Metabolic alkalosis develops in two stages. First, bicarbonate accumulates or hydrogen ions are lost. Second, the kidneys fail to excrete the excess bicarbonate, allowing the alkalosis to persist. Understanding both stages is the key to interpreting the ABG and choosing the correct treatment.
This article builds on Acid-Base Disorders Explained and complements Metabolic Acidosis Explained. If acid-base interpretation feels unfamiliar, read the foundation article first, then return here for the detailed metabolic alkalosis pathway.
Metabolic alkalosis occurs when plasma HCO3- rises, causing blood pH to rise above 7.45. The primary problem is metabolic; the lungs respond secondarily by retaining CO2.
Learning Objectives
- Define metabolic alkalosis and identify the primary abnormality
- Explain why bicarbonate rises and why pH rises
- Recognise the typical ABG pattern and expected respiratory compensation
- Explain why metabolic alkalosis persists once it has developed
- Classify the causes of metabolic alkalosis by mechanism
- Use urine chloride to separate chloride-responsive from chloride-resistant alkalosis
- Apply a stepwise clinical approach using worked case examples
What Is Metabolic Alkalosis?
Metabolic alkalosis is a primary acid-base disorder characterised by an increase in plasma bicarbonate concentration that raises blood pH above the normal physiological range. Unlike respiratory alkalosis, where the primary abnormality is a fall in carbon dioxide from hyperventilation, metabolic alkalosis begins with an increase in bicarbonate or the loss of hydrogen ions. The lungs subsequently respond by retaining carbon dioxide as a compensatory mechanism.
The elevated PaCO2 is not the cause of the disorder. It represents the body's attempt to reduce the severity of the alkalosis. Metabolic alkalosis is defined as a primary increase in HCO3- with blood pH above 7.45, and respiratory compensation occurring secondarily.
Students frequently confuse metabolic alkalosis with respiratory alkalosis because both raise pH. The simplest way to distinguish them is to identify which variable changed first: if bicarbonate rises first, it is metabolic alkalosis; if carbon dioxide falls first, it is respiratory alkalosis.
Metabolic alkalosis is commonly encountered in persistent vomiting, nasogastric suction, loop and thiazide diuretic therapy, hyperaldosteronism, ICU patients and those receiving excessive alkali. Understanding its mechanism is therefore important across emergency medicine, surgery, nephrology and intensive care.
Normal Acid-Base Physiology
Before understanding metabolic alkalosis, it helps to review how the body normally regulates blood pH. The body continuously produces acid during metabolism, yet arterial pH remains remarkably stable because three defence systems work together at different speeds: chemical buffers, the lungs, and the kidneys.
Chemical Buffers
The bicarbonate buffer system is the most important extracellular buffer. Hydrogen ions combine with bicarbonate to form carbonic acid, which splits into carbon dioxide and water; the carbon dioxide is then exhaled. This buffering acts within seconds.
The Lungs
The lungs regulate carbon dioxide. Because CO2 behaves as an acid, increasing ventilation removes acid while reducing ventilation retains it. Respiratory regulation occurs within minutes.
The Kidneys
The kidneys regulate bicarbonate. They can reabsorb filtered bicarbonate, generate new bicarbonate, and excrete hydrogen ions. Renal regulation develops over hours to days, which is why the kidneys ultimately determine whether alkalosis resolves or persists.
Metabolic alkalosis develops because bicarbonate becomes disproportionately elevated relative to carbon dioxide. The lungs then increase carbon dioxide in an attempt to restore this balance.
Why Does Bicarbonate Rise?
Most students memorise lists of causes without understanding why bicarbonate increases. In reality, almost every cause of metabolic alkalosis can be explained by three simple mechanisms.
Mechanism 1: Loss of Hydrogen Ions
Hydrogen ions represent acid. When hydrogen ions are lost, bicarbonate remains relatively increased. Persistent vomiting, nasogastric suction and renal hydrogen loss are classic examples. Loss of gastric hydrochloric acid removes hydrogen ions from the body, so the blood becomes more alkaline.
