Given that the mechanical ventilator only offers assist‑control (AC), synchronized intermittent mandatory ventilation (SIMV), and pressure‑support ventilation with CPAP (PSV‑CPAP) modes, and lacks a pressure‑support ventilation (PSV)‑only mode, should I use SIMV, and what are the appropriate ventilator settings?

Use PSV-CPAP Mode Instead of SIMV

If your ventilator has PSV-CPAP available, use this mode rather than SIMV for spontaneously breathing patients who need ventilatory support. PSV-CPAP is essentially pressure support ventilation with a backup CPAP level, which provides patient-triggered breaths with pressure augmentation—this is physiologically superior to SIMV for most clinical scenarios.

Understanding Your Available Modes

Based on the guideline evidence, here's what you actually have 1:

• AC (Assist/Control): Delivers full mandatory breaths whether patient-triggered or machine-triggered; every breath gets the same preset volume or pressure
• SIMV: Delivers mandatory breaths at a set rate, but allows unsupported spontaneous breaths between mandatory breaths
• PSV-CPAP: Patient triggers all breaths, ventilator provides pressure support; includes backup rate (typically 6-8 breaths/min) if patient becomes apneic 1

Why PSV-CPAP is Preferred Over SIMV

PSV-CPAP allows complete patient control of respiratory timing and rate while providing consistent pressure support for every breath, whereas SIMV creates an artificial split between supported mandatory breaths and unsupported spontaneous breaths 1.

Research demonstrates that PSV results in:

• Smaller increases in metabolic demand (15.7% vs 32.7% with SIMV) when reducing support 2
• Lower inspiratory effort (P0.1: 2.64 vs 4.01 cm H2O) 2
• Better patient-ventilator synchrony due to patient control of all breath timing 3

Recommended PSV-CPAP Settings

Initial Settings:

Pressure Support Level:

• Start at 10-15 cm H2O above PEEP
• Titrate to achieve tidal volume of 6-8 ml/kg predicted body weight 4
• Target respiratory rate <25-30 breaths/min

PEEP:

• Start at 5 cm H2O minimum (never use zero PEEP) 4
• Increase to 8-12 cm H2O if oxygenation inadequate
• Avoid PEEP >15 cm H2O unless severe ARDS

FiO2:

• Titrate to maintain SpO2 92-96% or PaO2 60-100 mmHg 5
• Start at 40-60% and adjust downward as tolerated

Backup Rate:

• Set at 6-8 breaths/min (usually automatic on PSV-CPAP mode) 1

Trigger Sensitivity:

• Set flow trigger at 2-3 L/min or pressure trigger at -1 to -2 cm H2O
• Adjust if auto-triggering or failure to trigger occurs

Monitoring Parameters:

Monitor these continuously 4:

• Tidal volume: Should remain 6-8 ml/kg PBW
• Respiratory rate: Target <30/min
• Plateau pressure: Keep <30 cm H2O (check with inspiratory hold)
• Driving pressure (Plateau - PEEP): Target <15 cm H2O 5
• Dynamic compliance: Trending improves detection of deterioration

When You Must Use SIMV Instead

Use SIMV only if:

1. Patient has inadequate respiratory drive and backup rate alone is insufficient
2. PSV-CPAP repeatedly fails due to apneic episodes exceeding backup capability
3. Severe patient-ventilator dyssynchrony occurs despite optimizing PSV settings

SIMV Settings (If Required):

Mandatory Rate:

• Start at 8-12 breaths/min
• Set lower (6-8/min) if patient has strong spontaneous effort
• Set higher (12-14/min) if weak respiratory drive

Tidal Volume (if volume-controlled):

• 6-8 ml/kg predicted body weight 4

Inspiratory Pressure (if pressure-controlled):

• Titrate to achieve tidal volume 6-8 ml/kg PBW

Pressure Support for Spontaneous Breaths:

• 8-12 cm H2O to support spontaneous breaths between mandatory breaths
• This prevents excessive work of breathing during unsupported breaths 6

PEEP:

• 5 cm H2O minimum 4

FiO2:

• Same targets as PSV-CPAP above

Critical Pitfalls to Avoid

With SIMV: The spontaneous breaths between mandatory breaths are often inadequately supported, leading to increased work of breathing and patient fatigue 6. Always add adequate pressure support (8-12 cm H2O) to spontaneous breaths.

With PSV-CPAP: If the patient becomes apneic, the backup rate may be insufficient. Monitor closely during initial hours and be prepared to switch to AC or SIMV if apneic episodes are frequent.

Lock-out periods: Some ventilators have long expiratory lock-out times that prevent patient triggering—this causes severe dyssynchrony. Shorten expiratory time if patient appears to be "fighting" the ventilator 1.

Excessive pressure support: Setting PSV too high (>20 cm H2O) can cause large tidal volumes, breath stacking, and barotrauma. Always verify delivered tidal volumes remain ≤8 ml/kg PBW.