Ventilator Modes Guide
A practical reference covering the most common mechanical ventilation modes used in critical care and respiratory therapy.
Understanding Mechanical Ventilation
Mechanical ventilation is a life-support intervention that assists or replaces spontaneous breathing in patients who cannot maintain adequate ventilation on their own. It is one of the most complex and consequential interventions a respiratory therapist manages. Understanding the available modes and when to use each one is fundamental to safe, effective respiratory care.
Every ventilator mode is defined by how it controls breath delivery. Three fundamental variables govern each breath: the trigger (what starts a breath), the limit (what controls the breath during delivery), and the cycle (what ends the breath). Different modes manipulate these variables to achieve specific clinical goals.
Modern ventilators offer a wide range of modes. While the names may vary slightly by manufacturer, the underlying principles are consistent. This guide focuses on the most widely used modes in adult critical care.
Core Ventilator Modes Explained
Assist-Control (AC)
Also called: CMV, VC-AC, PC-AC
In AC mode, the ventilator delivers a breath every time the patient triggers it, or at a set minimum rate if the patient does not trigger. Every breath — whether patient-triggered or machine-triggered — is fully supported to the set parameters. This is the most common mode for initial ventilator support.
Clinical advantages
Ensures full support with every breath. Good for patients with minimal respiratory drive.
Considerations
Risk of auto-PEEP and respiratory alkalosis if patient has a high spontaneous rate. Does not encourage weaning.
Synchronized Intermittent Mandatory Ventilation (SIMV)
Often combined with Pressure Support
SIMV delivers a set number of mandatory breaths synchronized with the patient's efforts, plus allows spontaneous breaths between mandatory cycles. Spontaneous breaths receive support only if pressure support is added. This mode requires more work from the patient between mandatory breaths.
Clinical advantages
Allows gradual reduction of mandatory breath rate as a weaning strategy.
Considerations
Higher work of breathing compared to AC. Evidence for SIMV as a superior weaning method is mixed.
Pressure Support Ventilation (PSV)
Also written as PS or PS-only mode
PSV augments every spontaneous breath with a set pressure. The patient controls the rate, inspiratory time, and tidal volume. The ventilator provides a boost to overcome the resistance of the circuit and ETT and supports the patient's respiratory muscles. PSV is commonly used for weaning and spontaneous breathing trials.
Clinical advantages
Maximizes patient comfort and synchrony. Reduces work of breathing without full control.
Considerations
Requires an adequate spontaneous respiratory drive. Not appropriate for apneic patients.
Pressure-Regulated Volume Control (PRVC)
Similar: Volume-Targeted Pressure Control, AutoFlow
PRVC is a dual-control mode that targets a set tidal volume but delivers pressure-limited breaths. The ventilator automatically adjusts the inspiratory pressure breath-by-breath to achieve the target volume, adapting to changes in lung compliance and resistance.
Clinical advantages
Provides the lung protection benefits of pressure-limited ventilation while guaranteeing tidal volume.
Considerations
May increase pressures during periods of high patient effort (reverse triggering risk).
BiLevel / APRV (Airway Pressure Release Ventilation)
Trade names: BiVent, DuoPAP
APRV maintains a high CPAP level (P-High) for most of the respiratory cycle, with brief releases to a low pressure (P-Low) to allow CO₂ washout. Spontaneous breathing is encouraged throughout the cycle. It is used primarily for severe ARDS and refractory hypoxemia.
Clinical advantages
Maintains alveolar recruitment. Allows spontaneous breathing at high airway pressures.
Considerations
Complex to set up and manage. Not appropriate for all patient populations. Can be poorly tolerated.
Continuous Positive Airway Pressure (CPAP)
Used invasively and non-invasively
CPAP maintains a constant positive pressure throughout the respiratory cycle without delivering mandatory breaths. The patient breathes spontaneously against this baseline pressure, which prevents alveolar collapse and reduces the work of breathing against collapsed airways.
Clinical advantages
Improves oxygenation by maintaining FRC. Useful for OSA and mild respiratory failure.
Considerations
Provides no ventilatory support. Not appropriate if patient cannot sustain adequate minute ventilation.
Initial Ventilator Settings
When initiating mechanical ventilation, the following general starting parameters are commonly used and then titrated based on patient response and ABG results:
Tidal Volume (VT)
6–8 mL/kg IBW
Lung-protective target
Respiratory Rate
12–16 breaths/min
Adjust for PaCO₂
FiO₂
Start at 1.0, wean rapidly
Target PaO₂ 80–100
PEEP
5 cmH₂O (minimum)
Titrate per oxygenation
I:E Ratio
1:2 (standard)
1:3–4 for obstructive disease
Peak Pressure Alarm
Plateau + 10 cmH₂O
Safety limit
These are general starting points. Settings must be individualized based on the patient's underlying condition, weight, ABG results, and institutional protocols.
Frequently Asked Questions
Which ventilator mode is best for ARDS?
Lung-protective ventilation using AC/VC mode with low tidal volumes (4–6 mL/kg IBW) and appropriate PEEP is the standard of care for ARDS, based on the ARDSNet protocol. APRV may be used in refractory cases per institutional protocol.
What is the difference between volume control and pressure control?
In volume control, a set tidal volume is delivered regardless of the pressure required. In pressure control, a set pressure is delivered and tidal volume varies based on lung compliance and resistance. Volume control guarantees minute ventilation; pressure control protects against excessive pressures.
What is auto-PEEP and why is it dangerous?
Auto-PEEP (intrinsic PEEP) occurs when the patient cannot fully exhale before the next breath begins, trapping air in the lungs. It increases intrathoracic pressure, reducing cardiac output and increasing barotrauma risk. It is most common in obstructive disease (COPD, asthma) or with high respiratory rates.
When should PSV be used instead of AC mode?
PSV is most appropriate when the patient has a reliable spontaneous respiratory drive and is being weaned from mechanical ventilation. It allows the patient to control rate and timing while still receiving pressure augmentation. It is a common choice during spontaneous breathing trials.
How do I know when a patient is ready to wean?
General criteria include: resolution or improvement of the underlying cause of respiratory failure, hemodynamic stability, acceptable gas exchange on low FiO₂ and PEEP, adequate spontaneous effort, and ability to protect the airway. A formal Spontaneous Breathing Trial (SBT) is then performed to assess tolerance.
Summary
- AC mode provides full support for every breath — ideal for initial ventilation.
- SIMV reduces mandatory breath rate to encourage spontaneous breathing during weaning.
- PSV supports spontaneous breaths — essential for weaning and SBTs.
- PRVC combines volume targeting with pressure limitation for lung protection.
- APRV is used for severe ARDS when conventional modes fail to maintain oxygenation.
- CPAP provides baseline pressure support without delivering mandatory breaths.
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