Medicine – Respiratory Alkalosis

Respiratory alkalosis occurs when alveolar ventilation is increased enough to cause excessive loss of carbon dioxide. Because carbon dioxide contributes to carbonic acid formation, a fall in arterial CO₂ causes the blood pH to rise.

The basic sequence is:

Hyperventilation → ↓ PaCO₂ → ↓ carbonic acid → ↑ pH.

Therefore the primary abnormality is:

Low PaCO₂ with alkalemia.

1. Mechanism

Carbon dioxide combines with water to form carbonic acid:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

If ventilation increases excessively, more CO₂ is exhaled.

This shifts the equilibrium to the left, reducing:

Hydrogen ion concentration.

The result is:

Respiratory alkalosis.

2. Psychogenic Hyperventilation

The original notes correctly include:

Psychogenic causes.

Anxiety, panic, pain, or emotional distress can cause rapid deep breathing.

This lowers PaCO₂ and may produce symptoms such as:

Light-headedness.

Dizziness.

Perioral tingling.

Tingling in the hands and feet.

Carpopedal spasm in more marked cases.

3. Why Tingling Occurs

Alkalosis increases the binding of calcium to albumin.

This reduces:

Ionised calcium.

Therefore acute respiratory alkalosis can produce neuromuscular symptoms such as:

Paraesthesia.

Muscle cramps.

Carpopedal spasm.

4. Pulmonary Disease

Pulmonary disorders commonly cause respiratory alkalosis when they stimulate ventilation.

Important examples include:

Pulmonary embolism.

Pneumonia.

Pulmonary oedema.

Interstitial lung disease.

Severe asthma early in an attack.

The common mechanism is:

Hypoxaemia or pulmonary receptor stimulation → hyperventilation → ↓ PaCO₂.

5. Pulmonary Embolism

A classic cause is:

Pulmonary embolism.

Patients often hyperventilate because of:

Hypoxaemia.

Pain.

Ventilation–perfusion mismatch.

Therefore the arterial blood gas may show:

Low PaCO₂ + respiratory alkalosis.

A normal or high PaCO₂ in a severely breathless patient may sometimes be more concerning because it can indicate respiratory fatigue.

6. High Altitude

At high altitude, atmospheric oxygen pressure falls.

This causes:

Hypoxaemia.

Peripheral chemoreceptors, especially in the carotid bodies, respond by stimulating:

Hyperventilation.

Therefore:

High altitude → hypoxaemia → hyperventilation → ↓ PaCO₂ → respiratory alkalosis.

7. Adaptation to Altitude

With time, the kidneys compensate by excreting more:

Bicarbonate.

This lowers plasma bicarbonate and allows continued hyperventilation without such a large rise in pH.

This renal compensation helps acclimatisation.

8. Right-to-Left Shunt

A right-to-left shunt allows deoxygenated venous blood to enter the systemic circulation without being fully oxygenated in the lungs.

This can cause:

Hypoxaemia.

Hypoxaemia stimulates ventilation and may therefore lead to:

Respiratory alkalosis.

The mechanism is indirect:

Shunt → hypoxaemia → hyperventilation → low PaCO₂.

9. Carbon Monoxide Poisoning

The original notes include:

Carbon monoxide poisoning.

Carbon monoxide binds haemoglobin with very high affinity and impairs oxygen delivery to tissues.

Patients may respond with:

Hyperventilation.

This can produce:

Respiratory alkalosis.

However, pulse oximetry may appear misleadingly normal because conventional pulse oximeters cannot reliably distinguish oxyhaemoglobin from carboxyhaemoglobin.

10. Salicylate Poisoning

Salicylates are a very important cause because they produce a characteristic mixed acid–base disturbance.

Early in toxicity, salicylates directly stimulate the:

Medullary respiratory centre.

This causes:

Hyperventilation

and therefore:

Respiratory alkalosis.

11. Later Salicylate Toxicity

As poisoning progresses, salicylates also cause:

Metabolic acidosis.

Therefore the classic pattern is:

Respiratory alkalosis + high-anion-gap metabolic acidosis.

This is an important examination association.

12. Fever and Sepsis

An important additional cause is:

Fever or sepsis.

Inflammatory mediators and increased metabolic demand can stimulate ventilation.

Therefore early sepsis may produce:

Respiratory alkalosis.

