From the Guidelines
Administering too much oxygen to a hypercapnic patient can be dangerous and should be avoided, with the goal of providing controlled oxygen therapy targeting oxygen saturations of 88-92% rather than the usual 94-98% for most patients. For hypercapnic patients, use low-flow oxygen delivery systems such as nasal cannula at 1-2 L/min or a Venturi mask set to deliver 24-28% oxygen 1. Avoid high-flow oxygen or non-rebreather masks unless absolutely necessary. The reason for this cautious approach is that hypercapnic patients (often those with COPD, obesity hypoventilation syndrome, or neuromuscular disorders) rely partly on hypoxic drive for respiratory stimulation. Excessive oxygen can remove this hypoxic stimulus, potentially worsening CO2 retention and leading to respiratory acidosis, confusion, and even respiratory failure. Additionally, oxygen-induced hyperoxia can cause V/Q mismatch and absorption atelectasis, further compromising gas exchange.
Key Considerations
- Monitor these patients closely with regular arterial blood gas measurements to assess both oxygenation and CO2 levels, and be prepared to provide ventilatory support if respiratory failure develops despite appropriate oxygen therapy 1.
- If a patient is suspected to have hypercapnic respiratory failure due to excessive oxygen therapy, the oxygen therapy must be stepped down to the lowest level required to maintain a saturation range of 88–92% 1.
- Sudden cessation of supplementary oxygen therapy can cause life-threatening rebound hypoxaemia with a rapid fall in oxygen saturations below the starting oxygen saturation prior to the start of supplementary oxygen therapy 1.
Management Strategies
- Use of oxygen alert cards to guide oxygen therapy in patients with chronic respiratory conditions 1.
- Limiting oxygen-driven nebulisers to 6 minutes in patients with COPD, and exploring the feasibility of introducing battery-powered, air-driven nebulisers or portable ultrasonic nebulisers 1.
- Stepping down oxygen therapy to the lowest level required to maintain a saturation range of 88–92% in patients with suspected hypercapnic respiratory failure due to excessive oxygen therapy 1.
From the Research
Mechanisms of Oxygen-Induced Hypercapnia
- Oxygen therapy can lead to various adverse effects, including hypercapnia, in patients with chronic obstructive pulmonary disease (COPD) and other chronic lung diseases 2.
- The mechanisms leading to oxygen-induced hypercapnia include the abolition of 'hypoxic drive', loss of hypoxic vasoconstriction, and absorption atelectasis, resulting in an increase in dead-space ventilation and the Haldane effect 2, 3.
Risk Factors for Hypercapnia
- Patients with COPD, morbid obesity, asthma, cystic fibrosis, chest wall skeletal deformities, bronchiectasis, chest wall deformities, or neuromuscular disorders are at risk of developing hypercapnia during supplemental oxygen therapy 2.
- The risk of hypercapnia is not restricted to COPD only, and it has been reported in other chronic lung diseases 2.
Oxygen Therapy Guidelines
- The international guideline recommends a target oxygen saturation of 88% to 92% in patients with acute exacerbations of COPD and other chronic lung diseases at risk of hypercapnia 2.
- Oxygen should be administered only when oxygen saturation is below 88% 2.
Clinical Implications
- High concentration oxygen therapy in hypercapnic COPD is associated with increased mortality 4.
- Over-oxygenation is common in hypercapnic COPD inpatients, and rates of oxygen prescription are suboptimal 4.
- Controlled oxygen therapy in patients with severe respiratory failure greatly reduces the risk of unwanted increase of PaCO2, but does not exclude it completely 5.
Treatment Options
- Long-term non-invasive positive pressure ventilation (NPPV) is increasingly used in chronic hypercapnic COPD, and it is feasible and effective in stable, non-exacerbated COPD patients with daytime hypercapnia 6.
- Home-NPPV with a PaCO2-reductive approach might be considered as an additional treatment option in patients with stable chronic hypercapnic COPD 6.