Page 1384 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 99: Electrolyte Disorders in Critical Care 957

(eg, limb or gut ischemia). In the latter situation, the use of intermittent Dietary intake:
hemodialysis will provide temporary correction followed by recurrent Ca 800 mg
hyperkalemia. CRRT offers a unique advantage in this situation as it PO 4 1200 mg
prevents rebound hyperkalemia. In cases of severe hyperkalemia from Mg 300 mg
a continuous potassium leak, one should consider mixed modalities:
initiating intermittent hemodialysis to rapidly correct the hyperkalemia,
followed by CRRT to prevent rebound hyperkalemia. 50% of total body mg
99% of total body Ca
There are multiple cases in the literature of patients with remark-
able neurologic recovery despite prolonged resuscitative efforts. 142,151,153 80% of total body PO 4
Patients with hyperkalemic cardiac arrest may have better outcomes
than generally associated with cardiac arrest and deserve aggressive and
prolonged resuscitative efforts.

CALCIUM
Net GI absorption:
Calcium is a divalent cation that regulates cellular movement, hormone Ca 200 mg
release, enzyme activity, and coagulation. Calcium also plays a role in PO 4 1000 mg
cell injury and death. 154,155 Ninety-nine percent of total body calcium is Mg 120 mg
located in the bones and teeth. Normally, cytosolic calcium is very low,
with a ratio of extracellular to intracellular ionized calcium of 10,000 : 1. 156
■ METABOLISM Unresorbed and GI secretion: Renal excretion:

Measuring Calcium: Normal serum calcium is 8.8 to 10.3 mg/dL. The Ca 600 mg Ca 200 mg
molecular weight of calcium is 40; in SI units the normal range is 2.2 PO 4 200 mg PO 4 1000 mg
Mg 120 mg
to 2.6 mmol/L (4.4 to 5.2 mEq/L). Forty percent of serum calcium is Mg 200 mg
protein bound, primarily to albumin. An additional 10% to 15% is com- FIGURE 99-11. Calcium, phosphorus, and magnesium have unique patterns of dietary
plexed to serum anions, such as bicarbonate, phosphate, and citrate. The intake, absorption, and degrees of mineralization and renal excretion. Values are typical for
remaining 45% is the physiologically active, ionized fraction. Normal adult males on an American diet.
157
ionized calcium is 4.0 to 5.2 mg/dL (1.0 to 1.3 mmol/L). Decreases in
albumin lower total serum calcium without affecting ionized calcium. increases calcium resorption. Typically two-thirds of filtered calcium
Likewise, increases in albumin or globulins cause meaningless increases is resorbed by the proximal tubules. Calcium resorption in the TALH
in total calcium, while the calcium regulatory mechanism maintains a is primarily passive, down an electrical gradient created by the Na-K-
normal ionized calcium. 157-159 Increases in pH enhance calcium binding 2Cl carrier and the renal outer medulla potassium (ROMK) channel
to albumin, lowering ionized calcium; decreases in pH have the opposite (Fig. 99-14). The distal convoluted tubule (DCT) is the only area where
effect. Free fatty acids, either from lipid infusions or endogenous lipol- calcium can be resorbed independent of sodium. PTH increases perme-
157
ysis, increase calcium binding by albumin, lowering ionized calcium. ability of the paracellular tight junctions to calcium. PTH and calcitriol
160
Despite widespread use of formulas to adjust total calcium for albumin both stimulate calcium resorption in the DCT.
and pH, these have been shown to be poor predictors of ionized calcium,
especially in critically ill patients. 161,162 In patients in whom total calcium ■ HYPOCALCEMIA
is borderline or there is suspicion of disordered protein-calcium bind- Hypocalcemia is common among ICU patients, with prevalence reported
ing, an ionized calcium level should be measured. 163-165
to be 20% to 88%. 165-168 Hypocalcemia is more frequent with increased
Calcium Regulation: Regulation of calcium balance begins with control severity of illness and is associated with increased mortality. 167,168
of dietary absorption. Net calcium absorption is 100 to 200 mg per day Etiologies: Broadly speaking, hypocalcemia occurs when calcium moves
(Fig. 99-11). Normally, people are in calcium balance and absorbed out of the vascular space faster than it can be repleted by intestinal
calcium is excreted in the urine. Parathyroid hormone (PTH), calcitriol,
calcitonin, estrogen, and testosterone regulate calcium balance. The
effects of estrogen and testosterone are complex, poorly understood, and Serum
will not be discussed here. Ca
PTH is a peptide hormone released from the parathyroid glands in Calcitriol
response to ionized hypocalcemia (Fig. 99-12). Elevated calcium, mag- – PO 4
nesium, and calcitriol all suppress PTH release. PTH minimizes urinary
calcium excretion, stimulates the conversion of 25-hydroxyvitamin D to Ca
1,25-dihydroxyvitamin D (calcitriol) by the kidney, and in conjunction iCa PTH + Calcitriol PO 4
with calcitriol, mobilizes calcium from bone.
Vitamin D is ingested or synthesized in the skin. In order for vita- Ca resorption
min D to become metabolically active it must be hydroxylated, first in Ca
the liver, and then in the kidney, to form calcitriol. Calcitriol increases PO 4 resorption PO 4
dietary absorption of both calcium and phosphorus and is active in bone
metabolism. Calcitriol inhibits PTH release.
Calcitonin is a 32-amino acid peptide that decreases serum calcium.
FIGURE 99-12. Decreased ionized calcium stimulates parathyroid hormone release from
Renal Handling of Calcium: Both ionized and complexed calcium, repre- the parathyroid gland. PTH acts on three targets: it stimulates 1-α-hydroxylase to increase
senting 55% to 60% of total calcium, are freely filtered at the glomerulus. calcitriol synthesis, which increases GI calcium and PO (phosphate) absorption (calcitriol also
4
Nearly all of this filtered calcium (98%) is resorbed by the tubules inhibits PTH secretion); in concert with calcitriol PTH stimulates bone resorption, releasing
(Fig. 99-13). In the proximal tubule, calcium is resorbed with sodium. calcium and PO ; PTH increases tubular resorption of calcium and decreases tubular resorption
4
Increased proximal sodium resorption, as occurs with volume depletion, of phosphorus, increasing serum calcium and lowering serum phosphorus.

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