Today's Veterinary Practice

JAN-FEB 2018

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PEER REVIEWED 62 FLUID THERAPY The classic Starling model of fluid dynamics did not take into account the endothelial glycocalyx, and it was previously thought that fluid was filtered at the arteriolar end of the capillary and subsequently resorbed on the venule side, with the rate of filtration and absorption related to both the hydrostatic pressure and the colloid oncotic pressure (COP) in the capillary and the interstitial space. 2 Newer research has shown that the COP of the subglycocalyx space replaces interstitial COP and that fluids are not absorbed on the venule side but rather are returned to the circulation via the lymphatic system. 2,4 TYPES OF FLUID LOSSES Fluid loss in small animals can be isotonic, hypotonic, or hypertonic. While osmolality is a measure of all solutes in solution, tonicity refers only to the effective solutes in solution; that is, those that cannot cross a membrane and so exert an osmotic pressure. Isotonic losses, such as those observed with vomiting and diarrhea, occur when the fluid being lost has an osmolality similar to that of plasma. With isotonic fluid loss, sodium concentrations are typically normal; however, if the animal is able to drink water, hyponatremia may result from replacement with a hypotonic fluid. Hypotonic losses occur when the type of fluid being lost has a higher concentration of water than plasma, such as with diabetes insipidus and panting. The loss of a hypotonic fluid, or water in excess of sodium, can result in hypernatremia. Although isotonic replacement fluids may benefit some patients with hypotonic losses, others benefit from fluids that contain more free water. Hypertonic losses, or loss of sodium in excess of water, are uncommon but can be seen with hypoadrenocorticism, third-space loss of fluid (such as pleural or peritoneal effusion), and diuretic administration. 1 Excess loss of hypertonic fluid can result in hyponatremia. Treatment of hypernatremia and hyponatremia will be discussed in Fluid Therapy: Part 2. DETERMINING FLUID DEFICITS Assessment of Hydration Dehydration refers to lack of fluid in the interstitial compartment, whereas hypovolemia describes lack of fluid in the intravascular compartment. Intracellular dehydration cannot be detected on physical examination and is primarily assessed via changes in sodium concentration. Although dehydrated patients have some hypovolemia, clinical changes do not occur until dehydration exceeds 10%. 1 The minimum percentage dehydration that can be detected on physical examination is 5%, and dehydration greater than 12% is likely to be fatal. 1 Percentage dehydration can be estimated by examining mucous membrane moisture, skin turgor ( FIGURE 2 ), eye position ( FIGURE 3 ), and corneal moisture, as described in TABLE 1 . 1 Overhydration of the interstitial compartment, which occurs in cases of fluid overload, can also be detected on physical examination. Signs of overhydration include chemosis, serous nasal discharge, increased skin turgor, peripheral edema, ascites, pleural effusion, and pulmonary edema. FIGURE 1. Distribution of body water within the various compartments. ECF, extracellular fluid. ECF 1/3 Intravascular fluid 1/4 of ECF Interstitial fluid 3/4 of ECF Intracellular fluid 2/3 FIGURE 2. Checking skin turgor on a dehydrated cat. Note the degree to which the skin can be tented away from the cat's body.

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