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:
| Mode | Type | Trigger | Cycle | Goal | Pro | Con |
|---|---|---|---|---|---|---|
| AC/CMV | VC/PC | Pt/Time | Vol/Time | Full support; Set $V_T$/$P_{insp}$ | $V_T$ assured (VC)/$P_{aw}$ limit (PC) | Asynchrony, baro/volutrauma |
| SIMV | VC/PC | Pt/Time (Mand); Pt (Spont) | Vol/Time (Mand); Flow (Spont) | Weaning, Pt sync | ↓Sedation, muscle use | ↑WOB if low |
| PSV | Pressure | Pt | Flow | Spont. support, weaning | Comfort, ↓WOB | Apnea risk, variable $V_T$ |
| PRVC | Hybrid | Pt/Time | Time | Target $V_T$ @ low $P_{aw}$ | $V_T$ assured, $P_{aw}$ protected | Slow response |
| %%{init: {'flowchart': {'htmlLabels': true}}}%% | ||||||
| flowchart TD |
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.
## 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.
