SIMV vs Spontaneous Breathing: Key Differences
No, SIMV (Synchronized Intermittent Mandatory Ventilation) is not the same as spontaneous breathing (Spont)—they are fundamentally different ventilatory modes with distinct mechanisms and clinical applications.
Core Distinctions
SIMV Characteristics
- SIMV delivers a preset number of mandatory breaths per minute while allowing the patient to take additional spontaneous breaths between these mandatory cycles 1
- Patient-triggered breaths delay the next machine-determined breath, creating synchronization between patient effort and ventilator delivery 1
- SIMV is sometimes referred to as spontaneous/timed (S/T) mode on non-invasive ventilation machines 1
- Provides a backup respiratory rate that prevents hypoventilation if the patient fails to initiate breaths 2
Spontaneous Breathing (Assisted Spontaneous Breathing/Pressure Support)
- The patient's respiratory effort triggers the ventilator both on and off, with respiratory frequency and timing entirely determined by the patient 1
- No mandatory breaths are delivered—if the patient fails to make respiratory effort, no ventilatory assistance occurs (though many manufacturers now incorporate a backup rate of 6-8 breaths/minute) 1
- Often termed pressure support (PS) when pressure is set rather than volume 1
Clinical Implications
When SIMV Offers Advantages
- SIMV prevents central apneas during sleep due to its backup rate, making it superior to pure spontaneous/pressure support modes for patients at risk of hypoventilation 2, 1
- Particularly beneficial in acute-on-chronic respiratory failure where assist-control or SIMV modes with backup ventilation prevent the periodic breathing that can occur with pressure support alone 1
- In patients with heart failure or those prone to central apneas, the backup rate prevents apneic episodes that fragment sleep 1
When Spontaneous Breathing (Pressure Support) Is Preferred
- For spontaneous breathing trials (SBTs), the American Thoracic Society/American College of Chest Physicians recommend conducting the initial SBT with inspiratory pressure augmentation (5-8 cm H₂O) rather than T-piece, as this approach shows higher success rates and trends toward lower ICU mortality 1
- During weaning, pressure support ventilation demonstrates superior outcomes compared to SIMV, with significantly lower failure rates (8% for PSV versus 39% for SIMV, p < 0.025) 3
Common Pitfalls to Avoid
The SIMV Weaning Trap
- SIMV has the poorest weaning outcomes among ventilator modes in randomized controlled trials 4, 3
- Patients show little adaptation to ventilator assistance with SIMV, making it a suboptimal weaning technique despite its theoretical appeal 4, 5
- The high internal resistance of demand-flow SIMV systems can markedly increase oxygen consumption (up to 27% higher) compared to pressure support 6
The Pressure Support Apnea Risk
- Pure pressure support without backup rate can cause central apneas during sleep if the level of support is too high, leading to hyperventilation, hypocapnia, and subsequent apnea when PaCO₂ drops below the apneic threshold 1
- This risk is particularly pronounced in patients with heart failure or those with prolonged circulation times 1
- Adding a backup rate (converting to SIMV/S-T mode) or adding dead space can prevent these central apneas 1
Practical Algorithm for Mode Selection
For acute respiratory failure requiring ventilatory support:
- Use SIMV (S-T mode) if the patient has risk factors for central apneas (heart failure, tendency to hyperventilate, sleep-disordered breathing) 1, 2
- Use pressure support if the patient has stable respiratory drive and you're conducting weaning trials 1, 3
For weaning from mechanical ventilation:
- Avoid SIMV as a weaning technique—use pressure support ventilation or spontaneous breathing trials instead 4, 3
- If using spontaneous breathing trials, conduct them with 5-8 cm H₂O pressure augmentation rather than T-piece 1
For sleep optimization in ICU patients: