Can high Fraction of Inspired Oxygen (FiO2) levels cause confusion?

High FiO2 and Confusion: Mechanisms and Management

High FiO2 can cause confusion through multiple mechanisms, particularly in patients with COPD or other risk factors for hypercapnic respiratory failure. This occurs primarily through worsening V/Q mismatch, decreased ventilatory drive, and other physiological mechanisms that lead to hypercapnia, which can cause altered mental status.

Mechanisms of Confusion with High FiO2

• High FiO2 can worsen V/Q mismatch in susceptible patients, particularly those with COPD, leading to hypercapnia and subsequent confusion 1
• When high-concentration oxygen is administered to patients with COPD and other conditions predisposing to hypercapnic respiratory failure, it can cause respiratory acidosis through multiple mechanisms 1
• The primary mechanism is the reversal of hypoxic pulmonary vasoconstriction, causing increased perfusion to poorly ventilated lung units, worsening V/Q mismatch 1
• Decreased ventilatory drive ("hypoxic drive") contributes to a lesser extent, as relief of hypoxemia reduces respiratory stimulation 1
• The Haldane effect further decreases carbon dioxide buffering capacity of hemoglobin when FiO2 is increased 1
• Absorption atelectasis can occur at FiO2 30-50%, resulting in increased shunt 1
• The higher density and viscosity of oxygen compared to air increases work of breathing, which can contribute to hypercapnia in exhausted patients 1

Patient Populations at Risk

• Patients with COPD (especially severe or moderate with previous respiratory failure) 1
• Patients with severe chest wall or spinal disease (e.g., kyphoscoliosis) 1
• Those with neuromuscular disease, severe obesity, cystic fibrosis, or bronchiectasis 1
• Patients with previously unrecognized COPD 1

Clinical Manifestations and Monitoring

• Hypercapnia from high FiO2 can lead to confusion, drowsiness, and other features of carbon dioxide retention 1
• Respiratory acidosis (pH < 7.35 or H+ > 45 nmol/L with PCO2 > 6.0 kPa) can develop rapidly, within 15 minutes of high-concentration oxygen therapy in acute COPD 1
• Careful monitoring of respiratory rate and heart rate is essential, as tachypnea and tachycardia are more common than cyanosis in hypoxemic patients 1
• Blood gas measurements are crucial when patients require increased oxygen concentration or show unexpected changes in clinical status 1

Management Recommendations

• For patients at risk of hypercapnic respiratory failure, target oxygen saturation should be 88-92% 1
• For patients not at risk, aim for SpO2 94-98% 1
• Start with 28% or 24% oxygen or 1-2 L/min nasal oxygen in patients at risk of hypercapnic respiratory failure 1
• Reduce FiO2 if SpO2 > 92% in at-risk patients 1
• When nebulized bronchodilators are needed for patients with hypercapnic acidosis, they should be given using an ultrasonic nebulizer or a jet nebulizer driven by compressed air, with supplementary oxygen via nasal cannulae to maintain target saturation 1
• Any increase in FiO2 must be followed by repeat blood gases within 1 hour (or sooner if conscious level deteriorates) 1
• Avoid sudden cessation of supplementary oxygen therapy as it can cause dangerous rebound hypoxemia 1

Special Considerations

• In patients with traumatic brain injury, exposure to high arterial oxygen or high supplemental oxygen has been independently associated with increased mortality at 6 months 2
• In subarachnoid hemorrhage patients, hyperoxemia does not have strong clinical evidence of causing further brain injury, but caution is still advised 1
• After cardiac arrest, when resources are available to titrate FiO2 and monitor oxyhemoglobin saturation, it is reasonable to decrease FiO2 when saturation is 100%, provided saturation can be maintained at 94% or greater 1
• Equilibration time after FiO2 changes varies by patient condition - in mechanically ventilated ICU patients, it takes approximately 6 minutes (±3.4) to reach 90% of the final equilibrated PaO2, with COPD patients taking longer (7.1 ±2.1 minutes) than non-COPD patients (4.4 ±2.0 minutes) 3

Prevention of Confusion Related to High FiO2

• Use the lowest effective FiO2 to achieve target saturation ranges 1
• Carefully titrate oxygen therapy based on patient-specific risk factors 1
• Implement appropriate changes to early warning systems for patients at risk of hypercapnic respiratory failure 1
• Monitor blood gases regularly when increasing oxygen therapy, especially in at-risk patients 1
• Consider non-invasive ventilation or invasive ventilation for patients developing respiratory acidosis despite appropriate oxygen therapy 1

By understanding these mechanisms and following appropriate management strategies, clinicians can minimize the risk of confusion and other complications associated with high FiO2 administration.