When should SIMV be used instead of assist‑control in an adult patient who has been mechanically ventilated for more than a week?

SIMV Should Generally Be Avoided in Prolonged Mechanical Ventilation

For adult patients mechanically ventilated for more than a week, SIMV is inferior to assist-control and should not be used as the primary weaning mode. Instead, use assist-control initially, then transition to daily spontaneous breathing trials (SBTs) with pressure support (5-8 cm H₂O) as the primary liberation strategy. 1, 2, 3

Why SIMV Is Not Recommended

• SIMV is explicitly inferior to both pressure support ventilation and T-piece weaning according to the American Thoracic Society and European Respiratory Society guidelines. 3

• The evidence base consistently demonstrates that SIMV prolongs weaning duration compared to other modes, making it a poor choice for patients already on prolonged ventilation (>7 days). 3

• No mode of ventilation has proven superior in terms of mortality outcomes in patients with respiratory failure, but assist-control provides complete ventilatory support and is appropriate at the outset of mechanical ventilation. 1

The Evidence-Based Approach for Prolonged Ventilation

Initial Management (Days 1-7)

• Start with volume-cycled assist-control ventilation when initiating mechanical ventilation, as it provides complete respiratory support and allows blood flow redirection to vital organs. 1

• Use low tidal volumes (4-8 ml/kg predicted body weight) and maintain plateau pressures <30 cm H₂O to prevent ventilator-induced lung injury. 1

Transition to Weaning (After Day 7)

• Implement a protocolized weaning strategy that includes daily readiness screening and spontaneous breathing trials rather than gradual SIMV weaning. 1, 2, 3

• Daily readiness criteria must include: patient arousable and able to follow commands, hemodynamic stability without vasopressors, resolution or improvement of the primary condition, adequate oxygenation (PaO₂/FiO₂ ≥200, PEEP ≤5 cm H₂O), and RSBI ≤105 breaths/min/L. 2, 3

Conducting the SBT

• Perform SBTs using pressure support of 5-8 cm H₂O with PEEP 5 cm H₂O rather than T-piece or CPAP alone, which increases SBT success rates from 76.7% to 84.6% and extubation success from 68.9% to 75.4%. 1, 2, 3

• Duration should be 30 minutes for standard-risk patients and 60-120 minutes for high-risk patients, as most failures occur within the first 30 minutes. 1, 2

• Stop the SBT immediately if SpO₂ <90%, heart rate >140 bpm or increases >20%, systolic BP >180 or <90 mmHg, respiratory rate >35 breaths/min, or increased anxiety/diaphoresis develops. 2, 3

Why This Approach Works Better Than SIMV

• Protocol-driven weaning reduces mechanical ventilation duration by approximately 25 hours and shortens ICU length of stay by about 1 day compared to physician-discretionary approaches. 2, 3

• The SBT-based approach has strong evidence (moderate-quality) supporting its use, whereas SIMV has been shown to be inferior in head-to-head comparisons. 1, 3

• One small study 4 showed SIMV+PS improved early oxygenation in moderate ARDS but found no differences in mortality, delirium, patient-ventilator asynchrony, or ventilation duration—outcomes that matter most for prolonged ventilation patients.

Post-Extubation Management for Prolonged Ventilation Patients

• Identify high-risk patients (age >65 years, COPD/CHF, hypercapnia during SBT, >14 days ventilation, ineffective cough) who should be extubated directly to prophylactic NIV. 1, 2, 5, 3

• Prophylactic NIV in high-risk patients reduces reintubation (RR 0.61,95% CI 0.48-0.79), mortality (RR 0.54,95% CI 0.41-0.70), and ICU length of stay by approximately 2.5 days. 2, 3

• For high-risk patients with prolonged intubation (>7 days), perform a cuff leak test and administer systemic steroids at least 4 hours before extubation if the test fails. 1, 5

Common Pitfalls to Avoid

• Do not use gradual SIMV weaning (progressive reduction of mandatory rate) as the primary liberation strategy—daily SBTs are superior. 2, 3

• Do not delay extubation in patients who pass their first SBT if they are low-risk; target an extubation failure rate of 5-10%. 5, 3

• Do not apply NIV only as rescue therapy after extubation failure in high-risk patients—it must be prophylactic to achieve mortality benefit. 2, 3

• Do not continue deep sedation during weaning attempts; implement daily sedation minimization or light-sedation protocols targeting a level where patients can follow simple commands. 2, 3