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A 56-year-old woman with a 25-year history of alcoholism and liver disease visits her physician complaining of abdominal swelling and back pain. The physician notes that she has severe ascites and administers a loop diuretic. The woman loses 5 L of fluid in a relatively short time. The data shown below were collected from the patient before receiving the diuretic and after the diuretic had caused the loss of fluid.
ph 7.4 - 7.49
hco3 24 - 35mEq/L
Pco2 40 - 48 mm Hg
Which of the following is the primary cause of the acid-base abnormality indicated in the above table?
A. Decreased HCO3-excretion
B. Decreased plasma aldosterone
C. Hyperkalemia
D. Hyperventilation
E. Loss of extracellular fluid
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The correct answer is E. Diuretic therapy has resulted in the type of metabolic alkalosis called "contraction alkalosis." Loop diuretics increase salt and water excretion by inhibiting tubular reabsorption of Na+ and Cl- in the kidney. The salt and water that are lost from the body contain very little HCO3- so that virtually all of the HCO3- contained in the ECF is retained in the body. In effect, the HCO3- present in the ECF (which includes the edema fluid) becomes more and more concentrated as urine containing relatively little HCO3- is excreted from the body.
For example, if the ECF volume before diuretic therapy were 16 L, the total amount of HCO3- in the ECF would be 384 mEq (16 L x 24 mEq/L = 384 mEq). The excretion of 5 L of HCO3--free urine would cause the HCO3- contained in the ECF to be concentrated into 11 L, thereby increasing the HCO3- concentration in the ECF to 35 mEq/L (384 mEq/11 L = 35 mEq/L). Therefore, it is the loss of extracellular fluid from the body that increases the concentration of HCO3- in the plasma.
Because only small amounts of HCO3- are normally excreted in the urine, decreasing HCO3-
excretion (choice A) would have a small effect on the concentration of HCO3- in the plasma.
Diuretics lead to increased levels of aldosterone (choice B) and decreased levels of potassium (ie, hypokalemia, not hyperkalemia, choice C) in the plasma by a variety of mechanisms.
Hyperventilation (choice D) decreases arterial PCO2; note that diuretic therapy has increased arterial PCO2
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