What conditions can cause respiratory acidosis?

Conditions Causing Respiratory Acidosis

Respiratory acidosis occurs when carbon dioxide accumulates in the body due to inadequate ventilation or gas exchange, resulting in elevated PaCO2 levels above 6.1 kPa (46 mmHg) and decreased blood pH below 7.35. 1, 2

Pathophysiology of Respiratory Acidosis

• Respiratory acidosis develops when carbon dioxide (CO2) combines with water (H2O) to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen ions (H+), leading to increased acidity in the blood 1
• In acute respiratory acidosis, the pH falls below 7.35 with elevated CO2 levels, while chronic respiratory acidosis may show normal pH due to renal compensation through bicarbonate retention 1
• Respiratory acidosis is characterized by elevated PaCO2 (>46 mmHg), and in chronic cases, the kidneys retain bicarbonate as a compensatory mechanism 2

Conditions Affecting Gas Exchange

• Severe pneumonia, pulmonary edema, and acute respiratory distress syndrome (ARDS) can impair alveolar gas exchange, leading to CO2 retention and respiratory acidosis 3
• Chronic obstructive pulmonary disease (COPD) exacerbations cause airflow limitation and ventilation-perfusion mismatch, resulting in CO2 retention 1, 3
• Asthma exacerbations with severe bronchospasm can lead to air trapping and inadequate ventilation, causing respiratory acidosis 3
• Pulmonary fibrosis and other restrictive lung diseases can reduce lung compliance and impair gas exchange 3

Disorders of the Chest Wall and Respiratory Muscles

• Kyphoscoliosis and other chest wall deformities restrict lung expansion, leading to alveolar hypoventilation and CO2 retention 4, 5
• Neuromuscular disorders affecting respiratory muscles (e.g., myasthenia gravis, Guillain-Barré syndrome, amyotrophic lateral sclerosis) impair the mechanical aspects of breathing 3, 5
• Spinal cord injuries, particularly those affecting the cervical and high thoracic regions, can compromise diaphragmatic and intercostal muscle function 3
• Severe obesity hypoventilation syndrome causes mechanical restriction of chest wall movement and diaphragmatic excursion 5

Central Nervous System Disorders

• Drug overdose, particularly with opioids, causes respiratory depression by direct action on brain stem respiratory centers, reducing responsiveness to carbon dioxide 4, 3
• Central sleep apnea and obesity hypoventilation syndrome involve abnormal respiratory drive leading to hypoventilation 5
• Brain injuries, strokes, or tumors affecting the respiratory centers in the medulla can impair central respiratory drive 2, 5
• Sedatives, anesthetics, and other CNS depressants can suppress respiratory drive and lead to CO2 retention 4, 3

Other Contributing Factors

• Increased dead space ventilation, where air moves in and out without participating in gas exchange, contributes to respiratory acidosis 3
• Increased CO2 production (e.g., severe hyperthermia, malignant hyperthermia, thyroid storm) can overwhelm normal ventilatory capacity 3
• Permissive hypercapnia, a ventilation strategy used in ARDS to prevent ventilator-induced lung injury, intentionally allows CO2 levels to rise 6
• Severe metabolic alkalosis can lead to compensatory hypoventilation and respiratory acidosis 5

Clinical Implications

• Respiratory acidosis primarily affects the central nervous and cardiovascular systems, potentially causing altered mental status, headache, and cardiac arrhythmias 3
• Acute respiratory acidosis is a medical emergency requiring prompt intervention to improve ventilation 3, 7
• Chronic respiratory acidosis is often associated with compensatory metabolic alkalosis due to renal bicarbonate retention 1, 2
• Respiratory acidosis is common in clinical practice, with studies showing approximately 20% of patients with acute exacerbations of COPD developing respiratory acidosis 1

Management Considerations

• Treatment should primarily address the underlying cause of alveolar hypoventilation 3, 6
• Ventilatory support (non-invasive or invasive) may be necessary to improve CO2 elimination 3
• Sodium bicarbonate administration for respiratory acidosis remains controversial and generally not recommended unless there is concurrent severe metabolic acidosis 6
• Hypercapnic acidosis is generally well-tolerated as long as tissue perfusion and oxygenation are maintained 6