C. Langdon Fielding
Fluid Therapy in the Field
In ambulatory equine practice, fluid administration can be a practical and life-saving therapeutic option. Treatment in a field setting may be preferable to hospitalization for reasons such as the temperament of the horse, financial constraints, or inability to transport the animal. Although there are some features of providing fluid therapy in the field that are unique to that situation, many of the principles are similar to fluid administration in a hospital setting.
Indications for Fluid Therapy
Dehydration is the primary reason for administering fluids, but there are a number of other important problems and clinical scenarios that can also be addressed with fluid administration.
Hypovolemia
Clinical signs of hypovolemia include tachycardia, poor pulse quality, pale mucous membranes, prolonged capillary refill time, decreased mentation, poor jugular refill time, cool extremities, and decreased urine output (Box 207-1). One consequence of hypovolemia is poor tissue perfusion that leads to anaerobic metabolism and accumulation of plasma lactate. Plasma lactate concentration can easily be measured in the field, and lactate concentration greater than 2.0 mmol/L supports a diagnosis of hypovolemia. Lactate can be measured with handheld analyzers, and results are available within minutes (one example is ACCUSPORT1). High values for packed cell volume (PCV) and total plasma protein concentration are also consistent with hypovolemia, but it must be recognized that these values can be affected by other disease processes and are considered less reliable indices of hypovolemia (Box 207-2). Additional tests that can be used in the field to support a finding of hypovolemia, although more labor or expertise intensive, include measurement of central venous pressure (negative values may be consistent with hypovolemia) and echocardiography.
Dehydration
Clinical signs of dehydration may include prolonged skin tenting and dry mucous membranes. Estimating the degree of dehydration on the basis of clinical signs can be inaccurate in a number of species, including horses. As discussed previously, increases in PCV and total plasma proteins are common with dehydration, but other factors (e.g., anemia, protein-losing enteropathies) can make these tests more difficult to interpret. High urine specific gravity (>1.030) can be a helpful diagnostic tool but becomes less reliable with renal tubular disease. History can be invaluable when dehydration is suspected because a horse may have lost access to water or the owners may have observed that water has not been consumed for a prolonged period.
Gastrointestinal Disease
Even in the absence of dehydration, fluid therapy is commonly used for gastrointestinal diseases such as feed impactions. The rationale for its use is to hydrate the intestinal contents or help prevent further reabsorption of fluid from the gastrointestinal tract. In some instances, fluid therapy may be used as a renal protective strategy because horses with colic often need repeated doses of nonsteroidal antiinflammatory medications. These horses also may not be maintaining normal hydration by voluntary water intake.
Renal Disease
In some horses with renal disease, fluids can be used to promote diuresis and protect the kidneys from further damage even if the animal is not dehydrated. Point-of-care testing devices may be used to identify high renal values in the field. If a urine sample can be obtained, measuring specific gravity can help confirm renal disease, depending on prior fluid administration and current hydration status. In the face of dehydration, a specific gravity of 1.008 to 1.014 would suggest isosthenuria and inadequate renal function. Even measurements of 1.014 to 1.020 in a dehydrated patient may indicate a problem with the kidneys. Field treatment of renal disease can be very practical because patients are often stable yet require continuous fluids for a prolonged period.
Toxicoses
In some instances, when intoxication is known to have occurred, fluid therapy may be used to address treatment for the specific toxin involved. For example, a horse overdosed with a nonsteroidal antiinflammatory medication may benefit from administration of an alkalotic fluid because this may help with excretion of the medication. Other examples might include blister beetle toxicosis, in which calcium supplementation may be beneficial if hypocalcemia is detected.
To Serve as a Vehicle for Other Additives
Particularly in the field, fluid therapy is often used in conjunction with fluid additives. In many cases, it is safer to administer a medication more slowly in intravenous fluids rather than as a fast, small-volume bolus. Examples include the administration of calcium gluconate for horses with synchronous diaphragmatic flutter, in which rapid treatment with undiluted calcium gluconate can pose a risk for cardiac arrhythmias. Some antimicrobials (e.g., oxytetracycline) may be more safely administered when given slowly and diluted in a bag of intravenous fluids.
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