Causes of Metabolic and Respiratory Alkalosis
Metabolic Alkalosis
Metabolic alkalosis results from either acid loss or alkali accumulation, with maintenance perpetuated by factors that prevent renal bicarbonate excretion 1.
Generation Mechanisms
Acid Loss:
- Gastrointestinal losses - Vomiting or nasogastric suction causing loss of gastric HCl 2
- Renal losses - Loop diuretics (furosemide) and thiazide diuretics causing hypokalemia and hypochloremia 3
Alkali Accumulation:
- Excessive oral or parenteral alkali intake 1
- Milk-alkali syndrome from excess calcium and alkali ingestion 1
Maintenance Factors
The kidney normally prevents metabolic alkalosis by excreting excess bicarbonate. Maintenance occurs when specific factors impair renal bicarbonate excretion 1:
- Volume contraction - The most common maintenance factor
- Hypochloremia - Chloride depletion prevents bicarbonate excretion
- Hypokalemia - Potassium deficiency enhances bicarbonate reabsorption 3
- Low GFR - Reduced filtration limits bicarbonate excretion
- Aldosterone excess - Primary hyperaldosteronism or secondary mineralocorticoid excess 2
- Elevated PaCO2 - Chronic hypercapnia maintains elevated bicarbonate 4
Specific Clinical Conditions
Genetic/Inherited Disorders:
- Bartter syndrome (Types 1-5) - Characterized by polyuria, hypochloremia, hypokalemia, and metabolic alkalosis with elevated urinary chloride 2
- Gitelman syndrome - Presents with hypocalciuria and hypomagnesemia 2
- Cystic fibrosis (Pseudo-Bartter syndrome) - Salt loss in sweat 2
Acquired Conditions:
- Congenital chloride diarrhea - Hypokalemic, hypochloremic metabolic alkalosis from watery diarrhea 2
- Post-hypercapnic alkalosis - Occurs when chronic hypercapnia is rapidly corrected (e.g., COPD patients placed on mechanical ventilation), leaving sustained high bicarbonate without respiratory compensation 4
- Licorice ingestion - Mineralocorticoid effect 1
Critical Pitfall
Thiazide and loop diuretics cause hypokalemia and metabolic alkalosis, which can exacerbate CO2 retention in patients with chronic lung disease 3. This must be prevented with adequate KCl supplementation and periodic electrolyte monitoring.
Respiratory Alkalosis
Respiratory alkalosis is caused by primary hypocapnia (PaCO2 <35 mmHg) resulting from disorders that increase alveolar ventilation 5.
Primary Mechanisms
Pulmonary Disorders:
- Conditions affecting gas exchange across the pulmonary capillary 5
- Pulmonary vasodilation occurs as a direct effect 6
- Hypoxemia-induced hyperventilation 7
Extrapulmonary Disorders:
- Chest wall and respiratory muscle disorders 5
- Inhibition of the medullary respiratory center 5
- Hyperventilation syndrome - Common in emergency department settings, diagnosed by exclusion 6
Metabolic Consequences
Respiratory alkalosis produces multiple metabolic derangements 6:
- Changes in potassium, phosphate, and calcium handling
- Development of mild lactic acidosis
- Altered renal ion handling
- Tachycardia, ventricular and atrial arrhythmias
- Ischemic and non-ischemic chest pain
- Changes in gastrointestinal perfusion, motility, and electrolyte handling
Therapeutic Context
Respiratory alkalosis is intentionally induced for treatment of elevated intracranial pressure 6. However, correction is otherwise best performed by addressing the underlying etiology rather than attempting to suppress ventilation.
Mixed Presentations
Mixed metabolic and respiratory alkalosis can occur, such as in superior mesenteric artery syndrome where chronic vomiting causes metabolic alkalosis while hypoxia-induced hyperventilation produces concurrent respiratory alkalosis 7.
Prognostic Significance
Severe metabolic alkalosis (pH ≥7.55) in critically ill patients is associated with significantly increased mortality 1. Even mild to moderate alkalemia (pH 7.45-7.55) carries prognostic significance, with primarily respiratory alkalosis showing higher 30-day mortality (aHR 1.21) compared to primarily metabolic causes 8.
Post-hypercapnic alkalosis specifically is associated with increased ICU stay, ventilator dependency, and mortality 4. While acetazolamide effectively improves alkalemia by inducing bicarbonate excretion, its impact on hard outcomes may be limited by patient complexity and underlying conditions.