A 71-year-old woman comes to the office for follow-up. She has a history of systolic heart failure and was hospitalized 2 weeks ago for an exacerbation, which was treated with intravenous diuretics. Since discharge the patient has been taking oral diuretics and says her shortness of breath has improved, but she is now experiencing muscle weakness and cramping. Blood pressure is 128/72 mm Hg and pulse is 80/min. Physical examination shows mildly decreased muscle strength in the lower extremities. Laboratory evaluation reveals a serum potassium level of 2.2 mEq/L. The patient receives potassium supplements, which improve the weakness. Another diuretic is added to her medical regimen to help prevent this adverse effect in the future. This additional medication predominantly acts on which of the following nephron segments?

Question

A 71-year-old woman comes to the office for follow-up.  She has a history of systolic heart failure and was hospitalized 2 weeks ago for an exacerbation, which was treated with intravenous diuretics.  Since discharge the patient has been taking oral diuretics and says her shortness of breath has improved, but she is now experiencing muscle weakness and cramping.  Blood pressure is 128/72 mm Hg and pulse is 80/min.  Physical examination shows mildly decreased muscle strength in the lower extremities.  Laboratory evaluation reveals a serum potassium level of 2.2 mEq/L.  The patient receives potassium supplements, which improve the weakness.  Another diuretic is added to her medical regimen to help prevent this adverse effect in the future.  This additional medication predominantly acts on which of the following nephron segments?

Quizplus · Accepted Answer

11ec160b_3b74_6dc2_9249_0fdb4a3e3259_MD0001_00 This patient has symptomatic hypokalemia, likely due to treatment with loop diuretics (eg, furosemide) used to reduce the symptoms of heart failure (eg, dyspnea, edema). The physician most likely added a potassium-sparing diuretic to this patient's treatment regimen to minimize potassium loss caused by loop diuretic use. All diuretics except for the potassium-sparing class cause potassium loss by increasing sodium delivery to the late distal tubule and cortical collecting duct, where aldosterone-induced sodium reabsorption occurs at the expense of potassium. Potassium-sparing diuretics act primarily by antagonizing this effect of aldosterone. Some potassium-sparing diuretics, such as amiloride and triamterene, function by directly blocking the epithelial sodium channels (ENaCs) in the apical membrane of principal cells. Others, including spironolactone and eplerenone, competitively inhibit the aldosterone receptor, causing downregulation of both the apical ENaCs and basolateral Na⁺/K⁺ pumps. Spironolactone and eplerenone (a more specific receptor antagonist than spironolactone with fewer side effects) are preferred in patients with heart failure because they can also prevent the deleterious effects of aldosterone-induced cardiac remodeling. (Choice A) Carbonic anhydrase inhibitors (eg, acetazolamide, dorzolamide) block sodium and bicarbonate reabsorption in the proximal convoluted tubule. These weak diuretics are typically used to treat altitude sickness or glaucoma. (Choices B and C) Osmotic diuretics (eg, mannitol) function by preventing sodium and water reabsorption, mainly in the proximal tubule and the descending limb of the loop of Henle. These are typically used for cerebral edema with increased intracranial pressure. (Choice D) Loop diuretics (eg, furosemide, bumetanide, torsemide) inhibit the Na-K-2Cl symporter in the thick ascending limb of the loop of Henle. This patient is likely already receiving this medication for symptomatic treatment of her heart failure; further dosage increase would worsen hypokalemia. (Choice E) Thiazide diuretics (eg, hydrochlorothiazide, chlorthalidone) act in the early distal convoluted tubule. These drugs are commonly used in the treatment of hypertension and are sometimes added to loop diuretics in patients with refractory edema, but they would likely worsen hypokalemia. Educational objective: All diuretics except for the potassium-sparing class cause potassium loss by increasing sodium delivery to the late distal tubule and cortical collecting duct, where aldosterone-induced sodium reabsorption occurs at the expense of potassium. Potassium-sparing diuretics (eg, spironolactone, amiloride) act on the late distal tubule and cortical collecting duct to antagonize the effects of aldosterone.

A 71-Year-Old Woman Comes to the Office for Follow-Up