BiPAP Oxygenation Management: FiO2 vs EPAP
Increase FiO2 first when trying to improve oxygenation on BiPAP, as this is the most direct and physiologically sound approach to address hypoxemia. 1
Physiological Rationale
The British Thoracic Society guidelines clearly establish that increasing FiO2 directly raises alveolar oxygen tension (PAO2), which increases arterial oxygen (PaO2) in poorly ventilated lung units (low V/Q ratio conditions). 1 This mechanism works regardless of whether the patient has hypoventilation, diffusion limitation, or V/Q mismatch. 1
EPAP (equivalent to PEEP in BiPAP) serves a different primary function: it recruits collapsed alveoli and maintains airway patency, which can improve oxygenation secondarily by improving V/Q matching. 2 However, this is not the first-line adjustment for hypoxemia.
Practical Algorithm for Oxygenation Management
Step 1: Increase FiO2
- Start by increasing supplemental oxygen flow by 1 L/min increments at intervals no shorter than 15 minutes until SpO2 targets are achieved. 3
- Target SpO2 90-96% in most patients (88-92% in type 2 respiratory failure). 3, 4
- FiO2 can be titrated up to 0.6-1.0 (60-100%) if needed. 3
Step 2: Consider EPAP Adjustment Only If:
- FiO2 requirements exceed 0.60 (to avoid oxygen toxicity). 1
- There is evidence of atelectasis or alveolar collapse requiring recruitment.
- The patient has significant intrinsic PEEP (common in COPD) that needs to be overcome. 2, 4
Step 3: EPAP Titration (When Indicated)
- Increase EPAP from baseline (typically 4-5 cmH2O) to 8-10 cmH2O in 1-2 cmH2O increments. 4, 5
- For hypoxemic respiratory failure requiring higher support, EPAP can be increased to 12-15 cmH2O. 3
Critical Caveat: The FiO2 Paradox with BiPAP
Higher EPAP (and IPAP) settings actually decrease the effective FiO2 delivered for a given supplemental oxygen flow rate due to increased intentional leak in the circuit. 1, 3, 6 This creates a counterintuitive situation where raising EPAP to improve oxygenation may paradoxically reduce the delivered oxygen concentration unless you simultaneously increase oxygen flow. 6
Specifically:
- When IPAP exceeds 12 cmH2O, oxygen flows should be at least 4 L/min to maintain adequate FiO2. 6
- The effective FiO2 varies with pressure settings but not with respiratory rate or pressure support level. 1, 3
- Oxygen should be connected close to the ventilator outlet (not at the mask) to maximize FiO2 delivery. 1, 6
When EPAP Should Be Your Primary Adjustment
There are specific clinical scenarios where EPAP adjustment takes priority:
Type 2 Respiratory Failure with Hypercapnia
- If the primary problem is ventilation (elevated CO2), increase IPAP first, not EPAP or FiO2. 4
- EPAP at 5 cmH2O helps overcome intrinsic PEEP in COPD but doesn't directly improve ventilation. 4
- Only increase EPAP to 8-10 cmH2O if re-breathing is suspected. 4
Cardiogenic Pulmonary Edema
- EPAP provides afterload reduction and improves cardiac function in acute heart failure. 1
- In this specific population, EPAP adjustment may be as important as FiO2 for improving oxygenation.
Monitoring and Reassessment
- Reassess within 1-2 hours of any adjustment to determine effectiveness. 3
- Pulse oximetry may overestimate actual arterial saturation, so targeting SpO2 90-94% provides a safety margin. 1, 3
- If oxygenation fails to improve despite FiO2 >0.60 and optimized EPAP, proceed to intubation rather than continuing to escalate non-invasive support. 3, 4
Common Pitfalls to Avoid
- Do not reflexively increase EPAP for hypoxemia without first optimizing FiO2, as this increases work of breathing and patient discomfort without directly addressing the oxygen deficit. 2
- Do not forget that increasing BiPAP pressures reduces effective FiO2, requiring simultaneous increase in oxygen flow. 1, 6
- Avoid excessive EPAP in patients with severe airflow obstruction, as this can worsen air trapping and hemodynamic compromise. 2
- In acute myocardial infarction, use BiPAP cautiously as some evidence suggests higher MI rates compared to CPAP. 1, 2