What are the recommended pressure and volume strategies for ventilated patients with Acute Respiratory Distress Syndrome (ARDS), status asthmaticus, and type 2 respiratory failure?

Ventilation Strategy for ARDS with Status Asthmaticus and Type 2 Respiratory Failure

Use lung-protective ventilation with tidal volumes of 4-8 mL/kg predicted body weight and plateau pressures ≤30 cmH₂O for all patients with ARDS, regardless of concurrent asthma or hypercapnia. 1, 2, 3

Core Ventilation Parameters

Volume Settings:

• Set tidal volume at 6 mL/kg predicted body weight (acceptable range 4-8 mL/kg PBW) 1, 2, 3
• Calculate predicted body weight: Males = 50 + 0.91 × [height (cm) - 152.4] kg; Females = 45.5 + 0.91 × [height (cm) - 152.4] kg 2, 3
• Never exceed 8 mL/kg PBW even if plateau pressures appear acceptable—both parameters must be optimized simultaneously 2, 3

Pressure Targets:

• Maintain plateau pressure ≤30 cmH₂O as an absolute ceiling 1, 2, 3
• Target driving pressure (plateau pressure - PEEP) ≤15 cmH₂O—this predicts mortality better than tidal volume or plateau pressure alone 2
• Accept permissive hypercapnia with pH >7.20 as a consequence of lung protection 3

PEEP Strategy Based on ARDS Severity

For Moderate-to-Severe ARDS (PaO₂/FiO₂ <200 mmHg):

• Use higher PEEP (typically >10 cmH₂O) 1, 2, 3
• Higher PEEP reduces mortality in this population (adjusted RR 0.90) 2
• This recommendation takes precedence despite concerns about air trapping in asthma 1, 4

For Mild ARDS (PaO₂/FiO₂ 200-300 mmHg):

• Lower PEEP may be appropriate 3, 4

Critical Consideration for Asthma Component:

• Set PEEP between 4-8 cmH₂O initially to counteract auto-PEEP and improve ventilator triggering 5
• Monitor closely for auto-PEEP and dynamic hyperinflation 5
• Allow adequate expiratory time (I:E ratio approximately 1:2 or 1:3) to prevent air trapping 5

Prone Positioning for Severe ARDS

For severe ARDS with PaO₂/FiO₂ <150 mmHg:

• Implement prone positioning immediately for at least 12-16 hours daily—this is a strong recommendation that reduces mortality (RR 0.74) 1, 2, 3
• Do not delay prone positioning even with concurrent asthma—early implementation improves outcomes 3

Managing the Dual Pathophysiology

The ARDS component dominates management priorities:

• Lung-protective ventilation prevents ventilator-induced lung injury which worsens both ARDS and outcomes 1, 4
• The inflammatory pulmonary edema of ARDS requires low tidal volumes regardless of underlying asthma 1

Addressing the asthma/obstructive component:

• Monitor for auto-PEEP by checking end-expiratory flow and pressure-time waveforms 5
• Adjust expiratory time to allow complete exhalation 5
• Bronchodilator therapy should continue but does not change core ventilation strategy 3

Managing Type 2 Respiratory Failure (Hypercapnia):

• Accept permissive hypercapnia as necessary to maintain lung-protective ventilation 3, 4
• Maintain pH >7.20 rather than normalizing PaCO₂ 3
• Do not increase tidal volumes above 8 mL/kg PBW to correct hypercapnia—this causes more harm than the hypercapnia itself 2, 3

Additional Interventions

Neuromuscular Blockade:

• For early severe ARDS with PaO₂/FiO₂ <150 mmHg, use neuromuscular blocking agents for up to 48 hours 2, 3

Corticosteroids:

• Administer systemic corticosteroids to mechanically ventilated patients with ARDS 2, 3
• This addresses both ARDS inflammation and asthma exacerbation 2

Fluid Management:

• Use a conservative fluid strategy in established ARDS without tissue hypoperfusion 2, 3

Oxygenation Targets:

• Target SpO₂ of 88-95% to avoid hyperoxia while maintaining adequate oxygenation 2, 3

Critical Pitfalls to Avoid

Do not prioritize normocapnia over lung protection:

• Increasing tidal volumes to correct hypercapnia will worsen ventilator-induced lung injury and increase mortality 2, 3
• The type 2 respiratory failure is managed with permissive hypercapnia, not higher tidal volumes 3, 4

Do not use excessive tidal volumes despite obstructive physiology:

• Even with concurrent asthma, tidal volumes >8 mL/kg PBW cause harm in ARDS 2, 3
• The temptation to use larger tidal volumes for "stiff lungs" or to overcome obstruction must be resisted 1, 4

Do not use high-frequency oscillatory ventilation:

• This is strongly recommended against and associated with harm 1, 2, 3

Do not perform aggressive or prolonged recruitment maneuvers:

• These are associated with harm, particularly the "staircase" incremental PEEP approach 2, 3, 5

Monitor for barotrauma:

• Patients with ARDS and obstructive disease are at higher risk when PEEP >10 cmH₂O 3
• This risk does not preclude higher PEEP in moderate-severe ARDS, but requires vigilance 1, 3