Page 187 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 177

intestine. Consequently, it is not a simple matter to deter- dosages of 20 to 30 ng/kg/day for 3 to 4 days and main-
mine the bioavailable elemental calcium content of a spe- tenance dosages of 10 to 20 ng/kg/day in most patients.
cific oral calcium salt. Oral calcium is usually administered The dose of calcitriol is divided and given twice daily to
at 25 to 50 mg/kg/day elemental calcium in divided ensure sustained priming effects on intestinal epithelium
doses. Oral calcium carbonate serves as an intestinal phos- for calcium transport. Calcitriol is commercially available
phate binder in addition to providing calcium for intesti- in 0.25- and 0.50-mg capsules (250 and 500 ng per cap-
nal absorption. It is advisable to continue oral calcium sule, respectively). It is likely that reformulation of
carbonate therapy for its intestinal phosphate-binding calcitriol in doses suitable for a variety of animal sizes will
effects if serum phosphorus concentration remains be necessary. It may be useful to prescribe calcitriol in liq-
increased. Lower serum phosphorus concentrations uid formulation so that small adjustments in dosage can
may allow increased endogenous synthesis of calcitriol be made accurately. A number of specialty pharmacies
because phosphate inhibits renal synthesis of calcitriol. reformulate human drugs for veterinary use and can cre-
Vitamin D preparations (see Table 6-5) include ate any calcitriol dose needed.
ergocalciferol, cholecalciferol, 25-hydroxycholecalciferol
(calcidiol), 1a-hydroxycholecalciferol, and calcitriol. CLINICAL FOLLOW-UP AND
Ergocalciferol and calcitriol are the preparations most POTENTIAL COMPLICATIONS
commonly used in veterinary medicine. Lifelong treat-
ment with some form of vitamin D metabolite is neces- Periods of hypocalcemia and hypercalcemia occur sporad-
sary for patients with primary hypoparathyroidism or ically in patients during initial efforts to manage serum
postoperative hypocalcemia that fails to resolve calcium concentration. Daily measurement of serum
spontaneously. tCa concentration during stabilization is necessary.
Ergocalciferol is favored by some because of its low Weekly serum calcium measurements should suffice dur-
cost, 465 but it has several features that make it the least ing maintenance therapy until target serum calcium con-
attractive agent for the treatment of hypocalcemia. centration has been achieved and maintained.
Ergocalciferol and its immediate metabolite, 25- Measurement of serum tCa concentration is
hydroxyergocalciferol, have low VDR avidity; thus, high recommended every 3 months thereafter in animals with
doses are necessary. Ergocalciferol is highly lipid soluble, permanent hypoparathyroidism. Serum calcium concen-
and several weeks are required to saturate body stores and tration should be adjusted to just below the reference
achieve a maximal effect. It also has a long half-life. Con- range. This not only lessens the likelihood that hypercal-
sequently, prolonged periods of hypercalcemia occur after cemia will develop but also reduces the magnitude of
overdose with ergocalciferol. In addition, there is extreme hypercalciuria that occurs in patients with PTH defi-
individual variation in the dose of ergocalciferol required ciency. Maintaining a mildly decreased serum calcium
to achieve a target serum calcium concentration. Use of concentration also ensures a continued stimulus for
loading doses reduces the time required to achieve a max- hypertrophy of the remaining parathyroid tissue in
imal effect (see Table 6-5). patients with postoperative hypoparathyroidism.
Calcitriol is the vitamin D metabolite of choice to pro- A change in dosage of vitamin D metabolites should
vide calcemic actions because it has the most rapid onset only occur after maximal effect has occurred and should
of maximal action and the shortest biologic half-life. be altered gradually. The time lag for maximal effect varies
Calcitriol is approximately 1000 times as effective as par- with the different vitamin D metabolites (see Table 6-5).
ent vitamin D and 500 times as effective as its precursor, Dosage increases of 10% to 25% are recommended when
calcidiol (25-hydroxyvitamin D), in binding to the VDR. serum calcium concentration is still below the target
The dose of calcitriol can be adjusted frequently because level. 446,447 Vitamin D metabolite and calcium salt sup-
of its short half-life and rapid effects on serum calcium plementation should be discontinued temporarily in
concentration. If hypercalcemia occurs, it abates quickly patients that develop hypercalcemia.
after dose reduction. The half-life of calcitriol in blood Hypercalcemia is a serious adverse effect of treatment
is 4 to 6 hours, whereas its biologic half-life is 2 to 4 days. that can result in death or renal damage causing acute or
Loading protocols for use of calcitriol in animals have not CRF. 111,115,314 Early signs of hypercalcemia should be
been reported, but it is logical to use a loading protocol explained to owners, who should be instructed to seek vet-
when more rapid correction of serum calcium concentra- erinary attention immediately if clinical signs suggest
tion is desirable. A calcitriol dosage of 30 to 60 ng/kg/ hypercalcemia. Clinical signs of hypercalcemia that
day has been recommended. 87,179 This dosage may be clients are likely to recognize include polydipsia, polyuria,
satisfactory as a loading dose, but in our experience it is anorexia, vomiting, and lethargy. Animals with
too high for chronic maintenance therapy. Calcitriol severe hypercalcemia require hospitalization. Fluids,
dosages for chronic maintenance therapy in humans furosemide, corticosteroids, bisphosphonates, calcitonin,
range from 10 to 40 ng/kg/day, and doses are divided or some combination may be required. All patients with
and given twice daily. 232,465,616 We have used loading symptomatic,vitaminDmetabolite-inducedhypercalcemia