Renal · Acid-Base Disorders
The facts most likely to be tested
Metabolic alkalosis is defined by an elevated serum bicarbonate (>24 mEq/L) and an elevated arterial pH (>7.45).
The most common cause of saline-responsive metabolic alkalosis is vomiting or nasogastric suctioning, leading to hypovolemia and chloride depletion.
Saline-responsive metabolic alkalosis is characterized by a low urinary chloride (<20 mEq/L) because the kidneys avidly reabsorb chloride to restore volume.
Saline-resistant metabolic alkalosis is associated with hypervolemia or normovolemia and presents with a high urinary chloride (>20 mEq/L).
Primary hyperaldosteronism (Conn syndrome) and Bartter/Gitelman syndromes are classic causes of saline-resistant metabolic alkalosis due to mineralocorticoid excess.
Loop diuretics and thiazide diuretics cause metabolic alkalosis by increasing distal delivery of sodium and stimulating hydrogen ion secretion in the collecting duct.
The compensatory response to metabolic alkalosis is hypoventilation, which increases the partial pressure of carbon dioxide (PaCO2) by approximately 0.7 mmHg for every 1 mEq/L rise in bicarbonate.
Vignette unlocked
A 45-year-old woman is brought to the emergency department due to generalized weakness and muscle cramps. She has a history of hypertension and has been taking an unknown 'water pill' for several months. Physical examination reveals a blood pressure of 155/95 mmHg and no peripheral edema. Laboratory studies show a serum sodium of 142 mEq/L, potassium of 2.8 mEq/L, bicarbonate of 34 mEq/L, and an arterial pH of 7.52. Her urinary chloride concentration is 45 mEq/L.
What is the most likely underlying mechanism for this patient's acid-base disturbance?
Mineralocorticoid excess
The patient presents with saline-resistant metabolic alkalosis (high urinary chloride) and hypertension, which points toward mineralocorticoid excess (e.g., primary hyperaldosteronism or diuretic abuse) rather than volume-depletion alkalosis.
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Etiology / Epidemiology
Driven by loss of H+ or gain of HCO3-. Common in vomiting, diuretic use, and hyperaldosteronism.
Clinical Manifestations
Often asymptomatic; look for hypoventilation (compensatory) and Chvostek sign due to hypocalcemia.
Diagnosis
Urine chloride is the gold standard to differentiate saline-responsive (<20 mEq/L) vs. saline-resistant (>20 mEq/L).
Treatment
Normal saline is first-line for volume depletion. Avoid in heart failure if fluid overloaded.
Prognosis
Correct underlying cause to prevent cardiac arrhythmias and seizures from severe pH shifts.
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Epidemiology & Etiology
Common in hospitalized patients due to nasogastric suctioning or loop/thiazide diuretics. Conn syndrome and Bartter syndrome are classic endocrine/genetic causes. Excessive bicarbonate intake (milk-alkali syndrome) remains a relevant clinical etiology.
Pertinent Anatomy
The kidney is the primary organ of compensation and maintenance. The proximal tubule reabsorbs the bulk of filtered bicarbonate, while the distal tubule regulates H+ secretion.
Pathophysiology
Maintenance requires both a generation phase and a renal failure to excrete excess bicarbonate. Volume depletion and hypokalemia stimulate the kidney to reabsorb HCO3- avidly. Secondary hyperaldosteronism exacerbates H+ loss in the distal nephron.
Clinical Manifestations
Patients present with hypoventilation as a compensatory response to raise pCO2. Neuromuscular irritability, including Trousseau sign and Chvostek sign, occurs due to decreased ionized calcium. Severe alkalemia can trigger ventricular arrhythmias and seizures.
Diagnosis
The urine chloride test is the gold standard for classification. Saline-responsive states show UCl < 20 mEq/L, while saline-resistant states show UCl > 20 mEq/L. Arterial blood gas confirms pH > 7.45 and HCO3 > 26 mEq/L.
Treatment
For saline-responsive cases, 0.9% Normal Saline is the first-line treatment to restore volume. In saline-resistant cases, treat the underlying endocrine disorder or use acetazolamide to promote bicarbonate excretion. Potassium repletion is mandatory as hypokalemia prevents correction.
Prognosis
Prognosis is excellent if the underlying cause is addressed. Monitor for hypokalemia and hypocalcemia during recovery. Failure to correct pH can lead to respiratory failure due to compensatory hypoventilation.
Differential Diagnosis
Vomiting: Urine chloride < 20 mEq/L
Diuretic abuse: Urine chloride > 20 mEq/L
Conn syndrome: Hypertension present
Bartter syndrome: Urine chloride > 20 mEq/L, normotensive
Milk-alkali syndrome: Hypercalcemia present