What is the difference between Synchronized Intermittent Mandatory Ventilation (SIMV) and Pressure Support Ventilation (PSV) in mechanical ventilation?

SIMV vs PSV: Key Differences in Mechanical Ventilation

SIMV provides guaranteed mandatory breaths with a backup rate that prevents central apneas, while PSV is entirely patient-triggered and offers no backup ventilation—making SIMV the safer choice for patients at risk of hypoventilation or inadequate respiratory drive. 1

Fundamental Mechanism Differences

SIMV (Synchronized Intermittent Mandatory Ventilation)

• Delivers a preset number of mandatory breaths per minute that are synchronized with patient-triggered breaths, ensuring a minimum minute ventilation regardless of patient effort 2
• Patient-triggered breaths delay the next mandatory breath, creating synchronization between patient effort and machine-delivered breaths 2
• Can be either volume-controlled (set tidal volume) or pressure-controlled (set inspiratory pressure) 1
• Prevents central apneas during sleep due to the presence of a backup respiratory rate 1

PSV (Pressure Support Ventilation)

• The patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath 2
• Provides no mandatory breaths—if the patient fails to make respiratory effort, no respiratory assistance occurs 2
• Most modern ventilators now incorporate a backup rate of 6-8 breaths per minute to address this safety concern 2
• Improves patient comfort and reduces ventilatory work by providing pressure-assisted breaths 3

Clinical Performance and Efficiency

Ventilatory Efficiency

• PSV delivers more efficient ventilation with lower ventilatory equivalent (better efficiency) compared to SIMV alone in healthy subjects 4
• SIMV combined with pressure support (SIMV+PSV) produces significantly greater minute volume and ventilatory equivalent than SIMV alone 4
• PSV provides a more balanced pressure and volume change form of muscle work to the patient 3

Tidal Volume Delivery

• PSV+VG (volume guarantee) delivers closer tidal volumes to set values (60% vs 49%) compared to SIMV+VG in preterm infants 5
• This improved accuracy occurs without increased risk of overventilation or adverse outcomes 5

Safety Profile and Clinical Outcomes

Risk of Hypoventilation

• SIMV is preferable for patients at risk of hypoventilation because the backup rate prevents central apneas 1
• PSV requires adequate spontaneous respiratory drive—patients with unreliable respiratory effort should not be placed on pure PSV 2

Sleep Quality and Patient Comfort

• SIMV provides better sleep quality compared to PSV due to prevention of central apneas 1
• PSV improves patient comfort during waking hours by allowing complete patient control of breathing pattern 3

Morbidity Outcomes

• Chronic lung disease occurred less frequently with PSV+VG compared to SIMV+VG in preterm infants, though overall mortality and major morbidity were similar 5
• Both modes show similar hemodynamic stability and gas exchange when properly titrated 6

Practical Clinical Application

When to Choose SIMV

• Patients with unreliable respiratory drive (neurological impairment, heavy sedation, sleep-disordered breathing) 1
• Initial phases of mechanical ventilation when respiratory stability is uncertain 7
• Patients requiring guaranteed minimum minute ventilation 2

When to Choose PSV

• Patients with intact respiratory drive who are spontaneously breathing 2
• Weaning from mechanical ventilation in stable patients with adequate respiratory effort 3
• Patients requiring improved comfort and reduced work of breathing 3

Critical Pitfalls to Avoid

• Never use pure PSV without backup rate in patients with unreliable respiratory drive—this can lead to life-threatening hypoventilation 2
• Be aware that ventilator terminology varies between manufacturers—what one calls "PSV" another may call "assisted spontaneous breathing" 2, 7
• SIMV alone (without pressure support) is less efficient than SIMV+PSV or PSV alone—consider adding pressure support to spontaneous breaths in SIMV mode 4
• Monitor for increased dead space ventilation when transitioning from controlled modes to SIMV, which may require increased minute ventilation to maintain normocapnia 6