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 factors
Patients with chronic hyponatremia receiving sodium bicarbonate
Severely dehydrated animal with lactic acidosis receiving sole administration of sodium bicarbonate to correct metabolic acidosis
Patients with severe respiratory failure (hypercapnia, pCO2 >60 mm Hg): administration of large amounts of sodium bicarbonate to these patients is believed to be contraindicated
Pathogenesis
Once infused, the sodium increases the strong ion difference and is shifting the equilibrium of the bicarbonate dissociation toward HCO3, therefore in turn raising the pH concurrently with the sodium levels.
In chronic hyponatremia, intracellular sodium concentrations have adapted and are similar to extracellular (plasma) concentrations. When the plasma sodium concentration is increased rapidly due to the administration of NaHCO3, the intracellular sodium concentration suddenly becomes lower than the plasma concentration. As water follows solute, water is drawn from the brain cells to extracellular fluid (plasma), causing osmotic demyelination syndrome (see also discussion on sodium above).
In severely dehydrated animals, the main acid–base disturbance is metabolic acidosis as a result of lactate accumulation because of hypoperfusion. If NaHCO3 is erroneously administered in an attempt to raise the pH, along with an inadequate amount of fluid administered, hypoperfusion and metabolic acidosis due to lactate accumulation persists. Acute hypernatremia can be caused if large amounts of NaHCO3 are infused in an attempt to rehydrate the animal with NaHCO3.
In the traditional approach to acid–base disturbance, dissociation of HCO3 produces CO2, which then has to be eliminated via the lungs. In patients with respiratory compromise, elimination can be decreased and may lead to respiratory acidosis. Following the physicochemical approach to acid–base disturbance, CO2 is an independent variable and therefore not influenced by the concentration of HCO3. Infusion of Na‐HCO3 therefore does not lead to elevated pCO2 concentrations in the blood or lungs. To err on the side of caution, administration of Na‐HCO3 to patients with respiratory compromise should be avoided.
Prevention
Determine acid–base and electrolyte status of patient and assess if Na‐HCO3 is truly the fluid of choice. The origin of acidosis should be identified. In equine medicine, the most common cause for metabolic acidosis is due to increased serum L lactate concentrations due to hypovolemia and endotoxemia, and therefore Na‐HCO3 is rarely indicated. These animals will benefit most from treatment of dehydration by administering replacement therapy of isotonic crystalloid fluids.
The second‐most common cause of metabolic acidosis is electrolyte derangements. The underlying electrolyte derangements should be identified and corrected. The most common causes of metabolic acidosis are hyponatremia and hyperchloremia. Sodium bicarbonate is therefore particularly useful in hyponatremic hyperchloremic patients where large amounts of sodium are needed without the addition of chloride. NaHCO3 is indicated if:
Hyponatremia concurrent with hyperchloremia
pH <7.2
If possible, an isotonic formulation (1.3%) of NaHCO3 should be administered:
Add 150 mmoL (13 g) of NaHCO3 to 1 L of sterile water
Alternatively, 150 mmol/L can be added to Lactated Ringer’s if sterile water is not available; note that this will result in a slightly hypertonic solution. There is no problem with Ca chelation.
Calculate the deficit based on:
Deficit mmol/L NaHCO3 = BW × –BE × 0.3 (adults) or 0.5 (foals <2 months)
Give half of the calculated amount over 30 min, the rest over 24 h
Alternatively, oral NaHCO3 can be given 0.3–0.5 g/kg q 12–24 h.
Diagnosis
Repeat blood gas analysis should be performed
