Ventilator Settings, Lung Injury & Protective Strategies
Ventilator Settings, Lung Injury & Protective Strategies
Clay Freeman, DNP, CRNA
Kristin Barkley, DNP, CRNA
Why ventilator settings matter
Ventilator modes define how breaths are delivered.
Ventilator settings define how safe or harmful those breaths become.
Poorly chosen settings can cause hypoxia, hypercapnia, hemodynamic compromise,
and ventilator-induced lung injury, even in patients with normal lungs.
Tidal Volume (VT)
Tidal volume determines alveolar ventilation and contributes directly to lung stress.
Recommended VT = 6–8 mL/kg IBW
- Excessive VT → volutrauma
- Inadequate VT → hypercapnia
Lung-protective strategies favor lower VT & PEEP, especially in patients with obesity, ARDS, restrictive disease, or during laparoscopy.
Respiratory Rate (RR)
RR controls minute ventilation and PaCO₂.
- Increasing RR → lowers PaCO₂
- Decreasing RR → allows longer exhalation
Normal RR = 8–12 breaths per minute
Excessive RR increases the risk of air trapping and auto-PEEP,
especially in obstructive lung disease.
I:E Ratio
The inspiratory-to-expiratory ratio affects mean airway pressure, oxygenation,
and carbon dioxide elimination.
- Normal: 1:2
- Decreasing ratio improves hypercapnia
- Inverse ratios (2:1, 1:1): increase mean airway pressure
- Longer expiratory times: improve CO₂ elimination
- Can set 2:1 – 1.8
Clinical implications
- Inverse ratio ventilation may improve oxygenation in ARDS.
- Prolonged inspiration increases risk of auto-PEEP.
- Obstructive disease requires long expiratory times to clear CO₂.
Positive End-Expiratory Pressure (PEEP)
PEEP prevents alveolar collapse at end expiration and improves oxygenation.
- Typical range: 5–10 cmH₂O
- Improves V/Q mismatch and hypoxia
- Increases functional residual capacity
Physiologic consequences
- Increased intrathoracic pressure → ↓ venous return
- May reduce preload and cardiac output
Clinical use
- Obesity
- Laparoscopic surgery
- Atelectasis (for recruitment)
- ARDS (high PEEP)
Peak Inspiratory Pressure (PIP)
PIP is the highest circuit pressure during inspiration.
PIP reflects:
- Airway resistance
- Inspiratory flow
- Alveolar pressure
Adjustable for Pressure Control Ventilation
Important distinction
High PIP does not always mean high alveolar pressure.
Resistance-related pressure (e.g., bronchospasm, kinked tube)
can obscure true lung stress.
Fresh Gas Flow (FGF) & FiO₂
Fresh Gas Flow
- Minimal flows reduce anesthetic waste.
- High flows may alter delivered pressures in PCV modes.
FiO₂
- Risk of oxygen toxicity and hyperoxia
- 💡 Absorption atelectasis
- Typical intraoperative target: 0.3–0.5
💡 Absorption Atelectasis Spotlight 💡
- Alveoli normally stay open because they contain a mix of gases (mainly nitrogen and oxygen).
- Nitrogen acts as a “splint” because it is poorly absorbed and helps maintain alveolar volume.
- When a patient breathes high concentrations of oxygen (especially near 100% FiO₂), nitrogen is washed out.
- Oxygen is rapidly absorbed into the pulmonary capillary blood.
- If ventilation to that alveolus is reduced or obstructed, the gas is absorbed faster than new gas enters → alveolar collapse.
Ventilator-Induced Lung Injury (VILI)
Mechanisms
- Volutrauma — excessive tidal volumes
- Barotrauma — excessive pressures
- Atelectrauma — repetitive alveolar collapse
- Biotrauma — inflammatory mediator release
Risk factors
- High VT
- High PIP
- Inadequate PEEP
- Prolonged mechanical ventilation
Protective Ventilation Strategies
- Lower tidal volumes
- Adequate PEEP
- Avoid unnecessarily high FiO₂
- Adjust RR to prevent auto-PEEP
- Monitor pressures and volumes continuously
Recruitment Maneuvers
Recruitment maneuvers temporarily increase airway pressure to reopen collapsed alveoli.
- May improve oxygenation
- Transient hypotension may occur (increased intrathoracic pressure)
- Use cautiously in hemodynamically unstable patients
Ventilator adjustments during laparoscopic surgery
Physiologic changes
- CO₂ insufflation (pneumoperitoneum) increases PaCO₂
- Diaphragmatic displacement occurs
- Reduced lung volumes (V/Q mismatch)
- Increased airway pressures
Clinical consequences
- Maintain ETCO₂ and PIP via ventilator adjustments — may need to switch to Pressure Control Ventilation for adequate ventilation
- Hypercarbia (PaCO₂ 55–70 mmHg) → myocardial depression, dysrhythmias, peripheral vasodilation, patient will be slow to wake/obtunded
- Increased pulmonary vascular resistance from increased CO₂ → RV strain
Mode considerations
- 💡 PCV-VG preferred over PCV
- VCV may result in rising PIP — Use caution!
💡 Pressure Controlled Ventilation with Volume Guarantee (PCV-VG) Spotlight 💡
- The ventilator uses pressure-controlled breaths with a decelerating flow pattern.
- You set a target tidal volume.
- The ventilator automatically adjusts inspiratory pressure breath-to-breath to achieve that volume based on lung compliance and resistance.
- There is a pressure limit, helping reduce barotrauma.
Why it’s useful:
- Combines the lung-protective benefits of pressure control with the reliability of guaranteed tidal volume.
- Helpful when compliance may change (e.g., laparoscopy, positioning, obesity, pediatrics).
Ventilator changes with patient positioning
Trendelenburg
- VCV: VT constant, PIP increases
- PCV: PIP constant, VT decreases
Reverse Trendelenburg
- Improved lung compliance
- Lower airway pressures
Prone positioning
- May decrease lung compliance
Ventilation in disease states
Obstructive disease
- Chronic CO₂ retention common
- Normalizing PaCO₂ may cause alkalosis
- Slower RR improves exhalation
- Avoid high VT and high inspiratory flow
Restrictive disease
- Low lung compliance
- Rapid, shallow breathing pattern
- Lower VT with higher RR
- High risk for barotrauma
SCOPE GUIDE
Strategies
- Adjust settings proactively during surgical changes.
- Use PEEP intentionally, not reflexively.
- Balance oxygenation with hemodynamics.
Clinical Optimization
- Monitor trends, not single values.
- Anticipate changes with positioning and pneumoperitoneum.
- Tailor ventilation to lung mechanics, not body size.
Pearls
- High pressures are warnings, not diagnoses.
- PEEP can both help and harm.
- Protective ventilation applies to all patients.
