Mechanical Ventilation Principles

MV Fundamentals - Breath of Life

Goals of Mechanical Ventilation:

• ↓ Work of Breathing (WOB), reduce muscle fatigue.
• Support adequate gas exchange ($O_2$ uptake, $CO_2$ removal).
• Reverse life-threatening hypoxemia (e.g., $P_aO_2$ < 60 mmHg).
• Reverse acute respiratory acidosis (e.g., pH < 7.25 due to ↑$P_aCO_2$).

Key Indications:

• Apnea or insufficient spontaneous ventilation.
• Acute Respiratory Failure:
  • Type I (Hypoxemic): $P_aO_2$ < 60 mmHg despite high $F_iO_2$.
  • Type II (Hypercapnic): $P_aCO_2$ > 50 mmHg & pH < 7.25.
• Airway protection (e.g., GCS < 8, risk of aspiration).
• Impending respiratory collapse (e.g., severe distress, muscle fatigue).
• Elective support (e.g., post-operative recovery, certain procedures).

⭐ PEEP (Positive End-Expiratory Pressure) is vital to prevent alveolar collapse (atelectrauma) and improve oxygenation by recruiting alveoli.

Ventilator Modes - Control Freaks & Helpers

• Volume Control (VC): Delivers preset tidal volume ($V_T$); airway pressure ($P_{aw}$) varies. Guarantees minute ventilation. Risk: Barotrauma.
• Pressure Control (PC): Delivers preset inspiratory pressure ($P_{insp}$); $V_T$ varies with lung mechanics. Limits $P_{aw}$. Risk: Hypoventilation.

Common Modes:

FullControl["💨 Full Control
• AC/CMV mode• VC and PC modes"]

PartialSupport["🌬️ Partial Support
• SIMV plus PSV• Patient efforts"]

Spontaneous["🫁 Spontaneous
• PSV/CPAP setting• Trialing weaning"]

FullControl -->|Weaning| PartialSupport PartialSupport -->|Transition| Spontaneous

style FullControl fill:#FDF4F3,stroke:#FCE6E4,stroke-width:1.5px,rx:12,ry:12,color:#B91C1C style PartialSupport fill:#FEF8EC,stroke:#FBECCA,stroke-width:1.5px,rx:12,ry:12,color:#854D0E style Spontaneous fill:#F1FCF5,stroke:#BEF4D8,stroke-width:1.5px,rx:12,ry:12,color:#166534

> ⭐ In Pressure Support Ventilation (PSV), cycling from inspiration to expiration typically occurs when inspiratory flow decreases to **25%** of peak inspiratory flow. This is a common default setting.


## Parameters & Monitoring - Fine-Tuning Breaths

*   **Key Parameters & Targets:**
    -   Vt: **6-8** ml/kg IBW (ARDS: **4-6** ml/kg).
    -   RR: **12-20**/min (target PaCO2).
    -   FiO2: Start **1.0**, titrate for SpO2 >**92%**.
    -   PEEP: Start **5** cmH2O, titrate.
    -   I:E Ratio: **1:2** to **1:3** (longer for obstruction, e.g., **1:4**).
    -   PIP: <**35-40** cmH2O.
    -   Pplat: <**30** cmH2O.
        > ⭐ Driving Pressure ($P_{plat}$ - PEEP) should be <**15** cmH2O to minimize Ventilator-Induced Lung Injury (VILI).
*   **Essential Monitoring:**
    -   Gas Exchange: ABG, SpO2, EtCO2 (PaCO2-EtCO2 gradient **2-5** mmHg).
    -   Lung Mechanics:
        +   Static Compliance ($C_{L}$): $C_{L} = V_T / (P_{plat} - PEEP)$.
        +   Airway Resistance ($R_{aw}$): $R_{aw} = (P_{IP} - P_{plat}) / Flow$. 
    -   Waveforms: Crucial for detecting asynchrony, auto-PEEP, leaks.
        ![Ventilator Pressure, Flow, and Volume Waveforms](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Anesthesiology_Critical_Care_Medicine_Mechanical_Ventilation_Principles/125a24f2-92ff-41bc-a085-d98991e94c9a.jpg)


## Complications & Weaning - Escape from the Machine

**Complications:**
*   Ventilator-Associated Pneumonia (VAP):
    -   Prevention: 📌 **HOST** bundle (**H**OB elevation **30-45°**, **O**ral care, **S**ubglottic suction, **T**rials of spontaneous breathing).
*   Barotrauma/Volutrauma: Pneumothorax risk; monitor peak/plateau pressures.
*   Oxygen Toxicity: From prolonged FiO2 > **0.6**; aim SpO2 **92-96%**.
*   Cardiovascular: ↓Preload, hypotension from ↑intrathoracic pressure.
*   Auto-PEEP: Air trapping, ↑work of breathing.

