Page 1380 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 99: Electrolyte Disorders in Critical Care 953

In the presence of ADH water flows
from the tubule so that the osmolality of
Tubular fluid leaving the TALH
is hypotonic to plasma the DCT approaches that of plasma.

CCD secretes potassium under
the influence of aldosterone.
K +
K + + The ratio of tubular K to peritubular
Cortex K + K + (approx. equal to venous) K is a
K
measure of aldosterone activity.
Outer
medulla

ADH-induced resorption of water in
Inner the medullary collecting duct increases
medulla the tubular fluid osmolality and potassium
by the same factor.

FIGURE 99-10. The transtubular potassium gradient measures the ratio of tubular potassium to interstitial potassium and quantifies the renal excretion of potassium. ADH, antidiuretic
hormone; CCD, cortical collecting duct; DCT, distal convoluted tubule; TALH, the thick ascending limb of the loop of Henle.

than total body potassium (eg, a 25% drop in serum potassium is due gastrointestinal tract ulcers and stenotic lesions. The microencapsulated
to less than a 25% drop in total body potassium). Sterns and colleagues extended-release formulations have the best compliance and low rates
analyzed the results of seven balance studies and found a linear relation- of GI side effects. 90-92
ship for potassium deficit and serum potassium (r = 0.893). The loss of Parenteral potassium should be used to correct symptomatic hypo-
100 mmol of potassium lowered the serum potassium by 0.27 mmol/L, kalemia or when patients are unable to take oral medications. Twenty
so a fall from 4 to 3 mmol/L represented a 370-mmol potassium deficit. to forty millimoles of KCl in 0.5-1 L of isotonic saline or 5% dextrose
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In Scribner and Burnell’s review, they estimated that a drop in potassium is a typical solution. Saline solutions are preferred as dextrose solutions
from 4 to 3 mmol/L represented a loss of 100 to 200 mmol of potassium, stimulate insulin release that can result in acute worsening of the hypo-
and a drop in serum potassium from 3 to 2 mmol/L represented an kalemia. 77,93 The use of saline with dilute concentrations of potassium
additional 200 to 400 mmol deficit. These estimates do not account for means that patients must get multiple liters of saline to correct even
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altered cellular distribution of potassium. In diabetic ketoacidosis, for modest potassium deficits, which may be contraindicated in volume-
example, serum potassium overestimates total body potassium, while overloaded patients.
β-agonist–induced hypokalemia underestimates total body potassium. Concentrated potassium solutions delivered at a rate of 10-40 mEq/h
In most cases of hypokalemia due to cellular redistribution, experts in smaller volumes are frequently used in the ICU setting. The use of
advise against treatment, as the hypokalemia is transient and treatment these solutions had generated fears about the possibility of precipitating
predisposes the patient to hyperkalemia. One exception to this is symp- arrhythmias from local, transient hyperkalemia near the infusion site or
tomatic periodic paralysis in which respiratory arrest due to hypokale- by causing peripheral vein irritation from caustic potassium solutions.
mia may occur, so emergent treatment is indicated. Caution should still Despite these concerns, the use of concentrated potassium infusions,
be used, as rebound hyperkalemia is common. 200 mmol/L, at a rate of 20 mEq/h in the ICU was shown to be safe and
The form of potassium used in repletion is most often potassium efficacious in both a retrospective study of 495 infusions and a prospec-
chloride. The chloride anion has some advantages over alternatives such tive study of 40 patients. 94,95 Twenty milliequivalents of KCl increased
as phosphate, bicarbonate, or citrate. The chloride anion is primarily the serum potassium by 0.25 mmol/L 1 hour after the infusion finished.
an extracellular anion, which minimizes the movement of potassium The peak rise in serum potassium, 0.48 mmol/L, was at the end of the
into the cell, maximizing the change in serum potassium. Chloride also infusion. ECG monitoring showed no change except for decreased ven-
does not increase the secretion of potassium at the collecting duct. The tricular ectopy. Potassium was infused safely through both peripheral
use of alternate potassium salts should be reserved for specific clinical and central sites. Infusion rates of greater than 20 mEq/h are best admin-
scenarios in which there is an indication for the anion (eg, citrate in istered through a central vein.
metabolic acidosis and phosphate in hypophosphatemia). Hypomagnesemia is a common cause of treatment failure. Patients
In patients who are asymptomatic, oral replacement is sufficient and doses who are resistant to potassium supplementation should have serum
from 40 to 100 mEq of KCl per day are typically sufficient to correct the magnesium measured, and if low, repleted. Patients with diuretic-
hypokalemia over several days. Increasing intake of potassium-rich foods induced hypokalemia often benefit from the initiation of a potassium-
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is less effective than potassium chloride supplements because the anions sparing diuretic. Amiloride has been shown to mitigate magnesium
associated with dietary potassium are primarily phosphate and citrate. losses associated with loop and thiazide diuretics.
Potassium chloride can be given as a liquid, in crystalline form (often Patients on amphotericin B often become hypokalemic. Both spirono-
marketed as a salt substitute), or pills with multiple formulations and lactone (100 mg twice a day) and amiloride (5 mg twice a day) have been
coatings. The bioavailability of all these formulations is identical, with shown to increase serum potassium and decrease the use of potassium
greater than 70% absorption. The liquid formulation has the fastest supplements in randomized prospective trials. 96,97
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absorption and lowest patient compliance of all formulations (due to In patients with recalcitrant vomiting (ie, bulimia) and associated
the bad taste). Wax-matrix extended-release tablets are associated with hypokalemia, one treatment strategy is to decrease the loss of hydrogen
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