What are the primary causes of respiratory acidosis?

Primary Causes of Respiratory Acidosis

Respiratory acidosis fundamentally results from inadequate alveolar ventilation leading to carbon dioxide accumulation, with PaCO2 rising above 6.1 kPa (46 mmHg) and pH falling below 7.35. 1, 2

Pathophysiologic Mechanism

Respiratory acidosis develops when CO2 combines with water to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen ions (H+), increasing blood acidity. 1, 2 This occurs specifically when carbon dioxide production exceeds elimination via the lungs due to alveolar hypoventilation. 3, 4

Primary Etiologic Categories

Central Nervous System Impairment

• Brain injuries, strokes, or tumors affecting the medullary respiratory centers impair central respiratory drive. 2
• Drug overdoses that suppress respiratory centers (opioids, sedatives, anesthetics). 3

Neuromuscular Dysfunction

• Spinal cord injuries compromise respiratory muscle function. 3
• Neuromuscular diseases affecting the respiratory muscle pump (myasthenia gravis, Guillain-Barré syndrome, muscular dystrophies). 3

Chest Wall and Pleural Abnormalities

• Chest wall deformities or trauma that mechanically restrict ventilation. 3
• Pleural effusions or pneumothorax limiting lung expansion. 3

Airways Disease

• COPD exacerbations are the most common clinical cause, with approximately 20% of acute exacerbations developing respiratory acidosis. 2
• Severe asthma with bronchospasm causing ventilatory failure. 3

Respiratory Parenchymal Disease

• Pulmonary edema (cardiogenic or non-cardiogenic) impairing gas exchange. 3
• Severe pneumonia or acute respiratory distress syndrome (ARDS). 3

Increased Dead Space

• Pulmonary embolism creating ventilation-perfusion mismatch. 3
• Mechanical ventilation with excessive dead space. 3

Clinical Distinction: Acute vs. Chronic

Acute Respiratory Acidosis

• pH falls below 7.35 with elevated PaCO2 and minimal bicarbonate elevation. 1, 2
• Occurs with sudden respiratory failure (Type II respiratory failure). 3
• Limited metabolic compensation through intracellular buffering only. 4

Chronic Respiratory Acidosis

• Sustained PaCO2 elevation with renal compensation producing markedly elevated bicarbonate (often >28 mmol/L). 5
• pH may normalize due to complete metabolic compensation requiring 3-5 days. 5
• Common in stable severe COPD patients. 1

Acute-on-Chronic Respiratory Acidosis

• Critical pitfall: Patients with chronic compensated respiratory acidosis can develop acute acidosis during exacerbations because their elevated bicarbonate was equilibrated to their baseline CO2 and cannot buffer a sudden further CO2 increase. 1
• This occurs frequently in COPD exacerbations despite pre-existing high bicarbonate levels. 1

Key Diagnostic Considerations

When evaluating respiratory acidosis, measure respiratory rate, observe chest/abdominal wall movement, and obtain arterial blood gas analysis. 1 The rapid shallow breathing pattern (increased rate with small tidal volumes) indicates respiratory muscle pump failure and inadequate alveolar ventilation. 1

Hypoxemia frequently accompanies respiratory acidosis, especially when patients breathe room air, due to concurrent alveolar hypoventilation. 4