For an intubated adult ICU patient with acute respiratory failure, should I start with Assist‑Control ventilation or Synchronized Intermittent Mandatory Ventilation, and when is it appropriate to switch to SIMV?

Assist-Control Ventilation Should Be Your Initial Mode for Intubated ICU Patients with Acute Respiratory Failure

For intubated adult ICU patients with acute respiratory failure, start with Assist-Control (A/C) ventilation rather than SIMV, as A/C provides more consistent ventilatory support, better sleep quality, and equivalent clinical outcomes without the theoretical disadvantages of SIMV. 1

Why Assist-Control Over SIMV

Clinical Outcomes Are Equivalent or Favor A/C

• Large observational studies show no mortality benefit with SIMV compared to A/C ventilation. A propensity-matched analysis of 1,578 mechanically ventilated patients across 349 ICUs found no difference in hospital mortality between SIMV-PS and A/C (OR 1.04,95% CI 0.77-1.42, p=0.78), despite treatment allocation bias that would have favored SIMV. 2

• A recent retrospective study of 345 ICU patients similarly found no significant differences between A/C and SIMV-PS in mortality (p=0.241), duration of mechanical ventilation (p=0.952), length of hospital stay (p=0.675), failed extubation (p=0.411), or need for tracheostomy (p=0.301). 3

• Even in moderate ARDS, where one might theoretically prefer patient-triggered breaths, a randomized trial showed SIMV+PS improved early oxygenation but provided no benefit in mortality, delirium incidence, patient-ventilator asynchrony, sedation requirements, or ventilation duration compared to A/C. 4

Sleep Quality Is Superior with Assist-Control

• The Society of Critical Care Medicine guidelines conditionally recommend using assist-control ventilation at night over pressure support ventilation to improve sleep in critically ill adults. 1

• Pooled analysis of three studies (n=61) demonstrated that A/C increased sleep efficiency by 18.33% (95% CI 7.89-28.76) and increased REM sleep time by 2.79% (95% CI 0.53-5.05) compared to pressure support modes. 1

• This sleep benefit matters for patient recovery and may reduce delirium risk, though the evidence quality is moderate to low. 1

SIMV Has Theoretical Disadvantages Without Proven Benefits

• SIMV creates a "lock-out" period between mandatory breaths that can prevent patient-triggered breaths and worsen synchrony. 1, 5 Setting a long expiratory time creates a prolonged lock-out period that may lead to poor patient tolerance. 1

• The mandatory breath rate in SIMV may be inadequate during periods of increased respiratory drive, forcing excessive spontaneous work of breathing between mandatory breaths. 1

• SIMV was historically promoted for weaning, but this theoretical advantage has not translated into clinical benefit in comparative studies. 6, 2

When to Consider SIMV (Rare Situations)

Patient-Ventilator Dyssynchrony Despite A/C Optimization

• If a patient demonstrates persistent dyssynchrony on A/C despite optimizing sedation, flow rates (60-100 L/min in distressed patients), and trigger sensitivity, consider switching to pressure support or adaptive modes rather than SIMV. 5

• One small study (n=10) suggested adaptive support ventilation (ASV) reduced sternocleidomastoid EMG activity and respiratory drive compared to SIMV-PS, but this doesn't support using SIMV over A/C—it supports using newer adaptive modes. 7

Specific Clinical Scenarios Where SIMV May Be Selected

• Postoperative patients or trauma patients with lower severity of illness were more likely to receive SIMV in observational data, though without outcome benefit. 2

• Patients with neuromuscular disease or chest wall disorders may benefit from controlled ventilation modes initially, as triggering may be ineffective. 1 In these cases, A/C with controlled breaths is preferable to SIMV.

Practical Algorithm for Mode Selection

Initial Mode Selection (Start Here)

1. Begin with A/C ventilation (volume or pressure control) for all intubated patients with acute respiratory failure 1

   • Set tidal volume 4-8 mL/kg predicted body weight
   • Target plateau pressure <30 cm H₂O for ARDS 1
   • Set respiratory rate to achieve target minute ventilation

2. Optimize A/C settings before considering mode change:

   • Match inspiratory flow to patient demand (60-100 L/min if distressed) 5
   • Minimize dynamic hyperinflation with longer expiratory times and lower rates 5
   • Consider PEEP 3-5 cm H₂O to offset intrinsic PEEP in obstructive disease 5
   • Use light sedation (dexmedetomidine preferred) to preserve synchrony 5

When to Transition Away from A/C

1. Switch to pressure support ventilation (not SIMV) when:

   • Patient is triggering all breaths consistently
   • Respiratory drive is stable
   • Ready to begin active weaning process
   • Patient demonstrates adequate spontaneous effort

2. Consider adaptive modes (PAV or NAVA) rather than SIMV if:

   • Persistent asynchrony despite A/C optimization 5
   • Available equipment and staff expertise exist 1

Avoid SIMV Unless Specific Contraindication to A/C

1. SIMV should not be your default mode because:
   • No mortality or clinical outcome benefit 3, 2
   • Inferior sleep quality compared to A/C 1
   • Creates lock-out periods that worsen synchrony 1, 5
   • More complex to manage without proven advantage

Critical Pitfalls to Avoid

• Don't use SIMV thinking it will facilitate weaning—this theoretical benefit has never been demonstrated in clinical trials, and direct transition from A/C to pressure support is equally effective. 2

• Don't set excessively long expiratory times in any assist mode, as this creates prolonged lock-out periods preventing patient-triggered breaths. 1, 5

• Don't confuse SIMV with pressure support ventilation—they are different modes. Pressure support (assisted spontaneous breathing) allows patient control of timing and is appropriate for weaning; SIMV combines mandatory breaths with spontaneous breaths and offers no advantage. 1

• Avoid excessive backup rates in assist modes that may override patient efforts and worsen synchrony. 5

• For patients with severe ARDS, prioritize proven interventions: low tidal volume ventilation (strong recommendation), prone positioning >12 hours/day for severe ARDS (strong recommendation), and higher PEEP for moderate-severe ARDS (conditional recommendation). 1 Mode selection (A/C vs SIMV) is less important than these fundamental strategies.