6 Complications of Fluid Therapy

       Angelika Schoster Dr.med.vet, DVSc, PhD, DVSc, DACVIM/DECEIM1 and Henry Stämpfli DVM, Dr.med.vet, DACVIM2

       1 Clinic for Equine Internal Medicine, University of Zurich, Switzerland

       2 Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada

List of Complications Associated with Fluid Therapy

Fluid Overload Using Crystalloid Solutions

      In healthy adult animals, the body is made of 60% of water. Two‐thirds of total body water is intracellular (ICF, intracellular fluid compartment) and one‐third is extracellular (ECF, extracellular fluid compartment). ECF is composed of interstitial and intravascular fluid (one‐third of body weight, ~8%) [1, 2]. Overhydration can have severe negative impacts on health and should be avoided.

       Definition

      Fluid overload occurs when the total body water is increased relative to the normal volume for a given patient. Fluid overload is caused by administration of excessive amounts of fluid or adequate amounts of fluid to a patient with impaired elimination, for example a patient with decreased urine output due to renal compromise. This condition is rare in adult horses with normal cardiac and renal function.

       Risk factors

       Pathogenesis

      If fluid plans are properly designed and followed, this complication is rare. It may occur more commonly in small patients (ponies and neonates) if the weight is estimated, as the margin of safety is smaller in these patients.

Starling’s law governs fluid shifts across capillary membranes. Hydrostatic pressures maintain an outward pressure, while oncotic forces aim to retain fluid in its respective compartment. Hydrostatic pressures are derived from body water on either side of the capillary. If large amounts of fluid are introduced into the intravascular space, hydrostatic pressure of the vasculature will increase. When the hydrostatic pressure becomes high enough to overwhelm counteracting forces in the interstitium, extravasation of fluid occurs. In addition, the main oncotic force of the vasculature, albumin, is often diluted in such situations. This lowers oncotic pressure in the vasculature bed, thus further promoting extravasation of fluid into the interstitium and resulting in edema formation [5]. In horses with hypoproteinemia (e.g. colitis, enteritis, colon torsion, post‐peritoneal lavage), the oncotic pressure of the vasculature is decreased even without prior dilution. In cases such as acute non‐oliguric renal failure, extravasation of fluid due to high hydrostatic pressure can also occur in patients with normal protein levels. Clinical signs of over‐hydration (see Diagnosis below) become noticeable once the amount of total water exceeds a critical value, which is individually different.

       Prevention

      A fluid plan should be formulated for each individual patient every 24 hours, taking into account dehydration, maintenance needs and ongoing losses. Care should be taken in animals with risk factors and response should be monitored closely. The weight of the animal should be measured if possible, not estimated, particularly in smaller horses (ponies, neonates).

      Before formulating a detailed fluid plan in severely hypovolemic patients, an initial resuscitation with a shock dose of a maximum of 90 ml/kg in bolus can be given. This amount can be administered safely in about 20–30 minutes to a 500 kg horse. In high‐risk patients, such as neonates, horses with suspected non‐oliguric renal failure or horses with severe systemic compromise or obvious signs of heart failure (distended jugular veins, jugular pulses, ventral edema, tachycardia, weak pulses), a more conservative approach is warranted and administration of 45–60 ml/kg over 30 minutes should be targeted initially. This should be followed by slowly replacing the remainder of the deficit over a 12–24 h period. Alternatively, hypertonic fluid (e.g. 5–7% hypertonic saline, 4 ml/kg) can be used as an initial bolus followed by crystalloid fluid therapy (minimum of 5–10 times the amount of hypertonic saline administered).

      A fluid deficit (dehydration or hypovolemia) can be assessed via clinical data such as heart rate, capillary refill time, moistness of mucous membranes, and skin tent, as well as laboratory data including hematocrit, plasma proteins and creatinine [6, 7]. Anything less than 5% of fluid loss cannot be diagnosed clinically, whereas severe dehydration 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: