Pressure Support, PRVC, and SIMV Explained

Covers pressure support mechanics and SBTs, PRVC's decelerating flow with volume targeting and pressure limits, plus SIMV's synchronized mandatory and pressure-supported breaths and pediatric versus adult use.

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🧭 REBEL Rundown

🗝️ Key Points

💨 Start with Breath Types: Controlled, assisted, and supported breaths are the foundation of all modes.
🛌 Comfort Over “Best Mode”: No mode improves mortality — focus on patient synchrony and comfort.
Know the Big 5 Modes: AC: All controlled or assisted (volume or pressure). PS: Fully spontaneous, great for SBTs. PRVC: Pressure-delivered, volume-targeted hybrid. SIMV: Mixed mode, less favored in adults. VS: Spontaneous mode with adaptive pressure.
⚠️ Watch for Pitfalls: PRVC may under-ventilate in agitation. SIMV often causes dyssynchrony.
🎯 Bottom Line: Master mode mechanics and match the vent to the patient — not the other way around.

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📝 Introduction

Mechanical ventilation can feel overwhelming, especially when faced with a sea of ventilator modes and unfamiliar terminology. In Part 2 of the series, we go beyond breath types and delivery mechanics to explore the most used modes in the ICU. We will break down each one; explaining how it works, when to use it, and why the goal isn’t the “best mode” but the most comfortable one for the patient.

️ Ventilator Modes Explained

Assist Control (AC)

Commonly mislabeled as “volume control” or “pressure control.”
Two main types:
- AC Volume: Delivers a preset tidal volume with each breath, whether machine-initiated (controlled) or patient-initiated (assisted).
- AC Pressure: Delivers a preset pressure; tidal volume varies based on compliance.
All breaths are either controlled or assisted.

Pressure Support (PS)

All breaths are spontaneous initiated by the patient.
The ventilator provides a preset level of pressure support, like a resistance band during a pull-up.
No set rate, but a backup mode (often AC) activates during apnea.
Commonly used for spontaneous breathing trials (SBTs) to assess extubation readiness.
- Typical goal: Patient breathing comfortably with PS ~5 cmH₂O and reasonable rate.

Pressure Regulated Volume Control (PRVC)

Also called autoflow or adaptive pressure ventilation.
A hybrid mode: Pressure-delivered, volume-targeted.
Delivers breaths with a decelerating flow waveform, mimicking physiologic breathing.
Adjusts pressure breath-to-breath to meet a target tidal volume with minimal required pressure.
Safety feature: Pressure limit (e.g., 30–35 cm H₂O). If exceeded, volume delivery stops early.
Pitfall: In agitated patients, rapid breathing may trick the ventilator into reducing pressure, causing under-ventilation.

Synchronized Intermittent Mandatory Ventilation (SIMV)

Less common in adult ICU but still commonly used in pediatrics.
Delivers a set number of mandatory (controlled or assisted) breaths.
Allows spontaneous, pressure-supported breaths between mandatory ones.
Example: SIMV 10 = 10 guaranteed AC breaths; additional breaths are spontaneous + supported.
Why it’s less popular: Found to be less effective than daily SBTs for weaning and frequent dyssynchrony from not giving enough PS (PS should target at least 2/3 of the AC breath volumes) .

Volume Support (VS)

A newer, fully spontaneous mode (like PS + PRVC).
Patient initiates all breaths.
The ventilator automatically adjusts pressure support to achieve a target tidal volume.
Think of it as the spontaneous cousin of PRVC—adaptive and volume-driven.

🚨 Clinical Bottom Line

Understanding ventilator modes starts with knowing breath types, delivery mechanics, and clinical goals. When it comes to choosing the right mode:

Focus less on the “best” mode and more on patient comfort and synchrony.
Recognize the strengths, limitations, and pitfalls of each mode.
Stay tuned for future episodes that dive into ventilator troubleshooting and advanced respiratory strategies.