**Weaning:**
*   Criteria: Improved underlying cause, stable hemodynamics, PaO2/FiO2 > **150-200**, PEEP ≤ **5-8** cmH2O, FiO2 ≤ **0.4**. Good cough. RSBI < **105** ($RSBI = RR / V_T$ in Liters).
*   Spontaneous Breathing Trial (SBT): **30-120** min on T-piece or low Pressure Support Ventilation (PSV).
*   Common Weaning Modes: PSV, T-piece trial.



```mermaid
%%{init: {'flowchart': {'htmlLabels': true}}}%%
flowchart TD

Start["<b>🫁 MV Patient</b><br><span style='display:block; text-align:left; color:#555'>• Mech ventilation</span><span style='display:block; text-align:left; color:#555'>• ICU admission</span>"]

Assess["<b>📋 Ready to Wean?</b><br><span style='display:block; text-align:left; color:#555'>• Assess criteria</span><span style='display:block; text-align:left; color:#555'>• Clinical screen</span>"]

ContinueMV["<b>👁️ Continue MV</b><br><span style='display:block; text-align:left; color:#555'>• Optimize support</span><span style='display:block; text-align:left; color:#555'>• Ongoing care</span>"]

RSBI["<b>📋 RSBI < 105?</b><br><span style='display:block; text-align:left; color:#555'>• Rapid shallow breathing</span><span style='display:block; text-align:left; color:#555'>• f/Vt ratio check</span>"]

SBT["<b>🔬 SBT</b><br><span style='display:block; text-align:left; color:#555'>• Spontaneous breath</span><span style='display:block; text-align:left; color:#555'>• Trial period</span>"]

Resume["<b>👁️ Resume Support</b><br><span style='display:block; text-align:left; color:#555'>• Resume settings</span><span style='display:block; text-align:left; color:#555'>• Further optimize</span>"]

Extubate["<b>💊 Extubate</b><br><span style='display:block; text-align:left; color:#555'>• Remove tube</span><span style='display:block; text-align:left; color:#555'>• Monitor airway</span>"]

Start --> Assess
Assess -->|Not Ready| ContinueMV
Assess -->|Criteria Met| RSBI
RSBI -->|Yes| SBT
RSBI -->|RSBI High| Resume
SBT -->|Tolerated| Extubate
SBT -->|Failed| Resume

style Start fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8
style Assess fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E
style ContinueMV fill:#EEFAFF, stroke:#DAF3FF, stroke-width:1.5px, rx:12, ry:12, color:#0369A1
style RSBI fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E
style SBT fill:#FFF7ED, stroke:#FFEED5, stroke-width:1.5px, rx:12, ry:12, color:#C2410C
style Resume fill:#EEFAFF, stroke:#DAF3FF, stroke-width:1.5px, rx:12, ry:12, color:#0369A1
style Extubate fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534

⭐ The Rapid Shallow Breathing Index (RSBI) is a critical predictor for weaning success; a value < 105 indicates a high likelihood of successful extubation.

High‑Yield Points - ⚡ Biggest Takeaways

• Tidal Volume (Vt): Target 6-8 mL/kg PBW, especially in ARDS, for lung protection.
• PEEP: Optimizes oxygenation and prevents alveolar collapse; titrate for best compliance.
• Plateau Pressure (Pplat): Keep <30 cmH2O to minimize barotrauma/volutrauma.
• Driving Pressure (ΔP): Aim for <15 cmH2O (Pplat - PEEP), critical in ARDS.
• Modes: Know ACVC, ACPC, SIMV, PSV indications and differences.
• Weaning: Daily Spontaneous Breathing Trials (SBT) are crucial for liberation from ventilation.