Medicine – Respiratory Acidosis

Respiratory acidosis occurs when alveolar ventilation is inadequate and carbon dioxide is retained. The primary abnormality is therefore:

↑ PaCO₂

with a resulting:

↓ pH.

The basic sequence is:

Hypoventilation → CO₂ retention → ↑ carbonic acid → ↑ H⁺ → acidosis.

1. Mechanism

Carbon dioxide combines with water to form carbonic acid:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

If ventilation falls, less CO₂ is exhaled.

As PaCO₂ rises, the reaction shifts to the right, increasing:

Hydrogen ion concentration.

Therefore:

HYPOVENTILATION → ↑ PaCO₂ → RESPIRATORY ACIDOSIS.

2. Chronic Obstructive Pulmonary Disease

The original notes correctly include:

COPD.

COPD can cause chronic alveolar hypoventilation and impaired CO₂ elimination, particularly in advanced disease.

Patients may therefore develop:

Chronic hypercapnia.

3. COPD and Chronic Compensation

When CO₂ retention persists for several days or longer, the kidneys compensate by retaining more:

Bicarbonate.

Therefore chronic respiratory acidosis may show:

↑ PaCO₂ + ↑ HCO₃⁻

with the pH closer to normal than in an acute episode.

4. Acute-on-Chronic Respiratory Acidosis

A patient with chronic COPD may have a chronically elevated PaCO₂ and bicarbonate.

If they then develop:

Infection.

Bronchospasm.

Sedative exposure.

Respiratory fatigue.

CO₂ may rise further.

This produces:

Acute-on-chronic respiratory acidosis.

5. Severe Asthma

The original notes correctly include:

Severe asthma.

Early in an asthma attack, patients often hyperventilate and may initially have:

Low PaCO₂.

Therefore early blood gases can show:

Respiratory alkalosis.

6. Rising CO₂ in Severe Asthma

As asthma becomes more severe, airflow obstruction and respiratory muscle fatigue may impair ventilation.

PaCO₂ may then become:

Normal or elevated.

In a severely breathless asthmatic patient, a rising PaCO₂ is concerning because it may indicate:

Impending ventilatory failure.

Therefore:

SEVERE ASTHMA + RISING CO₂ → DANGEROUS SIGN.

7. Obesity

The original notes include:

Obesity.

Obesity alone does not always cause respiratory acidosis.

The important syndrome is:

Obesity hypoventilation syndrome – OHS.

8. Obesity Hypoventilation Syndrome

OHS is characterised by obesity with chronic daytime:

Alveolar hypoventilation and hypercapnia

that cannot be fully explained by another cause.

Patients often also have:

Obstructive sleep apnoea.

9. Mechanism in Obesity Hypoventilation

Severe obesity increases the mechanical load on the respiratory system and reduces:

Chest-wall compliance.

Lung volumes.

Ventilatory efficiency.

This can eventually cause:

Chronic CO₂ retention.

Therefore:

OBESITY HYPOVENTILATION → CHRONIC RESPIRATORY ACIDOSIS.

10. Respiratory Depressant Drugs

The original notes correctly include:

Respiratory depressants.

These drugs reduce central respiratory drive.

Important examples include:

Opioids.

Benzodiazepines.

General anaesthetic agents.

Other sedative drugs.

11. Opioid Toxicity

Opioids suppress respiratory centres in the brainstem.

This can cause:

Slow respiration.

Reduced tidal volume.

Hypoventilation.

CO₂ retention.

Therefore:

OPIOID OVERDOSE → HYPOVENTILATION → RESPIRATORY ACIDOSIS.

12. Muscle Relaxants

Neuromuscular blocking drugs can impair the ability of respiratory muscles to contract.

If ventilatory support is inadequate, this results in:

Hypoventilation.

Hypercapnia.

Respiratory acidosis.

This is particularly relevant around:

Anaesthesia and critical care.

13. Neuromuscular Disorders

The original notes correctly include:

Neuromuscular disorders leading to hypoventilation.

Normal ventilation requires intact:

Brainstem respiratory centres.

Spinal pathways.

Peripheral nerves.

Neuromuscular junctions.

Respiratory muscles.

Failure at any of these levels can reduce ventilation.

14. Guillain–Barré Syndrome

Guillain–Barré syndrome can cause progressive:

Respiratory muscle weakness.

If the diaphragm and accessory respiratory muscles weaken sufficiently, alveolar ventilation falls.

This can result in:

Hypercapnic respiratory failure.

15. Myasthenia Gravis

A severe myasthenic crisis can cause weakness of:

Diaphragm.

Intercostal muscles.

Bulbar muscles.

This may lead to:

Hypoventilation and respiratory acidosis.

16. Motor Neurone Disease

Advanced motor neurone disease can weaken respiratory muscles.

Patients may develop:

Nocturnal hypoventilation first

followed later by:

Daytime hypercapnia.

This can produce chronic respiratory acidosis.

17. High Cervical Spinal Cord Disease

A high cervical spinal cord lesion can impair innervation of the:

Diaphragm and accessory respiratory muscles.

Severe lesions can therefore cause:

Ventilatory failure and CO₂ retention.

18. Central Nervous System Depression

Any condition that suppresses the brainstem respiratory centre can cause respiratory acidosis.

Examples include:

Head injury.

Brainstem stroke.

Sedative overdose.

Severe CNS disease.

