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le song tra
03-05-11, 04:47 PM
Definition

Hypokalemia is defined as a plasma [K+] < 3.5 mEq/L.

Etiology

decreased net intake

is seldom the sole cause of K+ depletion because urinary excretion can be effectively decreased to <15 mEq/d.
shift into cells

Movement of K+ into cells may transiently decrease the plasma [K+] without altering total body K+ content. These shifts can result from alkalemia, insulin, and catecholamine release, periodic paralysis
increased net loss: nonrenal Kali loss and renal kali loss

Nonrenal K+ loss. Hypokalemia may result from the loss of potassium-rich fluids from the lower GI tract, from the loss of upper GI contents is typically more attributable to renal K secretion from secondary hyperaldosteronism.excessive sweating

Renal K+ loss accounts for most cases of chronic hypokalemia
Augmented distal urine flow rate occurs commonly with diuretic use and osmotic diuresis, Bartter's and Gitelman's syndromes
Promote K+ loss by increasing the lumen-negative gradient ,
Distal Na+ reabsorption
Primary mineralocorticoid excess
Secondary hyperaldosteronism
Liddle's syndrome.

DIAGNOSIS
Clinical Presentation
The clinical features of K+ depletion vary greatly, and their severity depends in part on the degree of hypokalemia. Symptoms seldom occur unless the plasma [K+] is <3.0 mEq/L.
Fatigue, myalgias, and muscular weakness or cramps of the lower extremities are common. Smooth muscle function may also be affected and may manifest with complaints of constipation or frank paralytic ileus. Severe hypokalemia may lead to complete paralysis, hypoventilation, or rhabdomyolysis.
Diagnostic Testing
renal K+ excretion and the acid-base status can help identify the cause

Urine K+. The appropriate response to hypokalemia is to excrete <25 mEq/d of K+ in the urine. A transtubular potassium gradient (TTKG) can be calculated as follows
TTKG = (Urine K/Serum K) ÷ (Urine Osmolality/Serum Osmolality)
A TTKG < 2 suggests a nonrenal source, while a TTKG > 4 suggests inappropriate renal K+ secretion
Acid-Base status. Intracellular shifting and renal excretion of K+ are often closely linked with the acid-base statusThe finding of metabolic acidosis in a patient with hypokalemia thus narrows the differential significantly, implying lower GI loss, distal RTA, or the excretion of a nonreabsorbable anion from an organic acid (DKA, hippurate from toluene intoxication).

Electrocardiogram (ECG) changes associated with hypokalemia include flattening or inversion of the T wave, a prominent U wave, ST-segment depression, and a prolonged QT interval. Severe K+ depletion may result in a prolonged PR interval, decreased voltage, and widening of the QRS complex.


key words

Hypokalemia http://www.merckmanuals.com/professional/sec12/ch156/ch156f.html#sec12-ch156-ch156f-779
Hypokalemic periodic paralysis
http://www.nlm.nih.gov/medlineplus/ency/article/000312.htm

Excessive sweating:
http://www.merckmanuals.com/home/sec18/ch206/ch206c.html

Bartter's and Gitelman's syndromes http://www.uptodate.com/contents/bartters-and-gitelmans-syndromes
Distinguishing between Bartter’s syndrome and Gitelman’s syndrome
http://resources.metapress.com/pdf-preview.axd?code=yh63qt1663603857&size=largest

Liddle’s syndrome http://www.merckmanuals.com/professional/sec17/ch237/ch237c.html