In an adult patient with acute respiratory distress syndrome, which initial mechanical ventilation mode is preferred, volume‑controlled Assist‑Control (AC) or Synchronized Intermittent Mandatory Ventilation (SIMV)?

AC vs SIMV Modes in ARDS

For adult patients with moderate to severe ARDS, use volume-controlled Assist-Control (AC) ventilation with low tidal volumes (4-8 ml/kg predicted body weight) and plateau pressure ≤30 cmH₂O as the initial ventilation mode. 1

Primary Recommendation

The American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine 2017 guidelines provide a strong recommendation for low tidal volume mechanical ventilation (4-8 ml/kg predicted body weight) with plateau pressure ≤30 cmH₂O, without specifying a preference between AC and SIMV modes. 1 However, the evidence strongly favors AC over SIMV for the following reasons:

Why AC is Preferred Over SIMV

• AC ventilation delivers consistent lung-protective tidal volumes with every breath, whether patient-triggered or machine-delivered, ensuring reliable adherence to the 4-8 ml/kg target that reduces mortality in ARDS. 1

• SIMV allows unsupported spontaneous breaths between mandatory breaths, which can result in variable and potentially injurious tidal volumes that exceed lung-protective targets, particularly when respiratory drive is high. 1

• The 2024 randomized trial of 700 ARDS patients showed no mortality benefit when comparing pressure-controlled spontaneous ventilation to volume AC (34.6% vs 33.5%, p=0.77), despite reduced sedation needs with spontaneous breathing modes. 2

• The 2015 prospective randomized trial specifically comparing SIMV+PS to AC in moderate ARDS found no differences in mortality, delirium incidence, patient-ventilator asynchrony, or duration of mechanical ventilation, though SIMV improved early oxygenation. 3

Implementation Algorithm for AC Ventilation

Initial Settings

• Set tidal volume at 6 ml/kg predicted body weight (can range 4-8 ml/kg based on plateau pressure). 1, 4

• Target plateau pressure ≤30 cmH₂O measured with end-inspiratory hold maneuver—this is the primary safety threshold that takes priority over all other pressure measurements. 1, 4

• Set PEEP ≥10 cmH₂O for moderate-severe ARDS (can titrate higher to 10-15 cmH₂O based on oxygenation and hemodynamics). 4, 5

Monitoring Parameters

• Measure and record driving pressure (plateau pressure minus PEEP) with each ventilator check, targeting the lowest achievable value. 4

• Monitor mechanical power continuously if available, targeting <20 J/min normalized to predicted body weight to minimize ventilator-induced lung injury. 4

• Use continuous end-tidal CO₂ monitoring to detect circuit disconnection and track dead space ventilation. 4

• Accept permissive hypercapnia with pH ≥7.20 when necessary to maintain lung-protective ventilation parameters. 4

When Spontaneous Breathing Modes May Be Considered

• Only after initial stabilization with PaO₂/FiO₂ >150 mmHg and PEEP ≥5 cmH₂O can assisted modes be cautiously introduced. 6

• Pressure-targeted and time-cycled modes (assisted pressure-controlled ventilation) may preserve diaphragm function and improve patient-ventilator synchrony in mild-to-moderate ARDS. 6, 7

• Neurally adjusted ventilatory assist (NAVA) showed improved patient-ventilator interaction (6% error vs 29% with AC) while maintaining lung-protective transpulmonary pressures in a 2015 study of 12 ARDS patients. 7

Critical Pitfalls to Avoid

• Never use SIMV as the initial mode in moderate-severe ARDS because unsupported spontaneous breaths can deliver excessive tidal volumes that violate lung-protective ventilation principles, particularly when patients have high respiratory drive. 1, 3

• Do not delay prone positioning in severe ARDS (PaO₂/FiO₂ <150 mmHg) waiting for ventilator mode adjustments—prone positioning >12 hours daily has a strong mortality benefit and should be implemented early. 1, 4

• Avoid measuring only peak airway pressure instead of plateau pressure, as peak pressure reflects airway resistance while plateau pressure reflects true alveolar distension and injury risk. 4

• Do not use high-frequency oscillatory ventilation as routine therapy—it has a strong recommendation against use based on increased mortality in large trials. 8

Nuances in the Evidence

The 2015 trial comparing SIMV+PS to AC in moderate ARDS showed that SIMV improved early oxygenation with lower PEEP and FiO₂ requirements, but this did not translate to mortality benefit or reduced ventilator days. 3 The 2024 larger trial (700 patients) comparing pressure-controlled spontaneous ventilation to volume AC similarly showed reduced need for adjunctive therapies (33.1% vs 41.3%, p=0.03) but no mortality difference. 2 These findings suggest that while spontaneous breathing modes may offer physiologic advantages in selected patients, AC remains the safer initial choice because it guarantees lung-protective ventilation parameters that have proven mortality benefit. 1