A 6-month-old infant with congenital heart disease (ventricular septal defect) has been receiving furosemide (1 mg/kg/dose BID) for 3 weeks to manage heart failure symptoms. He is brought to clinic for routine follow-up. The mother reports he has been irritable, feeding poorly, and seems βweakβ over the last few days. On examination: mild dehydration (sunken eyes, dry mucous membranes), weight down 5% from baseline, blood pressure 85/50 mm Hg (normal for age). No edema. No tachypnea.
π― Task (examiner observed): Interpret the ABG, identify the acid-base disorder, determine the etiology (chloride-responsive vs chloride-resistant metabolic alkalosis), explain the pathophysiology of furosemide-induced metabolic alkalosis, and outline management including repletion of chloride and potassium.
β Q1 (Examiner): βInterpret this ABG: pH 7.50, HCO3 32, PCO2 48. What is the primary acid-base disorder?β
β Primary metabolic alkalosis with appropriate respiratory compensation.
Elevated HCO3 (32) and alkalemia indicate metabolic alkalosis. Elevated PCO2 (48) is the compensatory hypoventilation. Using the formula: expected PCO2 = 40 + 0.7 Γ (ΞHCO3) = 40 + 0.7Γ(8) = 45.6 mm Hg. Actual 48 is within range β simple metabolic alkalosis, not mixed.
β Q2 (Examiner): βWhat is the significance of the low urine chloride (10 mEq/L)? How does it help classify this metabolic alkalosis?β
β Urine chloride <15 mEq/L indicates chloride-responsive metabolic alkalosis. This means the patient has volume depletion and chloride deficiency, and the alkalosis will correct with administration of sodium chloride and potassium chloride. Causes: vomiting, NG suction, diuretics (loop or thiazide), post-hypercapnia, chloride-losing diarrhea, cystic fibrosis.
β Q3 (Examiner): βWhy does furosemide cause metabolic alkalosis? Describe the pathophysiology.β
β Furosemide causes metabolic alkalosis via multiple mechanisms:
1. Contraction alkalosis: Loop diuretics cause loss of chloride-rich, bicarbonate-poor fluid β reduction in ECF volume concentrates remaining bicarbonate.
2. Chloride depletion: Loss of chloride in urine impairs the kidney's ability to excrete bicarbonate.
3. Hypokalemia: K+ depletion increases H+ secretion in distal nephron β enhances alkalosis.
4. Secondary hyperaldosteronism: Volume depletion stimulates aldosterone β increases H+ and K+ excretion, worsening alkalosis and hypokalemia.
β Q4 (Examiner): βWhy is urine chloride preferred over urine sodium to assess volume status in metabolic alkalosis?β
β In metabolic alkalosis, the kidney excretes bicarbonate (negative charge) with sodium or potassium as cations. Thus, urine sodium may be high even in volume-depleted states because sodium is excreted with bicarbonate. Urine chloride remains low because chloride is reabsorbed avidly to maintain electroneutrality. Therefore, urine chloride is the most reliable indicator of volume status in metabolic alkalosis.
β Q5 (Examiner): βWhat electrolyte abnormalities are associated with this condition, and why are they dangerous?β
β Hypokalemia (K+ 2.9 mEq/L) β dangerous because it can cause cardiac arrhythmias (U waves, prolonged QT, torsades de pointes), muscle weakness, ileus, and nephrogenic diabetes insipidus. Hypochloremia β perpetuates alkalosis. Patients are also volume depleted. Alkalemia shifts potassium intracellularly, worsens ionized hypocalcemia (risk of tetany), and decreases cardiac output.
β Q6 (Examiner): βWhat is the treatment for this childβs metabolic alkalosis?β
β Chloride repletion is key: Administer NaCl and KCl intravenously (e.g., 0.9% normal saline + 20-40 mEq/L KCl). Volume repletion corrects contraction alkalosis, chloride allows renal excretion of bicarbonate, and potassium replacement corrects hypokalemia (which also helps alkalosis). Discontinue or reduce furosemide if possible. After repletion, the kidney will excrete the excess bicarbonate within 1-2 days.
β Q7 (Examiner): βWhy canβt you just give sodium bicarbonate or citrate to correct this alkalosis?β
β That would be contraindicated β giving bicarbonate would worsen the metabolic alkalosis. The patient already has excess bicarbonate. The treatment is to provide chloride (as NaCl or KCl) to allow renal elimination of bicarbonate, not to add more base.
β Q8 (Examiner): βWhat is the role of acetazolamide in metabolic alkalosis? When would you use it?β
β Acetazolamide (carbonic anhydrase inhibitor) causes renal bicarbonate loss, which can correct metabolic alkalosis. Used in refractory cases or when patients cannot tolerate volume repletion (e.g., heart failure, renal failure with volume overload). However, it worsens hypokalemia and must be used with potassium supplementation. Not first-line β reserved for severe, resistant alkalosis.
β Q9 (Examiner): βHow does hypokalemia contribute to the maintenance of metabolic alkalosis?β
β Hypokalemia stimulates renal ammonia production and increases H+ secretion in the collecting duct, worsening alkalosis. It also increases proximal tubule bicarbonate reabsorption. Therefore, metabolic alkalosis cannot be corrected until potassium is repleted. This is why KCl is essential, not just NaCl.
β Q10 (Examiner): βWhat is the expected urinary chloride level while the patient is actively receiving furosemide vs after stopping?β
β During active furosemide therapy: urine chloride is high (>20 mEq/L) because the drug blocks NaCl reabsorption in the loop of Henle. After furosemide effect wears off (hours later): urine chloride becomes low (<15 mEq/L) due to compensatory chloride retention from volume depletion. This patient has low urine chloride because they are not receiving furosemide at the time of urine collection (or are in the post-diuretic phase).
