Estimating Rehydration Needs

Constable and coworkers showed that the time required for cervical skin to return to its normal position after tenting and the degree of eyeball recession in dehydrated preruminant calves are reasonably accurate methods to determine the state of hydration for calves. Table 5-1, based on this work, is a guide for estimating dehydration. Rehydration by intravenous fluid administration is recommended once dehydration reaches 8%. When skin pinched on the neck takes 6 seconds to return to normal and an eyeball is recessed 4 mm, it indicates the 8% dehydration point has been reached. Similar quantitative studies of the relationship between clinical signs and degree of dehydration for mature ruminants have not been undertaken. In the absence of data to suggest otherwise, the values for estimating dehydration in calves by using skin tent is probably a reasonable guide for mature cattle. However, emaciation can cause eyeball recession and loss of skin turgor, thus making these tests more difficult to interpret in cattle that have recently lost substantial weight. The body weight of mature ruminants can change dramatically based on the amount of ingesta and water in the rumen; therefore estimates of percent dehydration measured as a percent of body weight are probably not very accurate. Therefore from a clinical standpoint, we predict that a cow that is 10% dehydrated will have a normal or near-normal hydration status if 10% of her body weight in fluids is restored. She may still be well under her “normal” body weight because of lack of rumen fill. On the other hand, a cow with fore-stomach distension from vagal indigestion or carbohydrate engorgement may gain weight from fluid sequestered in the third space compartment (inside the rumen) during the disease process but lose significant extra-cellular body water. Even if we could predict the hydration status of a ruminant patient with certainty, factors other than hydration must still be considered in planning and executing the rehydration process. At times an experienced veterinarian can or must break the 8% rule. Sometimes cattle with severe dehydration and normal gastrointestinal function will recover uneventfully with only oral or intraruminal rehydration or with a combination of intraruminal rehydration and a small amount of fluids administered intravenously. In these cases, breaking the rule saves substantial time and expense. However, endotoxemic or hypovolemic cattle or those in shock—with only mild or moderate dehydration—should receive intravenous fluids. Acute strangulating gastrointestinal disease and acute mastitis are examples of conditions that result in this situation. Rapidly correcting or preventing shock is particularly important if a standing surgical procedure is planned. Finally, patients with fatty liver, chronic or refractory ketosis, and pregnancy toxemia may benefit from intravenous glucose therapy regardless of hydration status.

TABLE 5-1 Guide for Estimating Dehydration of Calves

A flow rate of less than 80 ml/kg/hr has been recommended for calves because of studies of central venous pressure in clinically dehydrated calves. Similar studies have not been performed to determine the maximum safe flow rate in mature cattle or small ruminants. However, significant elevation in central venous pres-sure occurred when approximately 40 ml/kg/hr of an isotonic crystalloid was administered intravenously to dehydrated cattle with experimentally induced intes-tinal obstruction, even though no clinical signs were observed. Although this is a much slower flow rate than the maximal flow rate recommended for calves (80 ml/kg/hr) and dogs (90 ml/kg/hr), a 20 L/hr total flow rate for an average dairy cow is a volume difficult to achieve with a single 14-gauge intravenous catheter. Therefore in most situations, intravenous fluids can be administered to mature cattle through a 14-gauge catheter as quickly as they will flow. Exceptions include cattle with heart disease, oliguric renal failure, and hypoproteinemia and those in recumbency.

A significant energy penalty is incurred if fluids are intravenously administered cold and must be warmed by the patient, especially neonates. Commercially prepared solutions ideally should be used. However, optimal solutions for many ruminant conditions are not readily available in some countries. In those cases, sterilizing locally prepared fluids is recommended. All of the usual components of solutions for cattle can be heat-sterilized, except sodium bicarbonate, which must be filtered. Alternatively, sterile distilled water and reagent grade salts can be used to formulate solutions. Contamination should be carefully avoided during and after preparation, and solutions should be used immediately after preparation. Bacteria can be eliminated by autoclaving, but heat-stable endotoxins and pyrogens may be present in rural and municipal tap water. Therefore the use of pyrogen-free water is important.

Choice of Solution

Ideally, a rehydration solution is formulated after determining the individual patient’s needs by laboratory evaluation. However, this is usually not practical or necessary. In general, ruminants, especially cattle, have rather consistent acid-base and electrolyte abnormalities associated with particular diseases, especially surgical diseases. The following data illustrate that alkalizing solutions are not indicated in most sick cattle, especially those with surgical diseases.

In a study of over 500 cattle older than 1 month of age, blood gas and electrolyte determinations were made from patient venous blood samples. Dehydrated cattle were about twice as likely to have metabolic alkalosis as metabolic acidosis. If cattle with 3 diseases that are easily recognized (pneumonia, carbohydrate engorgement, and diarrhea) were excluded, only 16% of dehydrated cattle had metabolic acidosis. About 20% of the dehydrated cattle were hyponatremic or hypokalemic, whereas over 40% were hypochloremic. In another study of 350 sick cattle, approximately 60% had pH values within the reference range. However, many had compensated acidosis or alkalosis. About 53% had abnormally elevated concentrations of HCO₃⁻, whereas about 10% had decreased HCO₃− concentrations. The only conditions of mature cattle in which metabolic acidosis was more common than metabolic alkalosis were carbohydrate engorgement, urinary tract disease, small intestine strangulation/obstruction (torsion of the root of the mesentery, intussusception, etc.), and enteritis/diarrhea. Cattle with abomasal displacement or volvulus, vagal indigestion, cecal displacement, or torsion and (in our study and experience) intussusception are usually alkalotic.

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