Hypocalcemia is defined as a blood ionized calcium (iCa) level < 6.5 mg/dL (< 1.25 mmol/L) (normal 9 - 11.4 mg/dL).
Serum total calcium does not accurately predict ionized calcium status in many clinical conditions. Calcium homeostasis in hypoalbuminemic critically ill dogs should be evaluated by iCa concentrations rather than total calcium or calcium adjusted for albumin or total protein.
Calcium is a vital intracellular and extracellular ion involved in neuronal activation, muscle contraction, enzymatic reactions, hormone secretion, and bone matrix.
Normal calcium homeostatic mechanisms maintain extracellular calcium concentrations within a narrow normal range and changes often result in abnormal skeletal and cardiac muscle activity.
Extracellular calcium exists in three forms: ionized (the biologically active form), complexed (to plasma buffers), and protein-bound (mainly to albumin). Most commonly, total calcium is measured on serum biochemical analyses and represents the sum of all calcium fractions. Serum-ionized calcium (iCa) concentration is a more accurate measure of hypercalcemia than total serum calcium or corrected serum calcium concentrations.
In dogs with hypocalcemia, the low concentration of calcium causes an excitatory effect on nerve and muscle cells, lowering the threshold potential and discharging repetitively without provocation. Tetany occurs as a result of spontaneous repetitive firing of motor nerve fibers.
Hypoglycemia can occur concurrently in dogs.
Although commonly observed in young dogs associated with inadequate dietary calcium, a number of disease may also cause hypocalcemia, including:
- congenital rickets
- chronic renal disease
- metabolic alkalosis
- primary hypoparathyroidism
- diabetic ketoacidosis
- nutritional secondary hyperparathyroidism (rickets) 
- milk fever (eclampsia; puerperal hypocalcemia)
- following parathyroidectomy in dogs with primary hyperparathyroidism
- phosphate-containing enemas
- inflammatory bowel disease
- oxalate toxicity (e.g., lily, philodendron, etc.)
In dogs with nutritional secondary hyperparathyroidism, clinically affected dogs often present with joint pain due to osteopenia, which can be confirmed on radiographs.
Treatment involves addressing the acute hypocalcemia with parenteral calcium gluconate (10 mL of 10% calcium gluconate in 250 mL of 0.9% saline administered at 2.5 mL/kg/hr for 8 – 12 hours).
Severe and persistent hypocalcemia is associated with ventricular tachyarrhythmias and fibrillation, leading to sudden death.
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