β Q11 (Examiner): βWhat is the difference between loop diuretics and thiazide diuretics in terms of metabolic alkalosis risk?β
β Both cause metabolic alkalosis, but thiazides carry a higher risk of severe hyponatremia (in addition to hypokalemia and alkalosis). Loop diuretics impair urinary concentrating ability, providing some protection against severe hyponatremia. Both cause chloride-responsive metabolic alkalosis. Thiazides are more associated with hypokalemia and hypomagnesemia.
β Q12 (Examiner): βWhen do you consider that a patient on furosemide has a mixed acid-base disorder rather than simple metabolic alkalosis?β
β Mixed disorder exists if PCO2 is lower than expected (concurrent respiratory alkalosis) or higher than expected (concurrent respiratory acidosis). For metabolic alkalosis with HCO3 32, expected PCO2 ~45-47. If PCO2 is 35, that suggests additional primary respiratory alkalosis. If PCO2 is 55, that suggests additional respiratory acidosis (e.g., in a child with lung disease). Always assess compensation.
π’ Examiner probe: βWhat if the urine chloride was >20 mEq/L with high blood pressure?β β That suggests mineralocorticoid excess (hyperaldosteronism, Liddle syndrome). Very different management (spironolactone or amiloride, not saline).
π§ Management of Chloride-Responsive Metabolic Alkalosis
π TREATMENT PROTOCOL (Nelson Chapter 73.7):
1. DISCONTINUE OR REDUCE THE OFFENDING AGENT:
- Hold or decrease furosemide dose if clinically feasible
- Consider alternative diuretics (e.g., potassium-sparing diuretics) or ACE inhibitors if heart failure requires ongoing therapy
2. VOLUME REPLETION WITH CHLORIDE:
- IV fluids: 0.9% normal saline (NaCl) with KCl 20-40 mEq/L
- Rate: replace estimated deficit over 24-48 hours plus maintenance
- For this 6-month-old (~7 kg, 5% dehydrated = 350 mL deficit): give bolus if hypovolemic β 20 mL/kg isotonic saline β then maintenance + deficit over 24h with D5 0.9% NS + 30 mEq/L KCl
3. POTASSIUM REPLETION (critical):
- K+ 2.9 mEq/L requires aggressive repletion: IV KCl 0.5-1 mEq/kg/dose, max 40 mEq/L infusion rate
- Monitor K+ every 4-6 hours
- Do not give NaCl alone without KCl β alkalosis will not correct until hypokalemia resolved
4. MONITOR RESPONSE:
- Improvement: urine chloride will rise as volume repleted, then bicarbonate will fall
- Serum HCO3 normalizes over 1-2 days after adequate chloride and potassium
- Watch for overshoot (hypochloremia, acidemia) β rare
5. REFRACTORY CASES (rare in children):
- Acetazolamide 5-10 mg/kg/day (but worsens hypokalemia and causes metabolic acidosis)
- Hydrochlorothiazide + amiloride combination for long-term diuresis without alkalosis
π¨ DO NOT:
- Give sodium bicarbonate β will worsen alkalosis
- Use potassium-sparing diuretics in volume-depleted patient (risk of hyperkalemia, worsening hypovolemia)
- Treat with KCl alone without NaCl β chloride is also needed to correct the deficit
π EXPECTED RESPONSE:
- Within 6-12 hours of adequate NaCl and KCl repletion: urine chloride increases, urine pH decreases, serum bicarbonate begins to fall.
- Complete correction of metabolic alkalosis typically within 48 hours.
- Hypokalemia corrects over 24-72 hours (large intracellular deficit).
π Nelson Chapter 73.7 Β· Metabolic Alkalosis Core Concepts
βοΈ Metabolic Alkalosis Definition Primary increase in serum HCO3 (>28 mEq/L) with alkalemia (pH >7.45). Respiratory compensation: hypoventilation β βPCO2.
π Chloride-Resistant (Urine Clβ» >20) Hypertension: hyperaldosteronism, Cushing, Liddle, licorice. Normal BP: Bartter, Gitelman. Treatment: spironolactone, amiloride, or indomethacin.
π Hypokalemia in Alkalosis Causes: transcellular shift (K+ into cells), urinary losses (aldosterone-mediated). Treatment requires KCl, not just NaCl.
π Furosemide Mechanism Inhibits Na-K-2Cl cotransporter in TAL β loss of chloride-rich fluid β contraction alkalosis, hypokalemia, metabolic alkalosis.
π§ͺ Urine Chloride Key Point Superior to urine Na in metabolic alkalosis because Na+ can be excreted with HCO3- even when volume depleted. Low Cl- indicates chloride depletion.
π Nelson Chapter 73.7 β Key quote: βThe etiologies of a metabolic alkalosis are divided into two categories based on the urinary chloride level. The alkalosis in patients with a low urinary [Cl-] is maintained by volume depletion; thus volume repletion is necessary for correction of the alkalosis. Because Cl- losses are the dominant cause of the volume depletion, these patients require Cl- to correct the volume depletion and metabolic alkalosis; they are said to have Cl- - responsive metabolic alkalosis.β
β TOACS TAKE-HOME POINTS:
1. Metabolic alkalosis β check urine chloride β <15 = chloride-responsive β treat with NaCl + KCl.
2. Furosemide causes contraction alkalosis, hypokalemia, and secondary hyperaldosteronism.
3. Never treat with bicarbonate β it worsens alkalosis.
4. Potassium repletion is essential; alkalosis will not correct until hypokalemia resolves.
5. Monitor serum electrolytes and urine chloride to guide therapy.