Mechanism 2: Gain of Bicarbonate
Sometimes bicarbonate itself increases, such as with excess bicarbonate administration, milk-alkali syndrome, or large alkali loads. Most healthy kidneys rapidly excrete excess bicarbonate, so bicarbonate administration alone rarely causes sustained metabolic alkalosis unless renal excretion is impaired.
Mechanism 3: Renal Hydrogen Loss
The kidneys normally excrete hydrogen ions while conserving bicarbonate. Hyperaldosteronism, loop diuretics, thiazide diuretics and mineralocorticoid excess all exaggerate this process, leading to increased bicarbonate retention.
| Mechanism | Example |
|---|---|
| Hydrogen ion loss | Vomiting |
| Hydrogen ion loss | Nasogastric suction |
| Hydrogen ion loss | Diuretics |
| Hydrogen ion loss | Hyperaldosteronism |
| Bicarbonate gain | Excess bicarbonate therapy |
| Bicarbonate gain | Milk-alkali syndrome |
Notice that every mechanism ultimately produces the same biochemical abnormality: higher plasma bicarbonate. The causes differ, but the physiology is identical.
Why Metabolic Alkalosis Persists
This concept separates strong students from those who simply memorise causes. Developing metabolic alkalosis is relatively easy; maintaining it is much harder. Healthy kidneys efficiently remove excess bicarbonate, so if bicarbonate simply rises, the kidneys usually excrete it and pH returns to normal. Persistent metabolic alkalosis develops only when something prevents the kidneys from excreting bicarbonate — this is called the maintenance phase.
Factors That Maintain Metabolic Alkalosis
- Volume depletion: loss of extracellular fluid stimulates sodium retention, and because sodium reabsorption is linked to bicarbonate reabsorption, bicarbonate is also retained.
- Chloride depletion: chloride is essential for bicarbonate excretion. Low chloride impairs bicarbonate secretion by the distal nephron, which explains why saline often corrects metabolic alkalosis caused by vomiting.
- Potassium depletion: hypokalaemia promotes intracellular shifts of hydrogen ions and increases renal hydrogen secretion, generating further bicarbonate.
- Mineralocorticoid excess: aldosterone stimulates hydrogen secretion, potassium secretion and sodium reabsorption, a combination that strongly promotes metabolic alkalosis.
The initial cause and the maintenance mechanism are often different. Vomiting initiates metabolic alkalosis, but volume depletion and chloride depletion maintain it. This is why treatment targets both stopping the vomiting and replacing chloride and extracellular fluid.
ABG Findings in Metabolic Alkalosis
Once metabolic alkalosis develops, it produces a characteristic arterial blood gas pattern. Recognising this pattern is the first step before searching for the underlying cause.
| Parameter | Typical Change | Reason |
|---|---|---|
| pH | Up | Primary alkalosis |
| HCO3- | Up | Primary abnormality |
| PaCO2 | Up | Respiratory compensation |
| Base excess | Positive | Excess bicarbonate |
Bicarbonate is the body's major extracellular base. When it increases, the HCO3-/CO2 ratio rises, pushing pH above 7.45. The respiratory centre then detects the elevated pH and reduces ventilation, retaining CO2 to move pH back toward normal.
Worked ABG Example
| Parameter | Result |
|---|---|
| pH | 7.52 |
| PaCO2 | 48 mmHg |
| HCO3- | 36 mmol/L |
Interpretation: primary metabolic alkalosis with appropriate respiratory compensation.
Never diagnose metabolic alkalosis from bicarbonate alone. Always examine pH, HCO3- and PaCO2 together.
Respiratory Compensation
A common misconception is that respiratory compensation completely normalises pH. It does not — compensation only reduces the severity of the disturbance. As pH rises, the respiratory centre decreases ventilation, retaining CO2 and partially correcting pH.
Unlike metabolic acidosis, compensation for metabolic alkalosis is limited. Excessive hypoventilation causes hypoxaemia, hypercapnia and respiratory distress, so the body cannot keep reducing ventilation indefinitely. This is why metabolic alkalosis is never fully compensated by the lungs.
