Ventilator Modes Explained: AC/VC, SIMV and Pressure Support
How do AC/VC, SIMV and pressure support differ?
Ventilator modes describe how breaths begin, which variable the ventilator controls and how inspiration ends. In volume assist-control, supported breaths receive a set tidal volume while pressure varies. SIMV provides synchronized mandatory breaths with spontaneous breathing between them. Pressure support assists patient-triggered breaths, so tidal volume depends on effort and respiratory mechanics.
Key takeaways
- Read the mode, settings and actual delivered breaths together.
- The total respiratory rate differs from the set rate when the patient triggers extra breaths.
- Pressure-supported breaths depend on the patient initiating breathing.
What changes between volume assist-control and SIMV?
In volume assist-control, a patient-triggered breath receives the same type of volume-targeted support as a timed breath. The set rate supplies a minimum frequency, while additional patient triggers increase the total rate. Airway pressure changes with resistance and compliance.
SIMV supplies synchronized mandatory breaths and permits spontaneous breaths between them. Those spontaneous breaths often receive added pressure support. Look at the mandatory-breath type and the support on spontaneous breaths rather than treating SIMV as one fixed level of assistance.
How does pressure support differ?
Pressure support starts with the patient’s inspiratory effort. The ventilator supplies an inspiratory pressure above PEEP, and breath size varies with the support level, effort, airway resistance and lung mechanics. The mode alone does not guarantee an adequate minute ventilation.
Watch the total rate and delivered tidal volume. A patient taking shallow, rapid breaths has a different breathing pattern from someone taking slower, larger breaths, even when both are on the same displayed pressure-support setting.
What should you check besides the mode name?
Review PEEP, oxygen concentration, delivered tidal volume, total rate, airway pressures, gas exchange and synchrony. Tidal-volume selection uses predicted body weight, which relates to height, rather than actual weight alone. Lung mechanics and patient response guide the interpretation of the settings.
In a clinical interview, explain the breath first, then explain a concerning change. A rising pressure, falling volume or sudden desaturation needs its own causal assessment. Naming the mode without describing what the machine and patient are doing leaves the reasoning incomplete.
