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Fluid Compartment Deficits |  |
| Fluid deficit in the intravascular space results in poor perfusion and inadequate tissue oxygenation. This volume deficit results in a lower vessel wall tension and stimulation of the baroreceptors. Physical parameters that reflect perfusion status include heart rate, pulse intensity, capillary refill time, mucous membrane color, and rectal temperature. Fluids administered must remain in the intravascular space, increase the vessel wall tension, and obliterate the need for the
baroreceptor compensatory response. Most animals with an intravascular deficit (poor perfusion) also have concurrent extravascular deficits. This warrants the simultaneous administration of crystalloid solutions when colloids are used for correction of the intravascular deficit. |
| Fluid deficit in the extravascular space (interstitial and intracellular) causes dehydration. This results in tenting of the skin, dry mucous membranes, sunken eyes, and dullness of the cornea. Perfusion parameters should be evaluated simultaneously to determine whether poor perfusion is compounding the dehydration. To replenish the extravascular spaces, crystalloid fluids that are the same tonicity as plasma are used. Crystalloids are water-based solutions with low molecular
weight molecules that are freely permeable across the capillary membrane. The concentration of these molecules in relation to normal plasma indicates the tonicity of the fluid. If the concentration is similar to normal plasma, the solution is called isotonic. |
| Physical and laboratory parameters are used to quantify the degree of dehydration. Semidry oral mucous membranes, normal skin turgor, and eyes maintaining normal moisture indicate 4-5% dehydration. Dry oral mucous membranes, mild loss of skin turgor, and eyes still moist indicate 6-7%. Dry mucous membranes, considerable loss of skin turgor, and eyes retracted indicate 8-10%. Very dry oral mucous membranes, complete loss of skin turgor, severe retraction of the eyes, dull eyes, and
possible alteration of consciousness indicate 12%. |
| The physical guidelines for estimating dehydration are misleading in 2 common clinical situations. Chronically emaciated animals may have metabolized the fat from around the eyes and the collagen in the skin, resulting in poor skin turgor and sunken eyes despite normal hydration. Animals with rapid fluid loss into a third body fluid space (a space within the body cavity where fluid from the local interstitial and intravascular spaces leak) have rapid fluid shifts from the
intravascular compartments into these spaces before clinical evidence of interstitial fluid loss is seen. Both situations require evaluation of mucous membrane and eye moisture, PCV, and total solids before dehydration can be estimated. |
| The intravascular space can be replenished using crystalloid solutions alone. However, large quantities of crystalloids may be required to provide volumes sufficient to blunt the baroreceptor response after 1 hr of infusion. The extra interstitial volume for a short period does not cause a problem in most animals, and if renal function is normal, the additional volume is eventually excreted. However, the brain and heart do not have “forgiving” interstitial spaces in disease, and
additional interstitial fluid can cause organ failure. |
| Many conditions can increase capillary permeability and cause systemic inflammation (eg, parvoviral diarrhea, pancreatitis, septic shock, massive trauma, heat stroke, cold exposure, burns, snake bite, and systemic neoplasia). Increased vessel permeability is an additional factor that favors movement of crystalloid fluid from the intravascular space into the interstitial space and complicates maintenance of intravascular volume. A combination of a large molecular weight colloid (to
hold the water in the vessels) and crystalloids (to replenish the interstitial spaces) is recommended. In addition, many of these animals have a third body fluid space, most likely due to significant regional inflammation. The end result is massive fluid requirements, which make maintaining fluid balance (especially intravascularly) difficult. It is almost impossible to predict how much fluid will be required to maintain perfusion and hydration in these critical patients. Fluid
selection becomes critical, with the rate and volume titrated until the goals of resuscitation have been met (ie, normal heart rate, blood pressure, perfusion parameters). |