Worked Example
HCO3- = 36 mmol/L. Increase above normal: 36 − 24 = 12. Expected rise in PaCO2: 0.7 × 12 = 8.4. Expected PaCO2 ≈ 40 + 8 = 48 mmHg. The measured PaCO2 of 48 mmHg matches the expected value, confirming appropriate respiratory compensation.
| Finding | Consider | Examples |
|---|---|---|
| PaCO2 lower than expected | Additional respiratory alkalosis | Sepsis, anxiety, pain, pregnancy, mechanical ventilation |
| PaCO2 higher than expected | Additional respiratory acidosis | COPD, severe obesity, neuromuscular disease, respiratory failure |
Compensation never overcorrects. If measured PaCO2 differs substantially from the expected value, suspect a mixed acid-base disorder.
Causes of Metabolic Alkalosis
Instead of memorising a long list, classify the causes according to their underlying mechanism.
- Gastric acid loss — persistent vomiting and nasogastric suction, the most common cause. These patients usually have volume depletion, chloride depletion and hypokalaemia.
- Diuretics — loop and thiazide diuretics cause volume contraction, increased distal sodium delivery, increased hydrogen secretion and potassium depletion, mechanisms that reinforce one another.
- Mineralocorticoid excess — primary hyperaldosteronism, Cushing syndrome, ectopic ACTH and licorice ingestion increase hydrogen and potassium secretion while retaining bicarbonate.
- Alkali administration — excess bicarbonate, milk-alkali syndrome and citrate-containing blood transfusions usually require impaired renal bicarbonate excretion before sustained alkalosis develops.
- Rare causes — Bartter syndrome, Gitelman syndrome, severe potassium depletion, post-hypercapnic alkalosis and congenital chloride diarrhoea are less common but important in examinations.
| Cause | Main Mechanism |
|---|---|
| Vomiting | Hydrogen loss |
| NG suction | Hydrogen loss |
| Loop diuretics | Renal hydrogen loss |
| Thiazides | Renal hydrogen loss |
| Hyperaldosteronism | Aldosterone excess |
| Excess bicarbonate | Bicarbonate gain |
| Milk-alkali syndrome | Bicarbonate gain |
| Bartter / Gitelman syndrome | Renal hydrogen loss |
Although many diseases cause metabolic alkalosis, most do so through only three physiological mechanisms: hydrogen loss, bicarbonate gain, and renal bicarbonate retention. Understanding these mechanisms is more useful than memorising causes.
Chloride-Responsive vs Chloride-Resistant Metabolic Alkalosis
This distinction is central to the clinical evaluation of metabolic alkalosis. The kidneys require chloride to excrete bicarbonate, so when chloride is depleted, bicarbonate excretion falls and metabolic alkalosis persists. This is why chloride replacement often corrects alkalosis caused by vomiting or diuretics.
Urine Chloride
| Urine Chloride | Likely Diagnosis | Common Causes |
|---|---|---|
| <10–20 mmol/L | Chloride-responsive | Vomiting, NG suction, previous diuretic use, volume depletion |
| >20 mmol/L | Chloride-resistant | Primary hyperaldosteronism, current diuretic therapy, Bartter syndrome, Gitelman syndrome, severe mineralocorticoid excess |
Chloride-responsive alkalosis usually improves with normal saline, potassium replacement and correction of the underlying cause. Chloride-resistant alkalosis instead requires correcting the underlying disorder rather than simply administering saline.
A patient receiving loop diuretics may still have a high urine chloride because the drug continues to promote chloride loss. Always interpret urine chloride together with the medication history.
Worked Clinical Cases
The best way to master metabolic alkalosis is to apply the physiology to real scenarios: identify the primary disorder, assess compensation, then determine why the kidneys cannot excrete the excess bicarbonate.
Case 1 — Persistent Vomiting
A 24-year-old woman has 4 days of persistent vomiting with dizziness, dry mucous membranes and postural hypotension. ABG: pH 7.54, PaCO2 47 mmHg, HCO3- 38 mmol/L. Potassium 2.9 mmol/L, chloride low.
