Respiratory Acidosis Management
Diagnosis and Assessment
- A serum bicarbonate level >27 mmol/L can be used to decide when to measure PaCO2 in patients with low to moderate probability of having obesity hypoventilation syndrome, as recommended by the American Thoracic Society and American College of Chest Physicians 1, 2
- Hypercapnia is defined as a PaCO2 greater than 6.1 kPa (45 mmHg), which exceeds the normal physiological range of 4.6-6.1 kPa (34-46 mmHg) 2
- End-tidal CO2 (ETCO2) > 50 mmHg indicates hypoventilation, and a change in ETCO2 > 10 mmHg indicates significant respiratory change, with capnometry providing earlier detection of subclinical respiratory depression than pulse oximetry 3
- The following table summarizes the predictive values of standard bicarbonate levels for diagnosing obesity-hypoventilation syndrome:
| Standard Bicarbonate Level | Predictive Value |
|---|---|
| >27 mmol/L | Positive predictive value: up to 48.3% |
| <27 mmol/L | Negative predictive value: 99% |
Pathophysiology and Risk Factors
- Four primary mechanisms lead to hypercapnia: increased concentration of CO2 in inspired gas, increased CO2 production, hypoventilation or ineffective ventilation, and increased external dead space 2
- Patients with AECOPD or obesity-hypoventilation syndrome are at risk of carbon dioxide retention if given excessive oxygen, with 20-50% of patients affected 2
Treatment and Management
- Target oxygen saturation should be 88-92% to prevent worsening hypercapnia in patients at risk, as recommended by the American Thoracic Society and European Respiratory Society 2, 1
- Direct measurement of PaCO2 is recommended, as relying solely on SpO2 or bicarbonate can lead to misdiagnosis, according to the American Thoracic Society and American College of Chest Physicians 2, 1
- Treatment options include:
Monitoring and Complications
- Sudden cessation of supplementary oxygen can cause life-threatening rebound hypoxemia, as noted by the American Thoracic Society 2
- Bicarbonate elevation takes days to develop, so it won't be present in acute respiratory acidosis, and relying solely on SpO2 or bicarbonate without direct PaCO2 measurement can lead to misdiagnosis, as noted by the American College of Physicians and American Thoracic Society 1