What is the target partial pressure of oxygen (pO2) in chronic obstructive pulmonary disease (COPD) patients still on ventilatory support after cardiac surgery?

Target pO2 in COPD Patients on Ventilatory Support After Cardiac Surgery

For COPD patients still on ventilatory support after cardiac surgery, target an oxygen saturation of 88-92% (corresponding to a pO2 of approximately 60-75 mmHg), using controlled oxygen delivery devices such as 24% or 28% Venturi masks or nasal cannulae at 1-2 L/min. 1, 2

Rationale for Lower Oxygen Targets

• COPD patients are at significant risk of hypercapnic respiratory failure when given excessive oxygen therapy, which can lead to respiratory acidosis, coma, and increased mortality. 2, 3
• A randomized controlled trial demonstrated significantly lower mortality (RR 0.22) in COPD patients receiving titrated oxygen to maintain SpO2 88-92% compared to those receiving high-concentration oxygen in the prehospital setting. 1
• Patients with COPD on oxygen therapy with a PO2 >10 kPa (75 mm Hg) are at risk of CO2 retention and may be assumed to have excessive oxygen therapy. 1
• The mechanisms for carbon dioxide retention are complex and include abolition of hypoxic drive, loss of hypoxic vasoconstriction, absorption atelectasis leading to increased dead-space ventilation, and the Haldane effect. 3

Initial Oxygen Management in the Postoperative Period

• In the perioperative and postoperative setting, COPD patients with known significant disease should receive oxygen from a 24% or 28% Venturi mask or 1-2 L/min from nasal cannulae, aiming at a saturation range of 88-92%. 1
• Venturi masks are preferred over nasal prongs for maintaining consistent oxygenation in COPD patients with acute respiratory compromise, as they provide more reliable oxygen delivery and result in less time with SpO2 <90%. 4
• For patients with respiratory rates >30 breaths/min, increase Venturi mask flow rates by up to 50% above the minimum specified to compensate for increased inspiratory flow demands. 1, 2

Blood Gas Monitoring Protocol

• Obtain arterial blood gases within 30-60 minutes of initiating oxygen therapy (or sooner if clinical deterioration occurs) to assess for hypercapnia and acidosis. 1, 2, 5
• If pH and PCO2 are normal on initial blood gases, continue targeting 88-92% saturation unless there is no history of previous hypercapnic respiratory failure. 2, 5
• If PCO2 is elevated but pH is ≥7.35 (indicating chronic compensated hypercapnia), maintain the 88-92% target range. 2, 5
• If the patient develops respiratory acidosis (pH <7.35 with elevated PCO2), consider non-invasive ventilation if acidosis persists for more than 30 minutes despite standard medical management. 2

Critical Safety Considerations

• Never abruptly discontinue oxygen therapy in hypercapnic patients, as oxygen levels will fall significantly over 1-2 minutes while carbon dioxide levels take much longer to correct, potentially causing life-threatening rebound hypoxemia. 1, 2, 5
• If a patient is found to have respiratory acidosis due to excessive oxygen therapy, step down oxygen delivery to 28% Venturi mask or oxygen at 1-2 L/min from nasal cannulae rather than discontinuing it. 1
• The presence of normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen, as pulse oximetry will be normal in patients with normal PO2 but abnormal pH or PCO2. 1

Special Postoperative Monitoring

• Patients with COPD may develop hypercapnic respiratory failure during hospitalization even if initial arterial blood gases were satisfactory, requiring vigilant ongoing monitoring. 5
• For patients on long-term home oxygen therapy for severe COPD, a senior clinician should consider establishing patient-specific target ranges based on their baseline status. 2, 5
• Avoid high blood oxygen levels as excessive oxygenation increases the risk of respiratory acidosis, particularly when PaO2 exceeds 10.0 kPa (75 mmHg). 1, 5