PEEP vs FiO₂ Guide
How to manage oxygenation in mechanically ventilated patients — understanding PEEP, FiO₂, and when to prioritize each.
Two Levers for Oxygenation
When managing oxygenation in a mechanically ventilated patient, the respiratory therapist has two primary ventilator parameters to adjust: FiO₂ (fraction of inspired oxygen) and PEEP (positive end-expiratory pressure). Understanding when and how to use each — and how they interact — is fundamental to safe, effective ventilator management.
Both interventions increase PaO₂ and SpO₂, but through distinct mechanisms. FiO₂ directly increases the partial pressure of oxygen available for diffusion across the alveolar-capillary membrane. PEEP maintains alveolar recruitment, prevents end-expiratory collapse, increases functional residual capacity (FRC), and reduces intrapulmonary shunt. Understanding these mechanisms guides when each is more appropriate.
What Is PEEP?
PEEP is the airway pressure maintained at the end of exhalation during mechanical ventilation. In healthy lungs, this is effectively zero (atmospheric). Adding PEEP prevents alveoli from fully collapsing at end-expiration, which:
- Increases functional residual capacity (FRC)
- Recruits previously atelectatic lung units
- Reduces intrapulmonary shunt fraction
- Improves V/Q matching and oxygenation
- Reduces the work of reopening collapsed alveoli with each breath (atelectrauma)
Even a minimum PEEP of 5 cmH₂O is used in most intubated patients to prevent atelectasis caused by the endotracheal tube bypassing the glottis and eliminating physiologic PEEP.
What Does FiO₂ Do?
FiO₂ is the concentration of oxygen in the inspired gas mixture, ranging from 0.21 (room air) to 1.0 (100% oxygen). Increasing FiO₂ raises the alveolar oxygen partial pressure (PAO₂), increasing the driving gradient for oxygen diffusion into the blood.
FiO₂ improves oxygenation from all mechanisms of hypoxemia, including V/Q mismatch and diffusion impairment. However, it has limited effect on true shunt — areas where blood passes through completely unventilated lung (such as consolidated or atelectatic regions) do not benefit much from increased FiO₂ because there is no gas exchange occurring regardless of oxygen concentration.
This is a key clinical insight: if a patient is not responding to FiO₂ increases but responds to PEEP, the mechanism of hypoxemia is likely intrapulmonary shunt, and lung recruitment is more important than increasing the oxygen fraction.
Oxygen Toxicity and FiO₂ Limits
Prolonged exposure to high FiO₂ causes oxygen toxicity — free radical-mediated lung injury that can worsen the very respiratory failure being treated. The threshold for concern is generally FiO₂ ≥ 0.60 sustained for more than 24–48 hours.
Clinical Practice Principle
Always use the minimum FiO₂ necessary to achieve the oxygenation goal. When FiO₂ exceeds 0.60 and oxygenation remains inadequate, PEEP optimization should be prioritized over further FiO₂ increases. Target PaO₂ 55–80 mmHg or SpO₂ 88–95% in most ventilated patients (lower target for ARDS per ARDSNet).
In ARDS patients, the ARDSNet protocol specifically provides PEEP-FiO₂ tables to guide coordinated adjustments and minimize FiO₂ while maintaining adequate oxygenation.
ARDSNet PEEP-FiO₂ Tables
The ARDSNet trial established two PEEP-FiO₂ tables — one for lower PEEP and one for higher PEEP strategies:
Lower PEEP Table
| FiO₂ | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9–1.0 |
|---|---|---|---|---|---|---|---|
| PEEP | 5 | 5–8 | 8–10 | 10 | 10–14 | 14 | 14–18/24 |
Values represent PEEP in cmH₂O. Higher PEEP table uses more aggressive PEEP levels at equivalent FiO₂. Selection between tables is individualized based on patient response, hemodynamics, and recruitable lung volume.
Risks of Excessive PEEP
While PEEP improves oxygenation, too much PEEP can cause harm:
Hemodynamic Compromise
High intrathoracic pressure reduces venous return, decreasing cardiac preload and output. Can cause hypotension, especially in hypovolemic patients.
Overdistension / Volutrauma
Excessive PEEP overdistends compliant alveoli adjacent to consolidated regions, increasing driving pressure and injury to healthy lung.
Auto-PEEP Masking
External PEEP added to existing auto-PEEP (air trapping) can critically elevate total PEEP, worsening hemodynamics and barotrauma risk.
Increased ICP
High PEEP can increase intracranial pressure by impairing cerebral venous drainage. Use cautiously in patients with TBI or elevated ICP.
Practical Decision Framework
When oxygenation is inadequate, use this general approach:
Check the current settings
Confirm actual delivered FiO₂, set PEEP, and measured plateau pressure before making changes.
Identify the mechanism of hypoxemia
V/Q mismatch responds to both FiO₂ and PEEP. True shunt (e.g., atelectasis, ARDS consolidation) responds better to PEEP recruitment.
If FiO₂ is below 0.60
Increasing FiO₂ is the first and safest step. Less risk of hemodynamic effects than increasing PEEP.
If FiO₂ is 0.60 or above
Prioritize increasing PEEP rather than further FiO₂ increases to reduce oxygen toxicity risk. Increase PEEP in increments of 2–3 cmH₂O while monitoring plateau pressure and hemodynamics.
Reassess ABG and hemodynamics
After any change, reassess oxygenation, acid-base status, and hemodynamic effect before further adjustments.
Frequently Asked Questions
Is 100% FiO₂ safe short-term?
Brief use of FiO₂ 1.0 (such as during preoxygenation before intubation or during acute decompensation) is acceptable. The concern arises with prolonged exposure (> 24 hours) at high FiO₂. In acute emergency situations, airway and hemodynamics take priority over oxygen toxicity concerns.
What is the P/F ratio and how does it guide management?
P/F ratio = PaO₂ ÷ FiO₂. It normalizes oxygenation for the amount of supplemental oxygen being given. A P/F ratio below 300 defines ARDS. As P/F ratio falls below 200, higher PEEP strategies and prone positioning should be considered.
How does PEEP affect cardiac output?
PEEP increases intrathoracic pressure, which can reduce venous return (preload) and decrease cardiac output. This effect is more pronounced at higher PEEP levels and in volume-depleted patients. Hemodynamic assessment (blood pressure, heart rate, urine output, central monitoring) is essential when titrating PEEP.
What is intrinsic PEEP (auto-PEEP)?
Intrinsic PEEP occurs when incomplete exhalation leads to progressive air trapping. The total PEEP in the lung is the sum of set PEEP and auto-PEEP. Auto-PEEP is detected by performing an end-expiratory pause. In COPD and asthma patients on the ventilator, minimizing auto-PEEP through adequate expiratory time is critical.
Summary
- FiO₂ increases the fraction of O₂ available for diffusion — effective for V/Q mismatch.
- PEEP recruits alveoli and reduces shunt — more effective when atelectasis drives hypoxemia.
- Target minimum FiO₂ to avoid oxygen toxicity — wean FiO₂ below 0.60 when possible.
- Follow ARDSNet PEEP-FiO₂ tables in ARDS to coordinate adjustments safely.
- Excessive PEEP can cause hemodynamic compromise, overdistension, and worsened auto-PEEP.
- Always reassess with ABG and clinical parameters after any oxygenation adjustment.
Advanced Ventilator References in RTB2
RTB2 offers faster bedside references and advanced tools inside the mobile app — including ARDSNet PEEP-FiO₂ tables, oxygenation calculators, and lung-protective ventilation guides.
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