Study Notes: Mechanical Ventilation: Core Concepts

Mechanical ventilation is the process of breathing for a patient when they cannot maintain adequate ventilation on their own. To understand it, you must first know the fundamental variables that control breaths and the pressures that result from delivering them.

1. Basic Ventilation Terms

Term	Meaning
Respiratory Rate (RR)	Number of breaths delivered per minute
Tidal Volume (VT)	Amount of gas delivered in one breath
Minute Ventilation (VE)	Total amount of gas moved per minute (RR × VT)
FiO₂	Fraction of inspired oxygen (%) delivered by the ventilator

2. Tidal Volume (VT)

• VT = amount of gas with each breath (e.g., 500 mL = 0.5 L)

• Delivered from blended gases:

  • 21% medical air (room air) + 100% oxygen from wall source → ventilator mixes to reach the set FiO₂

• VT changes with the depth of breath; mechanical ventilation makes it precise and consistent

3. Respiratory Rate (RR)

• Number of breaths given per minute

• RR controls timing

  • Example: RR = 10 → 60 sec / 10 = 1 breath every 6 seconds

  • Example: RR = 20 → 1 breath every 3 seconds

4. Minute Ventilation (VE) — The Cornerstone

VE = RR /times VT

• Determines how well a patient blows off CO₂

• Hyperventilation = increased minute ventilation (VE), not just fast RR

  • Example: RR 24 but VT only 150 mL → VE = 3.6 L/min (NOT hyperventilation)

• Normal VE ≈ 5–8 L/min

5. Breath Control Concepts

Concept	Definition
Trigger	What starts the breath (Time, Pressure, or Flow)
Cycle	What ends inspiration and allows exhalation

• Time-triggered: ventilator initiates breath based on RR

• Patient-triggered: patient creates a negative pressure or flow change to trigger a breath

• Exhalation is always passive (both naturally and mechanically)

6. Airway Pressures

Pressure	What it Represents
PIP (Peak Inspiratory Pressure)	Highest pressure during inspiration. Reflects airway resistance + alveolar pressure
Plateau Pressure (Pplat)	Pressure in alveoli when flow is paused (inspiratory hold) — reflects alveolar pressure only
PEEP (Positive End-Expiratory Pressure)	Pressure left in lungs at end-exhalation to prevent alveolar collapse
MAP (Mean Airway Pressure)	Average pressure in the airways across the entire breath cycle
Driving Pressure	Pplat − PEEP (associated with ARDS outcomes)

7. Pressure Waveform Logic

• PIP > Pplat → airway resistance problem (bronchospasm, secretions, kinked tube)

• High Pplat → poor lung compliance (ARDS, pulmonary edema, fibrosis)

• PEEP prevents atelectasis by keeping alveoli partially open at end-exhalation

8. PEEP (Why It Matters)

• Prevents collapse between breaths

• Improves oxygenation by increasing functional residual capacity

• Too much PEEP = risk of barotrauma or hypotension

9. FiO₂

• Oxygen concentration delivered

• Ventilator blends O₂ and air to achieve the target FiO₂

  • Example: FiO₂ 40% ≈ mix of 100% O₂ + 21% medical air

• Goal: Use the lowest FiO₂ needed to maintain adequate oxygenation (to avoid oxygen toxicity)

10. Mode Reminder (Not Detailed Here)

• In Volume Control: VT is fixed, pressure varies

• In Pressure Control: Pressure is fixed, VT varies → minute ventilation may also vary

SUMMARY CONCEPT MAP

1. RR + VT → Minute Ventilation (CO₂ control)

2. FiO₂ + PEEP → Oxygenation

3. Pressures (PIP, Pplat, MAP, Driving Pressure) → Lung mechanics and safety

4. Trigger & Cycle → Breath timing and pattern

Mastering these concepts is the foundation of safe ventilation management.