Respiratory acidosis

Respiratory acidosis is an acid-base disorder characterised by a respiratory pathological process which decreases the pH of the blood. If the acidosis is severe, it can overcome the body's defense against acidosis, causes the pH in the blood to fall below 7.35. This is called acidaemia.

Metabolic acidosis is similar but is rather due to a metabolic problem. Respiratory alkalosis is sort of the opposite of respiratory acidosis.

Respiratory acidosis occurs due to impaired ventilation (hypoventilation), which causes pCO2 to increase (hypercapnia). This is called hypercapnic respiratory failure.

Etiology

Respiratory acidosis occurs due to hypercapnic respiratory failure.

Hypercapnic respiratory failure may occur due to (alveolar) hypoventilation or due to increased dead space.

Hypoventilation

Physiological ventilation requires a normal respiratory drive from the CNS, normal conduction of nerve impulses from the CNS to the respiratory muscles, normal function of the chest wall and respiratory muscles, normal conduction of air through the upper airways, and normal functioning of the lungs.

As such, hypoventilation may occur due to problems in several different organ systems. Problems with the central nervous system can impair the normal drive to ventilate, problems with the peripheral nervous system, respiratory muscles, chest wall, or upper airways may make the patient unable to breathe despite the respiratory drive, and problems with the lung can impair gas exchange to such a degree that any amount of ventilation is insufficient for gas exchange.

• Problems with the CNS
  • Medications which decrease respiratory drive (benzodiazepines, alcohol, opioids)
  • Stroke
  • Encephalitis
• Impaired neuromuscular transmission
  • Injury to the cervical spine between C3 and C5
  • Neuromuscular disorders (ALS, GBS, MG)
  • Tetanus
  • Botulism
  • Electrolyte disorders
• Impaired function of chest wall
  • Severe kyphosis or scoliosis
  • Pectus excavatum
• Impaired airways (rarely causes hypoventilation by itself)
  • Acute exacerbation of COPD or asthma
  • Foreign body aspiration
  • External airway obstruction (goitre, tumour)

Increased dead space (V/Q mismatch)

Increased dead spacing occurs when there is a ventilation/perfusion mismatch (V/Q mismatch) where regions of the lung are not perfused. When a part of the lung receives no perfusion, the alveoli in the area effectively become dead space (due to not having blood to exchange gas to). This can occur in case of:

• Pulmonary embolism (severe)
• Interstitial lung disease (severe)

Pathophysiology

Compensatory mechanism

The body cannot compensate for respiratory acidosis by stimulating ventilation, as ventilation is the problem in the first place. However, kidney compensation occurs similarly as for metabolic acidosis. Kidneys respond to acidosis by increasing urinary excretion of acids and decreasing urinary excretion of bicarbonate. This mechanism is slow, taking a few days to kick in.

Clinical features

Hypercapnia causes depressed consciousness, which may range from sluggishness to somnolence to coma.

Diagnosis and evaluation

Arterial blood gas is essential in the evaluation of acid-base disorders. It will give the pH, bicarbonate level, pCO2, pO2, and lactate levels. In respiratory acidosis, the pCO₂ level is elevated (> 45 mmHg). If there is acidaemia, the pH is < 7.35.

When kidney compensation has kicked in, bicarbonate levels start to increase. Appropriate compensation causes an increase of approximately 4 units of bicarbonate per 10 mmHg elevation in pCO₂.

Management

Treating the underlying cause is essential, but measures to improve hypoxaemia and hypercapnia are important as well, to prevent worsening. Oxygen supplementation, non-invasive ventilation, or invasive ventilation may be used.