Understanding Mechanical Ventilation: Tidal Volume and Pressure

This chapter explores mechanical ventilation's complexities, particularly tidal volume and the interplay between pressure and lung compliance. It highlights the two main breathing types and emphasizes the significance of these concepts in context to upcoming mechanical ventilation modes.

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🧭 REBEL Rundown

🗝️ Key Points

- 💨 Master the 3 Types of Breaths
  Control, Assist, and Spontaneous — know the difference before tackling ventilator modes.
- 📦 Breath Delivery: Volume vs. Pressure
  Volume-Targeted = fixed volume → monitor pressure
  📈 Pressure-Targeted = fixed pressure → monitor volume
- 🫁 Lung Compliance = Pressure-Volume Relationship
  Volume mode: ↑ pressure = ↓ compliance (stiff lungs)
  Pressure mode: ↓ tidal volume = ↓ compliance

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📝 Introduction

For many medical residents, the ICU can feel like stepping into a pressure cooker. At the heart of that stress often lies one intimidating machine: the ventilator. Rather than diving headfirst into complex ventilator modes, this episode lays a critical foundation by breaking down the basic building blocks of mechanical ventilation, something every clinician should master before moving on to more advanced concepts. Once you know the 3 types of breaths and how those breaths are delivered, you can more easily understand most of the mechanical ventilator modes.

🧮 The 3 Types of Breaths

To simplify things, we use a pull-up analogy to explain the types of ventilator breaths:

🫁 The 3 Types of Breaths…It's Like

😮‍💨 Breath Delivery: Volume vs. Pressure

Once you know the type of breath, the next key concept is how it’s delivered:
1. Volume-Targeted Delivery
- The ventilator delivers a fixed tidal volume (e.g., 400 mL) with each control or assist breath.
- What to monitor: Pressure. As lung compliance worsens, pressure increases.
- Risk: Barotrauma if the pressure becomes too high.
2. Pressure-Targeted Delivery
- The ventilator delivers air to a preset pressure (e.g., 15 cm H₂O).
- What to monitor: Tidal volume. As compliance drops, so does delivered volume.
- Adjustment: Modify pressure to maintain appropriate ventilation.

🧱 Putting It All Together: Lung Compliance

The relationship between pressure and volume is described by compliance:
- 📐 Compliance = Δ Volume / Δ Pressure
In volume mode:
- Rising pressure to achieve the same volume = decreased compliance (stiff)
- Decreasing pressure to achieve the same volume = increased compliance (loose)
In pressure mode:
- Dropping tidal volume at a constant pressure = decreased compliance (stiff)
- Rising tidal volume at a constant pressure = increased compliance (loose)