The mechanism is:

Reduced central ventilatory drive.

19. Upper Airway Obstruction

Severe upper-airway obstruction can impair ventilation.

Possible causes include:

Foreign body.

Severe obstructive sleep apnoea.

Upper-airway oedema.

Tumour.

If obstruction is severe enough, CO₂ retention can occur.

20. Chest-Wall Disorders

Conditions that restrict expansion of the chest can also cause chronic hypoventilation.

Examples include:

Severe kyphoscoliosis.

Marked chest-wall deformity.

These reduce effective ventilation and may eventually cause:

Chronic hypercapnia.

21. Acute Respiratory Acidosis

In acute respiratory acidosis, the kidneys have had little time to compensate.

Therefore bicarbonate increases only slightly.

A useful rule is:

For every 10 mmHg rise in PaCO₂, HCO₃⁻ rises by about 1 mmol/L acutely.

22. Chronic Respiratory Acidosis

If hypercapnia persists for several days, the kidneys increase:

Hydrogen ion excretion

and

Bicarbonate retention.

Therefore compensation becomes greater.

A useful rule is:

For every 10 mmHg rise in PaCO₂, HCO₃⁻ rises by about 3–4 mmol/L chronically.

23. Why the pH Improves in Chronic Disease

Renal bicarbonate retention buffers some of the excess hydrogen ions generated by persistent CO₂ retention.

Therefore chronic respiratory acidosis may have:

A substantially raised PaCO₂

but only:

A mildly reduced pH.

24. Symptoms of Hypercapnia

Raised CO₂ may cause:

Headache.

Drowsiness.

Confusion.

Flushed skin.

Tremor.

Asterixis.

In severe cases:

Reduced consciousness or coma.

25. Carbon Dioxide Narcosis

Severe hypercapnia can depress cerebral function.

This is sometimes termed:

CO₂ narcosis.

Patients may become:

Drowsy.

Confused.

Obtunded.

Comatose.

This represents severe ventilatory failure.

26. Respiratory Failure

Respiratory acidosis is particularly associated with:

Type 2 respiratory failure.

This is characterised by:

Hypercapnia

with or without significant:

Hypoxaemia.

27. COPD – Note Form

Mechanism:

Airflow obstruction + impaired alveolar ventilation.

↓

CO₂ retention.

↓

Respiratory acidosis.

Chronic COPD:

↑ PaCO₂.

↑ HCO₃⁻ due renal compensation.

Acute exacerbation:

Further CO₂ rise.

↓

Acute-on-chronic respiratory acidosis.

28. Severe Asthma – Note Form

Early attack:

Hyperventilation.

↓

Low PaCO₂.

↓

Respiratory alkalosis.

Severe/fatigued patient:

Reduced ventilation.

↓

Normalising or rising PaCO₂.

↓

Respiratory acidosis.

Therefore:

RISING CO₂ IN SEVERE ASTHMA IS A RED FLAG.

29. Obesity – Note Form

Important condition:

Obesity hypoventilation syndrome.

Mechanism:

Reduced ventilatory efficiency.

↓

Chronic alveolar hypoventilation.

↓

CO₂ retention.

↓

Chronic respiratory acidosis.

30. Drug Causes – Note Form

Opioids.

Benzodiazepines and other sedatives.

Anaesthetic agents.

Neuromuscular blockers.

Mechanism:

Reduced respiratory drive or respiratory muscle function → hypoventilation.

31. Neuromuscular Causes – Note Form

Guillain–Barré syndrome.

Myasthenia gravis.

Motor neurone disease.

Muscular dystrophy.

High spinal cord lesions.

All can cause:

Respiratory muscle weakness → hypoventilation → hypercapnia.

32. Additional Causes – Note Form

Important additions include:

CNS depression or brainstem disease.

Severe kyphoscoliosis.

Upper-airway obstruction.

Obstructive sleep-related hypoventilation.

Inadequate mechanical ventilation.

33. Important Corrections and Clarifications

The original definition is correct:

HYPOVENTILATION → ↑ CO₂ → RESPIRATORY ACIDOSIS.

COPD is a classic cause, particularly when advanced disease produces:

CHRONIC HYPERCAPNIA.

Severe asthma needs an important qualification:

Early asthma often causes:

RESPIRATORY ALKALOSIS.

A normalising or elevated PaCO₂ during a severe attack can indicate:

RESPIRATORY MUSCLE FATIGUE AND IMPENDING FAILURE.

The original term:

“Obesity”

is better refined to:

OBESITY HYPOVENTILATION SYNDROME.

“Muscle relaxants and respiratory depressants” should be thought of mechanistically as:

CENTRAL RESPIRATORY DEPRESSION OR FAILURE OF RESPIRATORY MUSCLE CONTRACTION.

Key Clinical Pattern

Remember:

RESPIRATORY ACIDOSIS = ↓ pH + ↑ PaCO₂.

Common causes include:

COPD.

SEVERE/FATIGUED ASTHMA.

OBESITY HYPOVENTILATION.

OPIOIDS/SEDATIVES.

NEUROMUSCULAR WEAKNESS.

CNS DEPRESSION.

CHEST-WALL RESTRICTION.

And the simplest rule is:

ANYTHING THAT REDUCES EFFECTIVE ALVEOLAR VENTILATION CAN CAUSE RESPIRATORY ACIDOSIS.