Mechanical ventilation is a life-saving intervention that supports or replaces spontaneous breathing. A firm understanding of ventilator settings and modes is critical for patient safety and optimized outcomes.
Initial Ventilator Settings
1. Mode
Assist-Control (AC) Mode: Common initial mode.
- Delivers preset tidal volume or pressure at a preset rate.
- If the patient initiates a breath, the ventilator completes it.
- Ensures minimum minute ventilation.
AC mode is ideal in early respiratory failure when full support is needed.
2. Tidal Volume (VT)
- Healthy lungs: 6–8 mL/kg of Ideal Body Weight (IBW).
- COPD: 6–8 mL/kg (lower end preferred).
- ARDS: 4–6 mL/kg IBW (lung-protective strategy to reduce barotrauma).
Avoid high VT to reduce the risk of volutrauma.
3. Respiratory Rate (RR)
- Initial: 12–16 breaths/min
- Adjust based on:
- pH/PaCO₂
- Patient’s spontaneous effort
4. FiO₂ (Fraction of Inspired Oxygen)
- Initially set at 100% post-intubation to avoid hypoxemia.
- Rapidly titrate down to the lowest level needed to maintain PaO₂ ≥ 60 mmHg or SpO₂ ≥ 90%.
Prolonged high FiO₂ can lead to oxygen toxicity and absorptive atelectasis.
5. PEEP (Positive End-Expiratory Pressure)
- Initial setting: 5 cm H₂O
- In ARDS or refractory hypoxemia: titrate up (e.g., 8–15 cm H₂O)
Mechanism: Prevents alveolar collapse, increases functional residual capacity (FRC), and improves oxygenation.
6. Sigh Breaths
- Not routinely used.
- If needed: Sigh = 1.5–2x VT, delivered 6–8 times/hour, especially in low VT strategies.
Calculations and Rules
Ideal Body Weight (IBW)
- Men: IBW = 50 + 2.3 × (height in inches − 60)
- Women: IBW = 45.5 + 2.3 × (height in inches − 60)
Used instead of actual body weight to avoid overestimating lung volume.
Minute Ventilation (VE)
- Formula: IBW (kg) × 100 mL/min
- Example: 60 kg patient → 60 × 100 = 6 L/min
Common Ventilator Settings Across Modes
| Setting | Description |
|---|---|
| FiO₂ | Fraction of inspired oxygen (21–100%) |
| PEEP | Maintains alveolar patency; initial 5 cm H₂O |
| Trigger Sensitivity | Determines how easily the ventilator detects a patient’s effort (flow or pressure) |
| I:E Ratio | Ratio of inspiratory to expiratory time (Normal: 1:2) |
Asthma/COPD: Prolong expiratory time (e.g., I:E = 1:3–1:4) to avoid air trapping.
Common Modes of Mechanical Ventilation
1. Volume Assist-Control (AC-VC)
- Delivers a set tidal volume at a fixed or triggered rate.
- Patient can trigger additional breaths.
- If patient fails to initiate, the ventilator delivers mandatory breaths.
High-Yield: Risk of barotrauma if lung compliance worsens.
2. Pressure Assist-Control (AC-PC)
- Delivers a preset pressure, not volume.
- Volume delivered varies with lung compliance and resistance.
Use in: ARDS (helps avoid volutrauma by limiting airway pressures).
3. Pressure Support Ventilation (PSV)
- Patient initiates every breath.
- Ventilator supports each breath with preset pressure.
- Tidal volume depends on patient effort and lung mechanics.
Best for weaning and patients with intact respiratory drive.
Inspiratory Flow
-
Normal flow: 50–60 L/min
-
Faster flow allows shorter inspiratory time, increases expiratory time.
Adjust in obstructive disease (e.g., asthma) to prevent auto-PEEP.
I:E Ratio (Inspiratory:Expiratory Time)
| Condition | Suggested I:E Ratio | Rationale |
|---|---|---|
| Normal | 1:2 | Mimics physiologic breathing |
| COPD/Asthma | 1:3 – 1:4 | Prevents air trapping, allows full exhalation |
| ARDS | 1:1 – 2:1 (inverse) | Improves oxygenation by increasing mean airway pressure |
Lung-Protective Ventilation (ARDSNet Protocol)
| Parameter | Target Range |
|---|---|
| VT | 4–6 mL/kg IBW |
| Plateau Pressure | < 30 cm H₂O |
| PEEP/FiO₂ | Guided by ARDSNet table |
| Permissive Hypercapnia | Acceptable if pH > 7.2 |
Minimize barotrauma and volutrauma while optimizing gas exchange.
High-Yield Clinical Pearls
- PEEP improves oxygenation, but excessive levels may ↓ CO and cause barotrauma.
- Auto-PEEP is common in obstructive lung diseases; allow adequate expiratory time.
- High FiO₂ > 60% for prolonged periods can cause oxygen toxicity.
- ARDS requires low VT, high PEEP, and often inverse I:E ratio.
- Sighs are rarely needed in lung-protective strategies unless using very low VT.
- Patient-ventilator synchrony is key: assess for dyssynchrony, agitation, tachypnea.
