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. 2
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. 3
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. 4
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, 5
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, 5
Set PEEP ≥10 cmH₂O for moderate-severe ARDS (can titrate higher to 10-15 cmH₂O based on oxygenation and hemodynamics). 5, 6
Monitoring Parameters
Measure and record driving pressure (plateau pressure minus PEEP) with each ventilator check, targeting the lowest achievable value. 5
Monitor mechanical power continuously if available, targeting <20 J/min normalized to predicted body weight to minimize ventilator-induced lung injury. 5
Use continuous end-tidal CO₂ monitoring to detect circuit disconnection and track dead space ventilation. 5
Accept permissive hypercapnia with pH ≥7.20 when necessary to maintain lung-protective ventilation parameters. 5
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. 7
Pressure-targeted and time-cycled modes (assisted pressure-controlled ventilation) may preserve diaphragm function and improve patient-ventilator synchrony in mild-to-moderate ARDS. 7, 8
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. 8
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. 2, 4
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, 5
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. 5
Do not use high-frequency oscillatory ventilation as routine therapy—it has a strong recommendation against use based on increased mortality in large trials. 9
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. 4 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. 3 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