Is SIMV (Synchronous Intermittent Mandatory Ventilation) combined with Pressure Support similar to PRVC (Pressure‑Regulated Volume‑Control)?

SIMV + Pressure Support vs PRVC: Key Differences

No, SIMV combined with Pressure Support is fundamentally different from PRVC (Pressure-Regulated Volume Control), and PRVC demonstrates superior performance with less patient-ventilator asynchrony and lower work of breathing.

Fundamental Mechanical Differences

SIMV + Pressure Support is a hybrid mode that combines two distinct breath types 1, 2:

• Mandatory breaths: Delivered at preset intervals, synchronized with patient effort when possible 3
• Spontaneous breaths: Occur between mandatory breaths, supported only by pressure support (patient-triggered and patient-cycled) 3, 2
• Can be configured as either volume-controlled or pressure-controlled for the mandatory breaths 1, 2

PRVC is a single, unified mode that 4:

• Delivers volume-targeted breaths using pressure control
• Automatically adjusts inspiratory pressure breath-by-breath to achieve a set tidal volume
• Maintains consistent tidal volume delivery while minimizing peak pressures
• Every breath receives the same level of support

Clinical Performance: PRVC is Superior

The most recent high-quality evidence demonstrates PRVC's advantages 4:

• Lowest incidence of patient-ventilator asynchrony compared to SIMV modes
• Significantly lower work of breathing: PTP (pressure-time product) for PRVC was 0.05-0.06 cm H₂Os vs 0.10-0.15 cm H₂Os for SIMV/VC + PS (P < 0.03)
• Particularly effective in injured lungs, where asynchrony and work of breathing remain minimal

Why SIMV + PS Performs Poorly

The inferior performance of SIMV + Pressure Support stems from several mechanisms 5, 6, 4:

Breath-to-breath variability 6:

• Unsupported spontaneous breaths during SIMV deliver lower tidal volumes than mandatory breaths
• Creates inconsistent minute ventilation despite similar total volumes
• Results in more variable oxygenation (SpO₂)

Increased respiratory workload 6, 4:

• Tachycardia and tachypnea are more common with SIMV + PS
• Lower and more variable oxygen saturation
• Higher pressure-time product indicating greater patient effort

Synchronization challenges 4:

• SIMV/VC + PS had the highest incidence of asynchrony among all modes tested
• The "lock-out" period required to prevent breath stacking can cause poor patient tolerance if set incorrectly 3, 7

When Each Mode is Appropriate

Use PRVC when 4:

• Guaranteed tidal volume is needed with minimal asynchrony
• Managing injured lungs (ARDS, pneumonia)
• Minimizing work of breathing is a priority
• Patient has adequate respiratory drive

SIMV + PS has limited modern indications 2, 8:

• Historically used for weaning, though this approach is now questioned
• May facilitate gradual reduction in mechanical support 8
• More complex to manage and requires understanding of synchronization windows 2

Critical Pitfall to Avoid

Do not assume SIMV + PS provides consistent ventilatory support 6: The spontaneous breaths between mandatory breaths receive only pressure support, which may be inadequate if the patient's effort is weak or inconsistent. This creates periods of hypoventilation that PRVC avoids through its breath-by-breath volume guarantee.

Practical Algorithm for Mode Selection

1. If volume guarantee with minimal asynchrony is needed: Choose PRVC 4
2. If combining SIMV with volume guarantee: Use SIMV/PRVC + PS rather than SIMV/VC + PS 4
3. Optimize pressure support level: When using any SIMV mode, adjust PS to optimize tidal volume on spontaneous breaths 4
4. Monitor for asynchrony: Watch for tachycardia, tachypnea, and variable SpO₂ as indicators of excessive work of breathing 6