Respiratory alkalosis
On this page
Definition
Respiratory alkalosis is a primary decrease in arterial carbon dioxide tension (PaCO₂) due to alveolar hyperventilation, leading to an increase in blood pH.Causes
Hypoxaemia or hypoxia:- High altitude
- Severe anaemia
- Pulmonary embolism
- Pneumonia
- Anxiety, panic attacks
- Pain, fever
- Pregnancy
- CNS disorders (stroke, subarachnoid haemorrhage, trauma, encephalitis)
- Drugs (salicylates*, progesterone, catecholamines)
- Overventilation (mechanical ventilation)
*salicylate overdose leads to a mixed respiratory alkalosis and metabolic acidosis. Early stimulation of the respiratory centre leads to a respiratory alkalosis whilst later the direct acid effects of salicylates (combined with acute renal failure) may lead to an acidosis
Pathophysiology
Hyperventilation causes excessive CO₂ excretion, leading to a fall in PaCO₂. This shifts the carbonic acid–bicarbonate equilibrium:CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
A decrease in hydrogen ion concentration raises the pH (alkalosis).
Compensation:
- Acute: limited renal compensation, small decrease in HCO₃⁻ (about 2 mmol/L for each 10 mmHg fall in PaCO₂)
- Chronic (>2–3 days): kidneys excrete more bicarbonate, larger decrease in HCO₃⁻ (about 4–5 mmol/L per 10 mmHg fall in PaCO₂)
Clinical Features
- Light-headedness, dizziness
- Perioral and peripheral paresthesiae
- Tetany, muscle cramps (due to decreased ionised calcium)
- Confusion, syncope (in severe cases)
- Chest tightness or dyspnoea (especially in anxiety-induced cases)
Arterial Blood Gas (ABG) Findings
| Parameter | Acute | Chronic |
|---|---|---|
| pH | Increased | Slightly increased or near normal (partial compensation) |
| PaCO₂ | Decreased | Decreased |
| HCO₃⁻ | Slightly decreased | More markedly decreased |