A low PaCO₂ may sometimes be one of the earliest blood-gas abnormalities.

13. Pregnancy

Pregnancy is another important physiological cause.

Progesterone stimulates the respiratory centre, causing a mild chronic increase in ventilation.

Therefore normal pregnancy commonly produces:

Mild respiratory alkalosis.

This is accompanied by renal compensation with a modest fall in:

Serum bicarbonate.

14. Liver Disease

Severe liver disease can also cause:

Hyperventilation

and

Respiratory alkalosis.

The mechanism is multifactorial and may involve altered central respiratory regulation and circulating mediators.

15. CNS Causes

Conditions affecting the central nervous system may stimulate respiration.

Examples include:

Stroke.

Subarachnoid haemorrhage.

Meningitis.

Encephalitis.

Head injury.

These can cause respiratory alkalosis through increased central respiratory drive.

16. Mechanical Ventilation

Respiratory alkalosis can also be iatrogenic.

If a mechanically ventilated patient receives excessive:

Minute ventilation,

too much CO₂ is removed.

This produces:

Low PaCO₂ and respiratory alkalosis.

17. Acute Compensation

In acute respiratory alkalosis, the kidneys have not yet had time to make major adjustments.

Therefore serum bicarbonate falls only modestly.

A useful rule is:

For every 10 mmHg fall in PaCO₂, HCO₃⁻ falls by about 2 mmol/L acutely.

18. Chronic Compensation

If respiratory alkalosis persists for several days, the kidneys increase bicarbonate excretion.

Therefore:

For every 10 mmHg fall in PaCO₂, HCO₃⁻ falls by about 4–5 mmol/L chronically.

This helps bring the pH back toward normal.

19. Clinical Features

Symptoms may include:

Dizziness.

Light-headedness.

Paraesthesia.

Perioral numbness.

Palpitations.

Muscle cramps.

Carpopedal spasm.

Severe alkalosis can occasionally contribute to:

Confusion or seizures.

20. Psychogenic Causes – Note Form

Anxiety/panic:

Hyperventilation.

↓

Low PaCO₂.

↓

Respiratory alkalosis.

May cause tingling and carpopedal spasm.

21. Pulmonary Causes – Note Form

Pulmonary embolism.

Pneumonia.

Pulmonary oedema.

Interstitial lung disease.

Early severe asthma.

Mechanism:

Hypoxaemia or pulmonary stimulation → hyperventilation.

22. Hypoxic Causes – Note Form

High altitude.

Right-to-left shunt.

Carbon monoxide poisoning.

Mechanism:

Reduced effective oxygen delivery → increased ventilation → low PaCO₂.

23. Toxicological Causes – Note Form

Salicylates:

Early direct respiratory-centre stimulation.

↓

Respiratory alkalosis.

Later:

Respiratory alkalosis + metabolic acidosis.

24. Additional Causes – Note Form

Pregnancy.

Sepsis/fever.

Liver disease.

CNS disease.

Excess mechanical ventilation.

These are important additions to the original list.

25. Important Corrections and Clarifications

The original definition is correct:

HYPERVENTILATION → LOW CO₂ → RESPIRATORY ALKALOSIS.

However, hyperventilation here means ventilation in excess of what is required for CO₂ production, not simply breathing quickly.

Pulmonary disease causes respiratory alkalosis mainly when it produces:

HYPOXAEMIA OR INCREASED RESPIRATORY DRIVE.

Right-to-left shunt causes respiratory alkalosis indirectly through:

HYPOXAEMIA.

Salicylate poisoning is particularly important because it commonly produces a:

MIXED RESPIRATORY ALKALOSIS + METABOLIC ACIDOSIS.

Key Clinical Pattern

Remember:

RESPIRATORY ALKALOSIS = ↑ pH + ↓ PaCO₂.

Common causes include:

PSYCHOGENIC HYPERVENTILATION.

PULMONARY EMBOLISM / OTHER HYPOXAEMIC LUNG DISEASE.

HIGH ALTITUDE.

RIGHT-TO-LEFT SHUNT.

CARBON MONOXIDE POISONING.

SALICYLATE TOXICITY.

SEPSIS.

PREGNANCY.

A useful final memory rule is:

Anything that drives ventilation excessively can cause respiratory alkalosis.