Understanding SIMV Pressure Control vs. Pressure Support and SIMV Volume Control
SIMV pressure control delivers mandatory breaths at a preset pressure with synchronized patient triggering, while pressure support provides patient-triggered assistance only, and SIMV volume control delivers mandatory breaths at a fixed tidal volume—the key distinction is that SIMV modes guarantee a minimum respiratory rate whereas pressure support does not. 1
Core Mechanical Differences
SIMV Pressure Control
- Delivers mandatory breaths at a preset number per minute regardless of patient effort
- Sets inspiratory pressure (not volume), with resulting tidal volume varying based on lung compliance, airway resistance, and chest wall mechanics 1
- Patient can trigger additional breaths, but these receive identical machine-delivered breaths to mandatory ones
- Includes a "lock out" period to prevent breath stacking 1
- Guarantees minimum minute ventilation through the mandatory rate
Pressure Support Ventilation (PSV)
- Entirely patient-triggered—both breath initiation and termination controlled by patient effort 1
- Respiratory rate and timing completely determined by the patient
- Sets pressure level, but no backup mandatory breaths (though modern ventilators often include 6-8 breaths/min backup) 1
- If patient stops breathing, no ventilation occurs (without backup rate)
- More efficient for respiratory muscle reloading—produces smaller changes in oxygen consumption and metabolic expenditure compared to SIMV 2
SIMV Volume Control
- Delivers mandatory breaths at preset tidal volume rather than pressure 1
- Ventilator generates whatever pressure necessary to achieve the set volume
- Same synchronized triggering and mandatory rate structure as SIMV pressure control
- Facilitates measurement of respiratory mechanics and driving pressure 3
- Results in fixed tidal volume but variable pressures based on thoracic mechanics 1
Clinical Implications
Patient-Ventilator Synchrony
Pressure support generally provides better synchrony because flow is not limited and the patient controls both breath timing and duration 1. SIMV modes may cause poor tolerance if expiratory time settings create prolonged lock-out periods 1.
Work of Breathing
- PSV produces lower occlusion pressures (P0.1) and inspiratory duty cycle compared to volume-controlled SIMV at equivalent support levels 2
- SIMV with pressure support added shows higher oxygen consumption and ventilatory equivalent than SIMV alone or assist-control 4
- PSV may be more suitable for progressive weaning due to smaller metabolic changes with support reduction 2
Tidal Volume Stability
Volume control maintains consistent tidal volumes regardless of changing lung mechanics, while pressure control allows tidal volume to vary 1. In pressure-controlled modes, deteriorating compliance or increasing resistance reduces delivered volume—this can be problematic during desaturations 5.
Airway Pressures
Both pressure-targeted modes (SIMV-PC and PSV) deliver lower peak and mean airway pressures than volume control for equivalent ventilation 6. This theoretically reduces barotrauma risk but doesn't guarantee fixed minute ventilation.
Common Pitfalls
Critical caveat with pressure support alone: Without adequate respiratory drive, the patient receives no ventilation. Always verify backup rate is activated in patients with unreliable respiratory effort 1.
SIMV pressure support combination variability: When combining SIMV with pressure support for spontaneous breaths, the pressure support level relative to mandatory breath pressure is highly variable and difficult to control—the actual ratio often differs significantly from clinician targets 7.
Volume control advantages for monitoring: Use volume control in early acute respiratory failure to facilitate driving pressure measurement and respiratory mechanics assessment, which are critical for lung-protective ventilation 3.
Lock-out period issues: In SIMV modes, inappropriate expiratory time settings can create excessively long lock-out periods, causing the ventilator to ignore patient triggers and reducing tolerance 1.