Fluid Therapy for Hyperkalemic Periodic Paralysis (Box 44.8)

K. Gary Magdesian

Hyperkalemic periodic paralysis (HYPP) is caused by a genetic defect in the sodium channel on the sarcolemma, resulting in

■ BOX 44.8

Fluid Guidelines for an Acute Hyperkalemic Periodic Paralysis Episode

1.

2. 2% to 5% dextrose (2% to 2.5% if ≥5 L are administered; 5% if 2 L are administered to a 500-kg horse)

3. 0.2 to 0.5 mEq/kg of calcium (0.2 to 0.5 mL/kg of 23% calcium gluconate total, diluted in fluids); calcium should be administered early (along with dextrose), since it is protective against arrhythmias. Dilution of 23% calcium gluconate should be to a final ratio of no more than 50 mL of calcium gluconate/L of fluid.

4. 0.5 to 1 mEq/kg of bicarbonate in second bag of normal saline, along with dextrose. Note that sodium bicarbonate and calcium should not be mixed in the same bag.

5. For refractory cases, insulin can be added to dextrose supplementation (regular insulin, 0.005 to 0.1 unit/kg/h).

intermittent signs of sweating, muscle fasciculations, stridor, and weakness.¹ Stressors such as anorexia, anesthesia, concurrent illness, trailering, and cold environmental temperatures can precipitate hyperkalemia and onset of clinical signs. Fluid therapy for horses with hyperkalemia should be low or free of potassium. Physiologic (0.9%) saline and isotonic sodium bicarbonate (1.3%) are both potassium-free options that have been used. Note that these two fluid types have potential effects on acid-base physiology, with saline producing a mild strong ion acidosis and sodium bicarbonate a metabolic alkalosis. There is some evidence in human medicine that the strong ion acidosis induced by the chloride excess in saline can actually increase potassium concentrations by causing shifts from the intracellular space.

Horses experiencing stressors such as forced withholding of feed before surgery or being off feed because of concurrent illness should be administered fluids with dextrose and/or calcium supplementation prophylactically at a maintenance rate. Supplementing with dextrose (2.5% to 3% dextrose at 1 L/h for an average-size horse, or 0.8 to 1 mg/kg/min of dextrose) may minimize the development of hyperkalemia and subsequent HYPP episodes. Calcium does not decrease serum potassium concentrations significantly but rather protects the heart from its adverse effects by raising threshold potential for the action potential on the sarcolemma.

Horses experiencing an active episode of hyperkalemia or showing clinical signs of HYPP should be administered 0.9% saline as the fluid choice, with the additives of calcium (0.2 to 0.5 mEq/kg of 23% calcium gluconate total, diluted as ≤50 mL 23% calcium gluconate/L of fluid) and dextrose (5% if 2 L of fluids are administered, or up to 2% to 2.5% if 5 L are administered). If signs persist, a second bag with sodium bicarbonate (0.5 to 1 mEq/kg of bicarbonate empirically) and dextrose should be added. Calcium and sodium bicarbonate should not be administered concurrently in the same fluid administration set because of a potential for precipitation. LRS also contains calcium, and therefore sodium bicarbonate should not be added directly to it.

Administration of two 5-L bags of saline to a 500-kg horse, with 2% to 2.5% dextrose, with the first bag containing calcium (0.5 mL/kg of 23% calcium gluconate total) and the second containing sodium bicarbonate (0.5 to 1 mEq/kg of bicarbonate) is a reasonable approach to treating an acute and severe episode. Each of these bags would be given sequentially. Some horses may not require this volume of crystalloid and can be treated with less; if less volume is desired, 4 to 6 mL/kg of normal saline can be administered.

Long-term management of horses with HYPP consists of a low-potassium diet, regular exercise, attempts to minimize stressors, and medications such as acetazolamide and phenytoin.² Horses undergoing chronic acetazolamide therapy could theoretically develop hyperchloremia and a tendency toward metabolic acidosis because of the effects of long-term inhibition of carbonic anhydrase. Administration of large volumes of 0.9% saline can compound hyperchloremia³; to minimize acidemia, it can be coadministered with isotonic sodium bicarbonate or other alkalinizing crystalloid when such horses require large-volume or long-term fluid therapy devoid of or low in potassium. Phenytoin can mask the clinical signs of HYPP, even when hyperkalemia is present; it must be emphasized that phenytoin should not be the sole prophylactic medication in these horses, because hyperkalemia may be left unchecked.²