Page 216 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice
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206 ELECTROLYTE DISORDERS
human patients with chronic renal failure treated with cal- The Henderson-Hasselbalch equation is derived from
cium-containing compounds. 37 It is very expensive, the formula for the dissociation constant of an acid. For
causes some adverse gastrointestinal effects, and has the the ionic species of phosphate of interest:
potential to bind other substances (e.g., bile acids, choles-
terol, vitamins) in addition to phosphorus. Initial reports pH ¼ pKa þ logð½PO 4 3 =½HPO 4 2 Þ
suggested that sevelamer was similar in effectiveness to
calcium acetate in binding phosphorus but with less risk 7:4 ¼ 12:4 þ logðxÞ
of hypercalcemia. 16 However, a recent study found cal- logðxÞ¼ 5:0
cium acetate superior to sevelamer in control of x ¼ 0:00001
hyperphosphatemia and calcium-phosphorus product. 133 ½PO 4 3 =½HPO 4 2 ¼ 0:00001
Sevelamer decreased serum bicarbonate concentrations in 2 3
½HPO 4 =½PO 4 ¼ 100, 000
this study, presumably as a result of the release and
absorption of the hydrochloride moiety. In another Thus, at a pH of 7.4, there are 100,000 molecules of
study, 58 once-daily administration was as effective as 2 3
HPO 4 for every molecule of PO 4 .
administration three times per day in some patients.
Lanthanum carbonate also contains no aluminum and 1
pH ¼ pKa þ logð½H 2 PO 4 =½H 3 PO 4 Þ
no calcium, is not absorbed from the gastrointestinal
tract, and acts as an efficient phosphorus binder. 50 Its 7:4 ¼ 2:0 þ logðxÞ
effects are similar to those of calcium carbonate but with- logðxÞ¼ 5:4
out risk of bone toxicity or hypercalcemia. 45 It binds to x ¼ 251, 189
phosphate at low and high pH, making it effective in both ½H 2 PO 4 1 =½H 3 PO 4 ¼ 251, 189
the stomach and small intestine. 63 Lanthanum is excreted
primarily in bile and should not accumulate in patients Thus, at a pH of 7.4, there are 251,189 molecules of
with renal failure, but its long-term safety is unknown. H 2 PO 4 1 for every molecule of H 3 PO 4 .
Phosphate binder effectiveness is monitored by mea-
suring fasting serum phosphorus concentration. The goal 2 1
pH ¼ pKa þ log ð½HPO 4 =½H 2 PO 4 Þ
is to maintain the serum phosphorus concentration in the
7:4 ¼ 6:8 þ logðxÞ
normal range. In normophosphatemic patients with early
logðxÞ¼ 0:6
renal insufficiency, one may monitor fasting FE Pi to deter-
x ¼ 4:0
mine the efficacy of phosphate restriction. Dogs with
2 =½H 2 PO 4 1 ¼ 4:0
½HPO 4
spontaneous chronic renal failure (mean serum creatinine
concentration, 2.3 mg/dL) had significantly higher FE Pi
From these calculations, it can be determined that, at a
values than control dogs (23% vs. 5%), respectively, and
2 ,
FE Pi decreasedinbothgroupsafter feedingofPrescription pH of 7.4, there will be 1,004,756 molecules of HPO 4
1 , and 10 molecules of
Diet K/D. 70 In one dog with chronic renal failure, FE Pi 251,189 molecules of H 2 PO 4
3 for every molecule of H 3 PO 4 . Therefore, it can
was below the mean value for the chronic renal failure PO 4
3
be seen that the amounts of H 3 PO 4 and PO 4 present
group despite increased serum PTH concentration.
in ECF at a pH of 7.4 can be safely ignored.
It has been suggested that FE Pi values less than 30% are
indicative of adequate phosphate restriction. 53 This
method is limited by the wide range of normal values for REFERENCES
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3-
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