How does Fraction of Inspired Oxygen (FIO2) change with changes in flow, Expiratory Positive Airway Pressure (EPAP), and Inspiratory Positive Airway Pressure (IPAP)?

How FIO2 Changes with Flow, EPAP, and IPAP in Non-Invasive Ventilation

When using non-invasive positive pressure ventilation (NPPV), the FiO2 decreases as IPAP increases, increases with higher oxygen flow rates, and is highest when oxygen is connected closest to the ventilator side of the circuit rather than near the patient interface.

Relationship Between FiO2 and Ventilator Parameters

Effect of Flow Rate on FiO2

• FiO2 increases with higher oxygen flow rates, though it is difficult to achieve FiO2 >0.30 without very high oxygen flows 1
• Typical oxygen flow rates through a T-connector result in approximately 31% FiO2 at 1 L/min, 37% at 2 L/min, 40% at 3 L/min, and 44% at 4 L/min 2
• Flow rates >4 L/min provide minimal additional increase in FiO2 and may cause patient-ventilator asynchrony by delaying triggering 2

Effect of IPAP on FiO2

• Increases in IPAP lead to decreases in FiO2 when all other variables remain constant 1
• Higher IPAP levels increase intentional leak in the circuit, which dilutes the oxygen concentration 2
• When IPAP levels exceed 12 cmH2O, oxygen flows should be at least 4 L/min to maintain adequate oxygenation 1

Effect of EPAP on FiO2

• Similar to IPAP, increases in EPAP can reduce FiO2 as the intentional leak increases with higher pressures 2
• The FiO2 does not appear to vary significantly with the amount of pressure support (difference between IPAP and EPAP) 2

Practical Considerations for Oxygen Delivery

Oxygen Connection Point

• The connection point of oxygen into the circuit significantly affects delivered FiO2 1
• Highest FiO2 is achieved when oxygen is added closest to the exhaust port on the ventilator side of the circuit 1
• A 3-orifice "T" shaped connector attached between the ventilator outlet and the hose allows for optimal oxygen delivery 2

Monitoring and Adjustments

• Continuous oxygen saturation monitoring is essential when using NIV, aiming for SpO2 88-92% in patients with acute hypercapnic respiratory failure 2
• Arterial or arterialised capillary blood gas analysis should be performed to assess pH, PaCO2, and PaO2 during NIV treatment 2
• When gas exchange improves with increased alveolar ventilation, NIV settings should be optimized before increasing FiO2 2

Special Considerations

Paradoxical Effects

• NPPV with low IPAP values and without oxygen supplementation can paradoxically lead to FiO2 <0.21 at the circuit-patient interface 1
• When respiratory frequency increases, the effective FiO2 may decrease due to increased minute ventilation and dilution of oxygen 2

Ventilator Types and Interfaces

• Different ventilator types and interfaces may affect FiO2 delivery differently 2
• In volume-targeted pressure support ventilation, the device automatically varies pressure support to deliver targeted tidal volume, which can affect FiO2 2

Clinical Implications

• For patients requiring high levels of ventilatory support (IPAP >12 cmH2O), higher oxygen flow rates are needed to maintain adequate FiO2 1
• A ventilator with an integral oxygen blender is recommended if oxygen at 4 L/min fails to maintain SpO2 >88% 2
• When adjusting ventilator settings, allow time for equilibration and reassess within 1 hour of any change in FiO2 or ventilator settings 2

Pitfalls to Avoid

• Increasing flow rates above 4 L/min may cause patient-ventilator asynchrony by delaying triggering 2
• Occlusion of the exhaust port (e.g., by sputum) can exacerbate hypercapnia through rebreathing 2
• Sudden increases in IPAP or EPAP without adjusting oxygen flow may lead to unexpected drops in FiO2 2
• When using high-flow oxygen with NIV, ensure the circuit has adequate mixing and provides a reservoir of enriched gas for inspiration 2