Complications in Equine Surgery. Группа авторов

of low potassium fluids (>48 h, hypokalemia)

       Administration of Na‐HCO3 (hypokalemia)

       Non‐oliguric renal failure (hyperkalemia)

       Pre‐existing potassium abnormalities such as rhabdomyolysis, ruptured bladder or hemolysis (hyperkalemia)

       Long‐term administration of diuretics (azetazolamide, e.g. for HYPP, hypokalemia)

       Anorexia for several days (hypokalemia, total body deficit of potassium)

       Reflux or diarrhea (usually hypokalemia)

       Pathogenesis

      Potassium is the most important intracellular electrolyte, as more than 98% of the body potassium is located intracellularly. Equine veterinarians are usually most interested in the extracellular amount of potassium. Potassium concentrations in blood are generally low and tightly maintained. Increases and decreases can occur rapidly. Small changes in serum potassium concentrations can lead to severe clinical signs that can be fatal. Potassium is important for cell membrane polarization. Abnormal serum concentrations of potassium therefore lead to changes in cell membrane potential, which affects primarily muscle and heart cells.

       Prevention

      Monitor blood potassium levels q24–48 h while administering fluid therapy. If a pre‐existing potassium abnormality is present and being corrected, aim for more frequent monitoring, every 6–12 hours. Fluids with adequate amounts of potassium should be administered.

      Replacement therapy can contain a potassium concentration similar to equine plasma (e.g. Lactated Ringer’ solution K⁺: 5 mmol/L). Fluids with higher amounts of potassium should not be used as replacement fluids as inadvertent administration of potassium can cause severe signs of hyperkalemia.

Maintenance fluids should contain higher amounts of potassium (13–20 mmol/L), particularly if the horse is not eating to avoid hypokalemia. If available, a commercial maintenance solution (e.g. Normosol^TM K⁺ 13 mmol/L) can be used; if unavailable, replacement fluids can be spiked with potassium chloride (20 mmol/L). Note that adding 20 mmol/L of KCl to Lactated Ringer’s will result in a total potassium concentration of 25 mmol/L, as LRS contains 5 mmol/L of potassium. Maintenance fluid should not be administered in volumes or rates higher than 2–4 ml/kg/day to avoid side effects due to potassium.

      Oral KCl administration assists in reestablishing body homeostasis of potassium in depleted anorexic horses (e.g. acute colitis and diarrhea) (500 kg horse, 30–50 g KCl PO q 12 h).

       Diagnosis

      Diagnosis is based on clinical signs and determination of blood concentrations of potassium. Hyperkalemia is clinically more relevant than hypokalemia. Clinical signs of hypokalemia are not well documented in horses and vary. Muscle weakness, diaphragmatic flutter, and intestinal hypomotility have been described. Clinical signs of hyperkalemia are mainly related to electrical conduction in the myocardium. Tall or peaked T‐waves, flattened P‐waves and prolongations of the QRS complexes appear on ECG and can lead to asystole. Initial changes can be detected at serum potassium levels of 6.2 mmol/L, and more pronounced and consistent signs are seen at serum potassium concentrations of 7–8 mmol/L [26].

       Treatment

      General hydration status and all other electrolytes should be assessed, as abnormalities in blood potassium concentrations rarely occur alone. In hypokalemia, the recommended potassium supplementation in the administered fluids depends on serum potassium levels.

       Serum K+ <2.5 mmol/L – substitute at 40 mmol/L

       Serum K+ 2.5–3 mmol/L – substitute at 30 mmol/L

       Serum K+ 3.0–3.5 mmol/L – substitute at 15–20 mmol/L

      In mild hyperkalemia (5–7 mmol/L), potassium free fluids should be administered and potassium levels monitored closely. If severe hyperkalemia (>7 mmol/L) is present and abnormalities are seen on ECG analysis, emergency treatments should be instituted and include:

       Intravenous 23% calcium gluconate, 0.5 ml/kg, given over 20 minutes diluted in isotonic IV fluids

       Intravenous dextrose 50%, 10 mg/kg/minutes, diluted to 5% (isotonic) in fluids and given over 30 minutes

       Intravenous insulin, 0.1–0.2 IU/kg/h, diluted in fluids and given over 30 minutes

       Expected outcome

       Depends on severity.

       Animals can die from cardiac effects.

       If treatment is instituted and the animal responds, full recovery is possible.

      Other Electrolyte Imbalances

       Definition

      Abnormal blood concentrations of magnesium (reference range 0.6–0.8 mmol/L), phosphorus (reference range 0.7–1.3 mmol/L) and calcium (reference range 2.9–3.3 mmol/L). Ionized calcium is more relevant than total calcium, as total calcium might be low due to hypoalbuminemia, whereas the concentration of ionized Ca is not affected by protein levels (ionized Ca reference range: 1.0–1.7 mmol/L).

       Risk factors

       Prolonged anorexia (low concentrations of all electrolytes)

       Reflux and diarrhea

       Renal disease

       Pathogenesis

      Prolonged fluid therapy and/or anorexia, as well as various primary diseases (e.g. renal disease, diarrhea, reflux and sepsis), can lead to abnormal levels of calcium, phosphorus and magnesium. The reader is referred to a more comprehensive review for further details (Torribio (2011) Vet. Clin. N. Am. Equine Pract; Stewart (2011) Vet. Clin. N. Am, Equine Pract,).

       Prevention

      To avoid electrolyte depletion while on fluid therapy, maintenance fluids should contain additional electrolytes other than sodium, chloride and potassium. If fluid therapy is anticipated for more than 48 hours, a maintenance solution should be used or electrolytes added to the replacement fluid. When fluid therapy is administered for more than 48 hours, electrolyte concentrations should be monitored daily, particularly in animals with diarrhea, reflux or renal disease.

       Diagnosis

      Diagnosis is based on clinical signs and measuring blood concentrations of electrolytes. Hypocalcemia can lead to abnormal muscle contractions evidenced by diaphragmatic flutter, intestinal hypomotility and weakness. Clinical signs of low phosphorus are variable but can include weakness. Clinical signs of hypomagnesemia are variable but can include weakness.

       Treatment

      The reader is referred to a more comprehensive review for further details (Toribio et al. (2011) Vet. Clin. N. Am.; Stewart (2011) Vet. Clin. N. Am.).

      Complications Due to Administration of Sodium Bicarbonate

       Definition

      Sodium bicarbonate is a fluid containing equal amounts of sodium and bicarbonate ions and can be indicated for correction of metabolic acidosis. Administration can result in undesired side effects such as hypernatremia or respiratory distress, and if used in the wrong patients without concurrent administration of isotonic fluids can result in worsening of the underlying acid–base abnormality.

       Risk