The raised pH and HCO3- confirm primary metabolic alkalosis, with the elevated PaCO2 representing appropriate compensation. Vomiting causes hydrogen loss; volume and chloride depletion then prevent bicarbonate excretion. Expected urine chloride is low (<10–20 mmol/L), indicating chloride-responsive alkalosis. Management prioritises isotonic saline, potassium replacement, chloride correction and treating the cause of vomiting.
Case 2 — Loop Diuretic Therapy
A 72-year-old man with heart failure on high-dose furosemide presents with fatigue and cramps. ABG: pH 7.49, PaCO2 46 mmHg, HCO3- 35 mmol/L; potassium and magnesium low.
Loop diuretics cause sodium, chloride and water loss, producing volume contraction and increased distal hydrogen and potassium secretion. Because the patient is actively on diuretics, urine chloride is often elevated despite true chloride depletion — an important limitation of the test. Management involves reviewing diuretic dosing, replacing potassium and magnesium, and optimising heart failure treatment.
Case 3 — Primary Hyperaldosteronism
A 48-year-old man has resistant hypertension despite three antihypertensives, with persistent hypokalaemia. ABG: pH 7.48, PaCO2 45 mmHg, HCO3- 34 mmol/L.
Excess aldosterone increases sodium reabsorption and hydrogen and potassium secretion, raising bicarbonate. Expected urine chloride is high (>20 mmol/L), indicating chloride-resistant alkalosis; saline alone will not correct it. Management focuses on confirming primary hyperaldosteronism, localisation studies, mineralocorticoid receptor antagonists, and surgery in selected patients.
| Feature | Vomiting | Loop Diuretics | Hyperaldosteronism |
|---|---|---|---|
| Blood pressure | Often low | Variable | Often high |
| Volume status | Depleted | Often depleted | Usually normal or expanded |
| Potassium | Low | Low | Low |
| Urine chloride | Usually low | Often high if current use | High |
| Saline response | Usually good | Variable | Usually poor |
Common Pitfalls
- Assuming every elevated bicarbonate means metabolic alkalosis — a raised bicarbonate may also be compensation for chronic respiratory acidosis. Always assess pH, PaCO2 and HCO3- together.
- Ignoring respiratory compensation — if PaCO2 is much lower than expected, consider a mixed disorder.
- Forgetting the maintenance phase — without impaired bicarbonate excretion, metabolic alkalosis is usually self-limiting. Always identify volume, chloride, potassium depletion or mineralocorticoid excess.
- Relying only on urine chloride — interpret it together with clinical history, current medications, volume status and blood pressure.
- Forgetting potassium — hypokalaemia is both a consequence and a maintaining factor of metabolic alkalosis. Failure to replace it delays recovery.
- Assuming saline corrects every patient — saline is effective mainly in chloride-responsive alkalosis, not in chloride-resistant disorders such as primary hyperaldosteronism.
- Treating the ABG instead of the patient — the ABG identifies the disorder; the underlying disease determines definitive treatment.
Metabolic alkalosis develops in two stages: generation and maintenance. Chloride depletion is often more important than bicarbonate excess. Compensation never completely normalises pH. Hypertension with metabolic alkalosis should prompt consideration of mineralocorticoid excess.
One Minute Revision
- Metabolic alkalosis is a primary increase in plasma bicarbonate resulting in elevated blood pH.
- Common causes: persistent vomiting, NG suction, loop and thiazide diuretics, hyperaldosteronism, excess bicarbonate administration.
- Typical ABG: pH up, HCO3- up, PaCO2 up (compensation).
- Persistence requires a maintenance factor: volume depletion, chloride depletion, potassium depletion, or mineralocorticoid excess.
- Urine chloride <10–20 mmol/L suggests chloride-responsive alkalosis; >20 mmol/L suggests chloride-resistant alkalosis.
- Clinical approach: confirm alkalosis → assess compensation → identify cause → measure urine chloride → treat the underlying disorder.
Frequently Asked Questions
This article is intended for medical education only. Metabolic alkalosis can indicate significant fluid, electrolyte or endocrine disturbance. Clinical management requires proper assessment, local protocols and senior or specialist input.