolyte and acid base status can be a valuable addition to evaluation of the emergency patient. Depending on the disease process, it is not uncommon to find electrolyte abnormalities with sodium and / or potassium. Lactate levels > 2 mmol/L suggest poor perfusion and inadequate tissue oxygenation. Monitoring lactate can help identify patients that may benefit from fluid therapy as well as a monitoring tool to determine if fluid therapy provided was adequate.

Dehydration vs Hypovolemia

It is important to recognize the difference between dehydration and hypovolemia. Dehydration is a decrease in interstitial fluid volume as evidenced by dry mucous membranes, and decreased skin elasticity. Hypovolemia is a decrease in circulating blood volume and evidenced by poor perfusion parameters. Severe dehydration can lead to hypovolemia; however, a hypovolemic patient does not have to be dehydrated (e.g., a previously healthy dog that has suffered trauma). The fluid deficit occurring with dehydration is corrected over hours (commonly 6 – 12 hours) while hypovolemia should be corrected in less than one hour.

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