Indications for Mechanical Ventilation and Ventilation Strategy
Indications for Mechanical Ventilation
Proceed to invasive mechanical ventilation when noninvasive support (HFNO or NIV) fails to improve or worsens within 1-2 hours, or when specific physiologic thresholds are crossed. 1
Primary Indications for Invasive Mechanical Ventilation:
Failure of noninvasive ventilation: Worsening arterial blood gases (ABGs) and/or pH within 1-2 hours of NPPV initiation, or lack of improvement after 4 hours 1
Severe acidosis: pH < 7.25 with hypercapnia (PaCO₂ > 60 mmHg or > 8 kPa) 1
Life-threatening hypoxemia: PaO₂/FiO₂ ratio ≤ 150 mmHg, particularly if < 100 mmHg despite optimized oxygen therapy 1
Severe tachypnea: Respiratory rate > 35 breaths/min despite optimal medical therapy 1
Respiratory distress with increased work of breathing: Persistent dyspnea and clinical signs of respiratory muscle fatigue despite oxygen and noninvasive support 1
Impaired mental status or inability to protect airway: Somnolence, inability to cooperate, or risk of aspiration 1
Cardiovascular instability: Hypotension, arrhythmias, or myocardial infarction complicating respiratory failure 1
Critical Timing Considerations:
Monitor closely during the first 1-2 hours of HFNO (requiring FiO₂ > 70% and flow > 50 L/min) or NIV trial 1
Do not delay intubation if the patient shows acute deterioration, as delayed intubation increases mortality risk 1
Obtain arterial blood gas measurements before initiating mechanical ventilation to guide therapy and document baseline status 1
Specific Ventilation Strategy
Use lung-protective ventilation with low tidal volumes (4-6 mL/kg predicted body weight) and plateau pressure < 30 cmH₂O as the cornerstone strategy. 1, 2
Initial Ventilator Mode and Settings:
Start with volume-cycled assist-control (AC) ventilation as the initial mode, which provides complete ventilatory support and prevents central apneas 2
Tidal volume: 4-6 mL/kg predicted body weight (PBW), calculated as:
- Men: 50 + 2.3 × (height in inches - 60)
- Women: 45.5 + 2.3 × (height in inches - 60) 2
Plateau pressure: Maintain ≤ 30 cmH₂O to prevent alveolar overdistension and ventilator-induced lung injury 1, 2
PEEP: Apply appropriate positive end-expiratory pressure; use higher PEEP for moderate-severe ARDS (PaO₂/FiO₂ < 150 mmHg) 1
For Moderate to Severe ARDS (PaO₂/FiO₂ < 150 mmHg):
Higher PEEP strategy: Optimize alveolar recruitment while monitoring plateau pressure 1
Prone positioning: Implement for > 12 hours per day 1
Deep sedation and analgesia: Apply within the first 48 hours of mechanical ventilation 1
Neuromuscular blockade: Consider for severe refractory hypoxemia, especially with ventilator-patient dyssynchrony 1
Alternative Modes for Refractory Cases:
Airway Pressure Release Ventilation (APRV): Consider as the primary alternative mode for patients with ARDS and ventilator asynchrony or refractory hypoxemia 3
Neurally Adjusted Ventilatory Assist (NAVA): Reserve for patients with intact respiratory drive and significant patient-ventilator asynchrony 3
Critical Pitfalls to Avoid:
Never use actual body weight for tidal volume calculations—always use predicted body weight 2
Avoid hyperventilation: Target normocapnia (PaCO₂ 40-45 mmHg) to prevent cerebral vasoconstriction and hemodynamic instability 2
Do not routinely use high-frequency oscillatory ventilation (HFOV) in ARDS—it may be harmful 1, 2
Rescue Therapies for Severe Refractory Hypoxemia:
ECMO consideration when:
ECMO should only be performed in specialized centers with appropriate expertise 1
Monitoring Parameters:
Continuously monitor: Plateau pressure, driving pressure (ΔP), oxygenation index (PaO₂/FiO₂), and patient-ventilator synchrony 2, 4
Obtain serial ABGs to guide ventilator adjustments and assess response to therapy 1
Use pulse oximetry and capnography to ensure adequate oxygenation and ventilation 1, 4