Time Constant in ARDS
The time constant (τ) in ARDS patients is typically prolonged to 0.5-1.5 seconds or longer, significantly exceeding the normal 0.3-0.5 seconds, requiring expiratory times of at least 3-5 time constants (1.5-7.5 seconds) to prevent auto-PEEP and dynamic hyperinflation. 1
Understanding the Time Constant in ARDS
The time constant (τ) represents the product of respiratory system resistance and compliance (τ = R × C), determining how quickly the lungs fill and empty during mechanical ventilation. In ARDS, this value becomes critically important for ventilator management.
Physiological Alterations in ARDS
ARDS causes decreased lung compliance due to alveolar flooding with plasma proteins, surfactant dysfunction, and increased lung weight from interstitial and alveolar edema, resulting in a "stiff" lung that requires higher pressures to achieve adequate ventilation 2
The inflammatory response leads to heterogeneous lung injury with areas of normal, collapsed, and consolidated lung tissue, creating regional variations in time constants throughout the lung 3, 4
Increased airway resistance from bronchospasm, secretions, and airway inflammation further prolongs the time constant, particularly affecting expiratory flow 2
Recommended Ventilation Settings
Tidal Volume and Pressure Limits
Use low tidal volumes of 4-8 mL/kg predicted body weight to minimize ventilator-induced lung injury (VILI), which has been shown to reduce relative mortality risk by 21% 2, 3, 4
Maintain plateau pressures below 30 cmH₂O (ideally <28 cmH₂O) to prevent alveolar overdistention and barotrauma 2, 1
Limit driving pressure (plateau pressure minus PEEP) to reduce stress and strain on the injured lung, with values below 15 cmH₂O associated with better outcomes 2
PEEP Strategy
For moderate to severe ARDS (PaO₂/FiO₂ <200 mmHg), use higher PEEP levels (typically 10-15 cmH₂O or higher) to maintain alveolar recruitment and prevent atelectrauma 2
Higher PEEP strategies in moderate/severe ARDS were associated with reduced mortality (adjusted RR 0.90; 95% CI 0.81-1.00) in individual patient data meta-analysis 2
PEEP should be titrated to optimize oxygenation while monitoring for hemodynamic compromise and overdistention, particularly watching for right ventricular dysfunction 2, 1
Respiratory Rate and Timing
Reduce respiratory rate to 25-30 breaths/min when possible to allow adequate expiratory time and prevent dynamic hyperinflation 1
Set inspiratory time to 40-50% of the respiratory cycle (approximately 0.7-0.85 seconds at RR 35) to allow accurate plateau pressure measurement 1
Include an end-inspiratory pause of 0.3-0.5 seconds for accurate plateau pressure assessment to confirm lung-protective ventilation 1
Ensure complete exhalation by monitoring expiratory flow waveforms to detect auto-PEEP, which indicates insufficient expiratory time 1
Permissive Hypercapnia
- Accept permissive hypercapnia with pH >7.20-7.25 rather than increasing minute ventilation excessively, as this reduces the risk of VILI from high tidal volumes or respiratory rates 1
Adjunctive Therapies for Severe ARDS
Prone Positioning
Implement prone positioning for at least 12-16 hours per day in all patients with severe ARDS (PaO₂/FiO₂ <150 mmHg or <100 mmHg), as this intervention has demonstrated mortality benefit 2, 1, 4
Prone positioning improves ventilation-perfusion matching and reduces ventilator-induced lung injury by redistributing transpulmonary pressure more evenly 2
The duration of prone positioning should be prolonged (>16-20 hours per 24-hour period) based on evidence suggesting longer duration correlates with better outcomes 2
Neuromuscular Blockade
- Consider neuromuscular blocking agents in severe ARDS to prevent patient-ventilator dyssynchrony and expiratory efforts that can cause derecruitment, particularly during procedures or when plateau pressures are difficult to control 1
Critical Pitfalls to Avoid
Do not use high-frequency oscillatory ventilation (HFOV) as it may worsen hemodynamics and increase mortality in ARDS patients 2, 1
Avoid maintaining a 1:2 I:E ratio at high respiratory rates without measuring plateau pressure, as this prevents confirmation of lung-protective ventilation 1
Monitor for right ventricular dysfunction when increasing inspiratory time or PEEP, as prolonged positive pressure can increase RV afterload in ARDS 1
Do not delay prone positioning in severe ARDS, as this may miss the therapeutic window for this effective mortality-reducing intervention 1
Check for auto-PEEP regularly, as inadequate expiratory time relative to the prolonged time constant can lead to dynamic hyperinflation, hemodynamic compromise, and barotrauma 1