Metabolic Alkalosis Explained
Metabolic alkalosis is an acid-base disorder caused by a primary increase in bicarbonate. For respiratory therapy students, the key pattern is a high pH with a high HCO₃. The lungs may compensate by decreasing ventilation, but oxygenation and ventilation must always be assessed carefully.
Core Metabolic Alkalosis Pattern
Metabolic alkalosis occurs when bicarbonate is high enough to push the pH above normal. Since HCO₃ is the metabolic component, the disorder is metabolic when pH and HCO₃ move in the same direction.
High, above 7.45
High, above 26 mEq/L
May be high if respiratory compensation is present
pH and HCO₃ move in the same direction
Normal Values to Know
| ABG Value | Normal Range | Role |
|---|---|---|
| pH | 7.35–7.45 | Determines acidotic, alkalotic, or normal status. |
| PaCO₂ | 35–45 mmHg | Respiratory component. |
| HCO₃ | 22–26 mEq/L | Metabolic component. |
| PaO₂ | 80–100 mmHg | Oxygenation status. |
How Compensation Works
In metabolic alkalosis, the respiratory system may attempt to compensate by decreasing ventilation. This retains CO₂, which helps move the pH back toward normal. However, compensation is limited because hypoventilation can worsen oxygenation.
HCO₃ is high, pH is high, and PaCO₂ remains normal.
HCO₃ is high, pH is still high, and PaCO₂ is also high.
HCO₃ and PaCO₂ are abnormal, but pH has returned to normal.
Do not assume a high PaCO₂ is always primary respiratory acidosis. It may be compensation for metabolic alkalosis.
Common Causes of Metabolic Alkalosis
Metabolic alkalosis often occurs when the body loses acid, gains bicarbonate, or develops electrolyte and volume changes that maintain alkalosis.
| Cause | Why It Causes Alkalosis | RT Clinical Connection |
|---|---|---|
| Vomiting or gastric suction | Loss of gastric acid raises bicarbonate effect. | Assess volume status and clinical context. |
| Diuretic therapy | Can cause volume contraction and electrolyte shifts. | Common in heart failure or fluid overload patients. |
| Excess bicarbonate administration | Raises metabolic base load. | Review medication and treatment history. |
| Hypokalemia | Potassium shifts can help maintain alkalosis. | Electrolyte abnormalities may affect respiratory muscle function and cardiac rhythm. |
| Post-hypercapnic alkalosis | HCO₃ may stay elevated after chronic CO₂ retention is rapidly corrected. | Important in ventilated COPD patients or rapid changes in PaCO₂. |
Example ABG
PaCO₂ 48 mmHg
HCO₃ 34 mEq/L
PaO₂ 84 mmHg
Step 1: pH is high, so the patient is alkalotic.
Step 2: HCO₃ is high and matches the high pH, so the primary disorder is metabolic alkalosis.
Step 3: PaCO₂ is high, showing respiratory compensation. Since pH is still abnormal, this is partial compensation.
Step 4: PaO₂ is normal.
Partially compensated metabolic alkalosis with normal oxygenation.
Common Student Mistakes
High PaCO₂ may be compensating for high bicarbonate.
Clinical history often makes the ABG pattern easier to recognize.
Compensatory hypoventilation may affect oxygenation, so always check PaO₂.
Patients with chronic CO₂ retention may keep elevated HCO₃ after PaCO₂ is lowered quickly.
Try Free Interactive ABG Cases
Practice metabolic alkalosis and other ABG patterns through guided clinical case studies with step-by-step feedback.