What are the lab values indicative of contraction metabolic alkalosis?

Laboratory Values for Contraction Metabolic Alkalosis

Contraction metabolic alkalosis is characterized by elevated serum bicarbonate (>28 mEq/L), elevated arterial pH (>7.45), hypochloremia (<95 mEq/L), and hypokalemia (<3.5 mEq/L), typically accompanied by evidence of volume depletion.

Key Laboratory Findings

Primary Acid-Base Parameters

• Arterial pH: Elevated (>7.45)
• Serum bicarbonate (HCO3-): Elevated (>28 mEq/L)
• PaCO2: Compensatory increase (respiratory compensation)
• Base excess: Positive

Electrolyte Abnormalities

• Serum chloride: Decreased (<95 mEq/L)
• Serum potassium: Decreased (<3.5 mEq/L)
• Serum sodium: May be normal, decreased, or increased (depending on volume status)
• Urine chloride: Low (<10-15 mEq/L) in contraction alkalosis
• Serum calcium: May be decreased due to albumin binding in alkalotic state

Volume Status Indicators

• Blood urea nitrogen (BUN): Elevated
• Serum creatinine: May be elevated
• BUN/creatinine ratio: Increased (>20:1)
• Urine specific gravity: Increased
• Hematocrit: May be elevated due to hemoconcentration

Pathophysiologic Mechanisms

Contraction metabolic alkalosis develops through a two-step process:

1. Generation phase: Loss of hydrogen ions and chloride (typically through vomiting, nasogastric suction, or diuretic use)

2. Maintenance phase: Volume contraction leads to:

   • Increased proximal tubular bicarbonate reabsorption
   • Enhanced distal sodium reabsorption in exchange for potassium and hydrogen ions
   • Aldosterone stimulation due to volume depletion
   • Hypokalemia, which further promotes bicarbonate reabsorption

Diagnostic Approach

The diagnosis of contraction metabolic alkalosis can be confirmed by:

1. Arterial blood gas: Shows elevated pH and bicarbonate with compensatory increase in PaCO2

2. Serum electrolytes: Reveal hypochloremia and hypokalemia

3. Urine chloride:

   • Low (<10-15 mEq/L) in contraction alkalosis
   • High (>20 mEq/L) in other causes of metabolic alkalosis (e.g., primary hyperaldosteronism)

4. Calculation of anion gap: Usually normal in pure contraction metabolic alkalosis

Clinical Correlation

Contraction metabolic alkalosis is commonly seen in:

• Excessive vomiting or nasogastric suction
• Diuretic therapy (especially loop and thiazide diuretics)
• Post-hypercapnic states
• Severe dehydration

The severity of laboratory abnormalities typically correlates with the degree of volume depletion, with more pronounced alkalosis seen in more severely volume-depleted patients.

Treatment Considerations

Treatment should focus on:

1. Volume repletion with chloride-containing solutions (0.9% NaCl)
2. Potassium repletion
3. Addressing the underlying cause

Laboratory values typically normalize rapidly with appropriate fluid and electrolyte replacement, confirming the diagnosis of contraction metabolic alkalosis.