A 76-year-old woman with a history of essential hypertension has become increasingly lethargic with nausea and loss of appetite over the past week. On physical examination her vital signs show T 37 C, P 86/min, RR 23/min, and BP 120/70 mm Hg. There are no abnormal findings. Laboratory studies show sodium 131 mmol/L, potassium 5.3 mmol/L, chloride 93 mmol/L, CO2 12 mmol/L, glucose 63 mg/dL, creatinine 1.2 mg/dL, and urea nitrogen 18 mg/dL. An arterial blood gas shows pH 7.3, pCO2 30 mm Hg, pO2 84 mm Hg, and HCO3 13 meq/L.

Questions:

4.1 What is suggested by these findings?

She has a metabolic acidosis without an anion gap. There is a mild respiratory alkalosis to compensate.

She has hyperkalemia. Her transtubular K+ concentration gradient (TTKG) is 4. In response to administration of mineralocorticoid, there is no significant change in the TTKG. Her plasma renin is increased and her serum aldosterone is increased.

4.2 What may be causing these findings? Describe the pathophysiology.

She may be receiving a potassium-sparing diuretic such as triamterene. Triamterene would inhibit the effect of aldosterone by inhibiting the sodium-potassium exchange in the collecting ducts.