Contraction Alkalosis

Definition

Contraction alkalosis refers to:

Metabolic alkalosis that results from reduction in extracellular fluid volume, which concentrates serum bicarbonate and enhances renal bicarbonate reabsorption.

 Pathophysiology

1️⃣ Basic Mechanism

Contraction alkalosis occurs due to two simultaneous mechanisms:

A. Hemoconcentration of Bicarbonate

• Loss of Na⁺ + water from extracellular space
• Total body bicarbonate remains initially unchanged
• Because ECF volume shrinks → HCO₃⁻ concentration rises

👉 Example
If bicarbonate = 24 mmol/L in 14 L ECF
Loss of 4 L fluid → same bicarbonate now distributed in 10 L
→ Concentration rises

B. Enhanced Renal Bicarbonate Retention

This is the more important sustaining mechanism

Volume contraction activates:

1. Renin–angiotensin–aldosterone system (RAAS)
2. Increased proximal tubular sodium reabsorption
3. Increased hydrogen ion secretion
4. Increased bicarbonate reabsorption

2️⃣ Renal Physiology Behind Contraction Alkalosis

✔ Proximal Tubule

Volume depletion → Angiotensin II activation → stimulates:

• Na⁺/H⁺ exchanger (NHE3)
• Increased hydrogen secretion into tubular lumen
• Increased bicarbonate reabsorption into blood

✔ Distal Nephron

Aldosterone causes:

• Increased Na⁺ reabsorption
• Increased:
• H⁺ secretion
• K⁺ secretion

👉 Leads to:

• Alkalosis maintenance
• Hypokalemia (frequently associated)

✔ Chloride Depletion

Most contraction alkalosis is also:

👉 Chloride responsive alkalosis

Because:

• Low chloride reduces ability of kidney to excrete bicarbonate
• Bicarbonate secretion in collecting duct requires chloride exchange

 Common Clinical Causes

1️⃣ Diuretic Therapy (Most Common ICU Cause)

Especially:

• Furosemide
• Bumetanide
• Hydrochlorothiazide

Mechanism:

• NaCl + water loss
• Volume contraction
• RAAS activation
• Potassium loss → worsens alkalosis

2️⃣ Gastrointestinal Chloride Loss

✔ Vomiting

✔ Nasogastric suction

Loss of:

• HCl
• Sodium
• Chloride
• Water

Result:

• Volume contraction
• Chloride depletion
• Bicarbonate retention

3️⃣ Post-Hypercapnic State

After rapid correction of chronic respiratory acidosis:

• Kidneys have retained bicarbonate
• CO₂ suddenly falls
• Leads to metabolic alkalosis

4️⃣ Mineralocorticoid Excess States

• Primary hyperaldosteronism
• Cushing syndrome

Mechanism:

• Increased hydrogen and potassium secretion

 Laboratory Findings

ABG

• ↑ pH
• ↑ HCO₃⁻
• Compensatory ↑ PaCO₂

Electrolytes

Typical triad:

✔ Hypokalemia
✔ Hypochloremia
✔ Volume depletion

Urine Chloride 

👉 Contraction alkalosis usually = low urine chloride

 Why Volume Contraction Maintains Alkalosis (Conceptual Summary)

Volume depletion causes:

1. Angiotensin II → proximal bicarbonate reabsorption
2. Aldosterone → distal hydrogen secretion
3. Chloride depletion → impaired bicarbonate excretion
4. Hypokalemia → intracellular H⁺ shift

All promote persistent alkalosis.

 Management

Step 1 – Identify Type of Metabolic Alkalosis

👉 Check urine chloride

Step 2 – Treat Underlying Cause

✔ Chloride Responsive (Most Contraction Alkalosis)

Treatment:

1. Isotonic saline
2. Potassium replacement
3. Stop diuretics if possible

Mechanism:

• Restores volume
• Restores chloride
• Allows kidney to excrete bicarbonate

✔ Severe or Refractory Cases

Consider:

• Acetazolamide
• Carbonic anhydrase inhibitor
• Promotes bicarbonate excretion
• Useful in ventilated ICU patients

✔ Life-Threatening Alkalosis (pH > 7.55)

Rare but may require:

• Hydrochloric acid infusion (ICU setting)
• Dialysis (renal failure patients)