From the Guidelines
A blood gas test is strongly recommended to determine improvement in CO2 levels when a person with chronic respiratory failure secondary to COPD is placed on non-invasive ventilation (NIV). According to the most recent and highest quality study 1, an arterial blood gas (ABG) should be obtained before initiating NIV to establish baseline values, and then repeated within 1-2 hours after starting NIV to assess the initial response. Further ABGs should be performed at 4-6 hours and 24 hours to monitor ongoing improvement.
Key Points to Consider
- The primary goal is to observe a reduction in PaCO2 (partial pressure of carbon dioxide) and normalization of pH, indicating effective ventilation.
- Typically, successful NIV therapy should show a decrease in PaCO2 by at least 5-10 mmHg and improvement in pH toward normal range (7.35-7.45) within the first few hours.
- This monitoring is crucial because persistent hypercapnia despite NIV may indicate treatment failure and the potential need for invasive mechanical ventilation.
- Blood gas analysis provides objective data on ventilation status that cannot be obtained through clinical assessment or pulse oximetry alone, as oxygen saturation does not reflect CO2 retention, which is the primary concern in COPD patients with respiratory failure.
Clinical Application
As stated in 1, clinical evaluation of the patient should include assessment of patient comfort, conscious level, chest wall motion, accessory muscle recruitment, coordination of respiratory effort with the ventilator, respiratory rate, and heart rate. Patients receiving NIV should be reviewed regularly to assess their response to treatment and to optimise the ventilator settings.
Timing of Blood Gas Measurements
The need for arterial blood gas analysis will be governed by the patient’s clinical progress, but should be measured in most patients after 1–2 hours of NIV and after 4–6 hours if the earlier sample showed little improvement, as recommended in 1. If there has been no improvement in PaCO2 and pH after this period, NIV should be discontinued and invasive ventilation considered.
Oxygen Saturation Monitoring
Oxygen saturation should be monitored continuously for at least 24 hours after commencing NIV and supplementary oxygen administered to maintain saturations between 85% and 90%, as suggested in 1.
From the Research
Blood Gas Analysis in NIV for COPD Patients
- Blood gas analysis is recommended to determine improvement in CO2 levels in patients with chronic respiratory failure secondary to COPD who are put on Non-Invasive Ventilation (NIV) 2, 3, 4.
- Arterial blood gas (ABG) analysis can help assess the effectiveness of NIV in improving respiratory function and reducing CO2 levels 2, 3.
- Studies have shown that NIV can reduce PaCO2 levels in COPD patients, with significant improvements observed after 3-6 months of treatment 3, 4.
Timing and Frequency of Blood Gas Analysis
- Blood gas analysis should be performed within 15 minutes of arrival and commencement of NIV, as recommended by guidelines 2.
- Repeat measurements should be taken after 1-2 hours of NIV initiation to assess improvements in respiratory function and CO2 levels 2, 5.
- Regular monitoring of blood gas levels can help identify patients who are at risk of late failure of NIV, allowing for timely interventions and adjustments to treatment 5.
Factors Associated with Improvement in CO2 Levels
- The use of NIV has been shown to reduce PaCO2 levels in COPD patients, with improvements observed in both stable and post-exacerbation patients 6, 3, 4.
- Factors such as nosocomial pneumonia, heart rate, and consciousness can affect the success of NIV and improvement in CO2 levels 5.
- The use of telemedicine and home initiation of NIV can also improve outcomes and reduce costs in COPD patients 4.