of around 12% may result in death. Dehydration in percent (%) times body weight gives the amount of fluid in liters to be replaced over a specific time:
Ongoing losses due to diarrhea or reflux should ideally be measured. If this is not possible, then losses can be estimated. Maintenance fluid requirement is 2–3 ml/kg/h (50–75 ml/kg/day) in adult horses and 3–4 ml/kg/h in foals (75–100ml/kg/day), who have a higher total tissue water amount.
Additional fluid sources such as enteral fluids, or use of different fluids such as plasma, colloids, or parenteral nutrition solution with their high osmolality, have to be factored into the fluid equation. Once the desired amount of fluid for the next 24 hours is calculated it should be given as a continuous rate infusion (CRI). Ideally in all, but certainly in smaller animals (ponies or foal), a fluid pump should be used.
The adequacy of fluid therapy should be monitored every 6–12 hours. The fluid plan should be adjusted accordingly every 12–24 hours. Monitoring parameters include hematocrit and plasma proteins, serum creatinine and lactate. Serial measurements have to be performed, as single hematocrit values can be influenced by splenic contractions and low protein concentrations can be due to primary hypoproteinemia rather than overhydration. Urine output is a good marker for hydration status. When adequate urine output (min, 1 mL/kg/h, approx. 500 mL/h or 12 L/day for a 500 kg horse) occurs after initiation of fluid therapy and urine specific gravity returns to normal (reference range 1,020–1,040), dehydration is likely resolved and fluid rates should be reduced to cover maintenance and ongoing losses. Repeated weighing of the patient as an objective determination of adequate fluid administered has limited value. For instance, horses with colitis may accumulate fluid in the colon, and gain weight rapidly while still being dehydrated. Continuous daily weight gain should alert for fluid overload in a horse with normal hydration status; however, severely dehydrated horses usually appropriately gain weight. Other techniques providing a more accurate estimation of fluid therapy include central venous pressure monitoring, bioimpedance analysis and pulse pressure variation [8–11]. These techniques are not routinely used in practice and are usually restricted to large referral or university hospitals.
Diagnosis
Diagnosis of fluid overload is based on clinical signs and laboratory data. Acute fluid overload often leads to signs of pulmonary edema, while chronic fluid overload is often associated with signs of heart failure. Pulmonary edema leads to impaired oxygenation; clinical signs include tachypnea, tachycardia, coughing, respiratory distress, “wet” lung sounds on auscultation and serous or frothy nasal discharge (see Figure 6.1). Signs observed with chronic fluid overload include lethargy, tachycardia, peripheral edema formation on the ventral midline (see Figure 6.2), distal limbs, the sheath in geldings or the head when carried low, and rarely chemosis (see Figure 6.3). Additional signs seen can be restlessness, shivering, colic, ascites, pleural effusion, and large amounts of urine voided. On laboratory analysis, hematocrit and plasma proteins are often below normal range. Arterial blood gas analysis can be performed to assess oxygenation in patients with suspected pulmonary edema. Blood pressure can be elevated. Other negative effects of fluid overload include interstitial tissue edema, gastrointestinal motility disturbances, acute respiratory distress syndrome, abdominal compartment syndrome, delayed wound healing and increased mortality [12, 13].
Figure 6.1 Frothy nasal discharge due to pulmonary edema from fluid overload in a horse.
Treatment
Once fluid overload is recognized, measures should aim at reducing the total amount of body fluid. Treatment options depend on severity of the case. If mild signs of pulmonary (mild tachypnea but no signs of respiratory distress or nasal discharge) or of cardiovascular impairment (mildly elevated heart rate but no overt signs of heart failure) are present and renal function is normal, the kidneys are likely to excrete the excessive amounts of fluid as long as no additional excessive fluid amount is administered.
Figure 6.2 Ventral edema as a consequence of fluid overload in a horse.
Figure 6.3 Chemosis as a consequence of fluid overload in a horse.
If severe clinical signs of pulmonary, cardiovascular or any renal function impairment are present, additional treatments should be initiated.
Discontinue or decrease administration of fluid, depending on whether the underlying clinical problem requires additional fluid therapy (e.g. electrolyte imbalances).
Increase renal excretion of fluid: Furosemide 1–2 mg/kg IV as a bolus. In case of severe pulmonary edema, up to 4 mg/kg.
Drain excessive fluid from pleural and peritoneal spaces if present.
Reassess hydration status initially every 2–4 hours, later every 6–12 hours, using the clinical and laboratory parameters described above until hydration status is normal.
Expected outcome
The outcome depends on the inciting cause and underlying disease. If the inciting cause (such as inadvertent over‐administration to a healthy patient) can be resolved, prognosis is good. If renal failure is the cause for fluid overload, prognosis is poorer and guarded. Horses with pulmonary edema can die within a short period of time, or can recover fully depending on severity and initiation of treatment.
Complications Associated with the Type of Crystalloid Fluid Infused
Fluid therapy can lead to acid–base and electrolyte imbalances when given to a healthy animal, but also overcorrections of pre‐existing abnormalities can lead to severe side effects if not performed correctly. Sodium and potassium mainly, but also chloride, calcium, magnesium and phosphor homeostasis, are important.
Many different crystalloid fluids are available commercially, containing varying concentrations of different electrolytes and base equivalents. Few formulations are currently available in 3–5 L bags, while 1 L bags usually are available but are often cost‐prohibitive and cumbersome to be administered to a normal sized horse. Depending on the country and legislation, these fluids differ slightly in their composition. Every clinic/hospital/practitioner should attempt to get an overview of formulations available in his/her country for administration to horses and should know content and concentrations including osmolality of the fluids.
Replacement fluid therapy should be considered separately from maintenance fluid therapy, especially the type of fluid chosen. In general, replacement fluids (e.g. Lactated Ringer’s, isotonic saline, Normosol‐RTM, Plasmalyte ATM)
