Lung Protective Ventilation

Part of: Mechanical Ventilation

Ventilator-Associated Lung Injury (VALI)

• Most frequently complicates ALI and ARDS
• Types include:
  • Barotrauma: caused by excessively high airway pressures
  • Volutrauma: caused by excessive $V_{\text{T}}$
  • Biotrauma
  • Cyclic atelectasis
• Oxygen toxicity
  • For patients on bleomycin, oxygen toxicity can occur at lower $F_{\text{i}}\mathrm{O}{}_2$

Barotrauma

• Manifestations
  • Pneumothorax
  • Pneumomediastinum
  • Subcutaneous emphysema
  • Systemic gas embolism
  • Cystic barotrauma
• Risk factors
  • High $P_{\text{plateau}}>32.5\pm 2.5\text{cm}\mathrm{H}{}_2\mathrm{O}$
  • High minute ventilation
  • Non-homogenous parenchymal disease (e.g. ARDS)
  • Necrotising lung pathology
  • Secretion retention

Biotrauma

• Release of proinflammatory cytokines in response to supranormal intraalveolar pressures; occurs in the absence of physical damage to lung architecture (as in barotrauma)
• Most clinically relevant manifestation is pulmonary and interstitial oedema

Protective Ventilation Principles

• Principle feature is low tidal volumes
• Permissive hypercapnoea: $P_{\text{a}}\mathrm{CO}{}_2$ is allowed to climb, with a resulting drop in arterial pH
  • Contraindicated in: increased ICP, haemodynamic instability, right heart failure, severe metabolic acidosis
• Open lung ventilation: strategy that combines low tidal volumes and high PEEP

ARDS Protocol

ARDS Ventilation Protocol

1. Choose ventilation mode (typically AC or SIMV)
2. Start with $V_{\text{T}}$ of 6 mL/kg IBW
3. Start with PEEP at ≥ 8 cm $\mathrm{H}{}_2\mathrm{O}$
4. Set the I:E ratio of 1:2
5. Measure and record $P_{\text{plateau}}$ every 4 hours and after any changes in $V_{\text{T}}$ or PEEP

• If $P_{\text{plateau}}$ >30 cm $\mathrm{H}{}_2\mathrm{O}$ → ↓ $V_{\text{T}}$ in 1 mL/kg increments until $P_{\text{plateau}}$ ≤30 cm $\mathrm{H}{}_2\mathrm{O}$ or to minimum of 4 mL/kg IBW
• If $V_{\text{T}}$ < 6 mL/kg IBW and $P_{\text{plateau}}$ <25 cm $\mathrm{H}{}_2\mathrm{O}$ → ↑ $V_{\text{T}}$ by 1 mL/kg IBW increments to a max of 6 mL/kg

6. Adjust the RR and $V_{\text{T}}$ according to pH goals:

• If pH <7.30 → consider ↑ RR to as high as 35 breaths/min while monitoring for development of auto-PEEP
• If pH <7.15 and RR ≥ 35 breaths/min → consider ↑ $V_{\text{T}}$ and suspending $P_{\text{plateau}}$ limit

7. Adjust I:E ratio to avoid auto-PEEP and dyssynchrony
8. Adjust PEEP to maximise alveolar recruitment while avoiding over-distention:

• ↑ or ↓ PEEP in increments of 2-3 cm of $\mathrm{H}{}_2\mathrm{O}$
• Select PEEP that gives the best compliance

9. Adjust the $F_{\text{i}}\mathrm{O}{}_2$ to achieve $S_{\text{p}}\mathrm{O}{}_2$ of 88-95% and/or $P_{\text{a}}\mathrm{O}{}_2$ of 55-80 mmHg

Improving CO₂ Clearance

• Expiration is a passive process depending on the pressure generated by the recoil of the chest wall and lung tissue
• In the presence of significant airway resistance (e.g. Asthma Exacerbation or COPD Exacerbation) not enough pressure is generated to adequately empty the lung in reasonable amounts of time
• Decrease the I:E ratio (e.g. to 1:3 or 1:4):
  • Increases clearance of $\mathrm{CO}{}_2$
  • Decreases gas trapping and auto-PEEP
  • Poorer oxygenation because of decreased mean airway pressure (although usually offset by intrinsic PEEP)
  • Decreased haemodynamic impact of positive pressure