What are the causes of respiratory acidosis?

Causes of Respiratory Acidosis

Respiratory acidosis results from inadequate alveolar ventilation leading to CO2 retention, caused by three primary mechanisms: disorders affecting gas exchange across the pulmonary capillary, disorders of the chest wall and respiratory muscles, and inhibition of the medullary respiratory center. 1

Primary Pathophysiologic Mechanisms

Respiratory acidosis develops when the respiratory muscles fail to achieve adequate alveolar ventilation despite high levels of diaphragmatic activity, resulting in arterial CO2 accumulation and subsequent acidosis (pH ≤7.35). 2 The fundamental problem is that the load on the respiratory system exceeds the capacity of the respiratory muscle pump. 2

1. Pulmonary Parenchymal and Airways Disease

• COPD exacerbations are the most common cause, where hyperinflation during exacerbation contributes to respiratory muscle compromise and a rapid shallow breathing pattern develops with increased respiratory rate but small tidal volumes at the expense of adequate alveolar ventilation. 2
• Acute asthma can cause acute Type II respiratory failure. 3
• Pulmonary edema impairs gas exchange across the pulmonary capillary. 3
• Increased dead space reduces effective ventilation. 3

2. Neuromuscular and Chest Wall Disorders

• Neuromuscular diseases (NMD) including motor neurone disease (particularly amyotrophic lateral sclerosis), acid maltase deficiency, Limb Girdle muscular dystrophy, and Myotonic Dystrophy cause progressive respiratory muscle weakness. 4
• In some NMDs, diaphragm involvement precedes locomotor disability, with presentation as acute-on-chronic hypercapnia being typical. 4
• Bulbar muscle involvement in muscular dystrophies causes sleep-disordered breathing from both respiratory muscle weakness and upper airway obstruction, with nocturnal hypoventilation gradually progressing to daytime hypercapnia. 4
• Chest wall deformities such as severe kyphoscoliosis create high impedance to lung inflation. 4
• Spinal cord injury disrupts neuromuscular control of ventilation. 3

3. Central Nervous System Depression

• Drug overdose inhibits the medullary respiratory center. 3
• Central nervous system events that suppress respiratory drive. 3

4. Increased CO2 Production

• While less common as a primary cause, increased carbon dioxide production can contribute to respiratory acidosis when ventilatory capacity is already compromised. 3

Clinical Context and Presentation

In NMD/CWD, any elevation of PaCO2 may herald an impending crisis, as patients have reduced respiratory reserve. 4 Minor infections such as coryza can be provocative, with progressive hypercapnia developing over 24-72 hours. 4

Approximately 20% of patients hospitalized for COPD present with or develop hypercapnic respiratory failure, which indicates increased risk of death. 2 Many COPD patients are hypercapnic at baseline, and the development of acidosis indicates acute-on-chronic hypercapnic respiratory failure. 2

Important Clinical Pitfalls

• Do not wait for acidosis to develop in NMD/CWD patients - NIV should be considered in any breathless/acutely unwell patient with these conditions before respiratory acidosis develops. 4
• Some patients with chronic conditions may present with marked chronic hypercapnia as an unexpected finding, potentially with established pulmonary hypertension, chronic hypoxemia, and polycythemia. 4
• Tolerance of acute and chronic hypercapnia varies considerably between patients - some are excessively sleepy with minimal PaCO2 elevation while others remain alert despite severe hypercapnia. 4