Study Note: Mechanical Ventilation

1. Definition and Purpose

Mechanical ventilation (MV) is the use of a machine (ventilator) to assist or completely take over the process of breathing when a patient is unable to maintain adequate gas exchange on their own.

Purpose:

• Support oxygenation (↑ O₂ delivery to tissues)

• Support ventilation (remove CO₂)

• Decrease the work of breathing

• Allow lung protection and rest

• Facilitate recovery in severe respiratory failure or anesthesia

2. Indications for Mechanical Ventilation

A. Respiratory Failure

1. Hypoxemic (Type I) – PaO₂ < 60 mmHg on room air

   • Causes: ARDS, pneumonia, pulmonary edema, atelectasis, pulmonary embolism

2. Hypercapnic (Type II) – PaCO₂ > 45 mmHg with pH < 7.35

   • Causes: COPD, asthma, drug overdose, neuromuscular disorders, CNS depression

3. Mixed (Type III) – e.g., severe ARDS or trauma

B. Airway Protection

• Coma, head injury, drug overdose, seizures, aspiration risk

C. Surgery / Anesthesia

• For controlled ventilation during general anesthesia

D. Shock or Multiorgan Failure

• To decrease metabolic demand and oxygen consumption

3. Basic Physiology of Ventilation

Normal breathing involves negative intrathoracic pressure generated by diaphragm contraction.
Mechanical ventilation delivers positive pressure to inflate the lungs.

Goal: Maintain adequate oxygenation (PaO₂) and ventilation (PaCO₂) without causing lung injury.

4. Basic Ventilator Components

• Gas source: Oxygen and air

• Flow/pressure sensors: Measure tidal volume and airway pressures

• Valves: Control inspiration and expiration

• Humidifier: Prevents mucosal drying

• Alarms: Detect leaks, disconnections, or high pressures

5. Key Ventilator Parameters

Parameter	Definition	Normal Range / Typical Value
FiO₂	Fraction of inspired O₂	21–100%
Vt (Tidal Volume)	Volume delivered per breath	6–8 mL/kg (ideal body weight)
RR (Respiratory Rate)	Breaths per minute	12–20/min
PEEP (Positive End-Expiratory Pressure)	Pressure left in alveoli at end expiration to prevent collapse	5–10 cmH₂O (can go higher in ARDS)
PIP (Peak Inspiratory Pressure)	Maximum pressure during inspiration	< 30–35 cmH₂O
Plateau Pressure	Pressure in alveoli after inspiration (no flow)	< 30 cmH₂O
I:E Ratio	Inspiratory:Expiratory time	1:2 (normal), may be adjusted in ARDS or asthma
Minute Ventilation (Ve)	RR × Vt	5–10 L/min

6. Modes of Mechanical Ventilation

A. Controlled Modes (Full Support)

1. Volume-Controlled Ventilation (VCV)

   • Delivers set Vt regardless of pressure

   • Used in most ICU cases

   • Advantage: Predictable CO₂ removal

   • Risk: High airway pressure → barotrauma

2. Pressure-Controlled Ventilation (PCV)

   • Delivers breath until a set pressure is reached

   • Volume varies depending on compliance

   • Advantage: Limits airway pressure, protects lungs

   • Risk: Unstable tidal volume if compliance changes

B. Assisted / Supported Modes (Partial Support)

1. SIMV (Synchronized Intermittent Mandatory Ventilation)

   • Combines mandatory breaths with spontaneous ones

   • Useful during weaning

   • Can add Pressure Support (PS) for comfort

2. Pressure Support Ventilation (PSV)

   • Patient initiates every breath; ventilator assists to a preset pressure

   • No fixed rate or tidal volume

   • Used for weaning or noninvasive ventilation

3. CPAP (Continuous Positive Airway Pressure)

   • Constant positive pressure throughout breathing cycle

   • Patient breathes spontaneously

   • Used for sleep apnea and mild respiratory failure

C. Advanced / Special Modes

• BiPAP: Noninvasive; has inspiratory (IPAP) and expiratory (EPAP) pressures

• APRV (Airway Pressure Release Ventilation): For ARDS; long inspiratory phase for alveolar recruitment

• PRVC (Pressure-Regulated Volume Control): Hybrid mode; pressure adjusted automatically to deliver set Vt

7. Oxygenation vs. Ventilation

Function	Controlled By	Key Parameters
Oxygenation	FiO₂ + PEEP	↑ PEEP = ↑ alveolar recruitment
Ventilation (CO₂ removal)	RR + Vt	↑ RR or Vt → ↓ PaCO₂

8. ABG Monitoring in Ventilation

ABG Finding	Problem	Correction
↓ PaO₂	Hypoxemia	↑ FiO₂, ↑ PEEP
↑ PaCO₂	Hypoventilation	↑ RR, ↑ Vt
↓ PaCO₂	Hyperventilation	↓ RR, ↓ Vt

9. Complications of Mechanical Ventilation

A. Pulmonary

• Barotrauma: Pneumothorax from high pressures

• Volutrauma: Alveolar overdistension

• Atelectrauma: Repeated alveolar collapse and reopening

• Oxygen toxicity: From prolonged FiO₂ > 60%

• VAP (Ventilator-Associated Pneumonia)

B. Hemodynamic

• ↓ Venous return → ↓ Cardiac output → Hypotension (due to ↑ intrathoracic pressure)

C. Other

• Diaphragmatic atrophy (disuse)

• Ventilator dependence

• Tracheal injury or stenosis (from cuff pressure)

10. Lung Protective Strategy (Especially in ARDS)

Goal: Prevent ventilator-induced lung injury (VILI)

Strategy	Target
Low tidal volume	6 mL/kg (ideal body weight)
Limit plateau pressure	< 30 cmH₂O
PEEP optimization	To prevent alveolar collapse
FiO₂ titration	Keep SpO₂ 88–95%
Permissive hypercapnia	Allow mild ↑ PaCO₂ if pH > 7.2

11. Weaning from Ventilation

Readiness Criteria

• Hemodynamically stable

• Adequate oxygenation (FiO₂ ≤ 40%, PEEP ≤ 5)

• Good mental status

• Strong cough and minimal secretions

Weaning Methods

1. Spontaneous Breathing Trial (SBT):

   • CPAP or low PSV for 30–120 minutes

   • Watch RR, HR, BP, SpO₂, and effort

2. Gradual SIMV / PSV reduction

Failure Signs

• RR > 35, HR ↑ > 20%, SpO₂ < 90%, anxiety, diaphoresis, paradoxical breathing

12. Ventilator Graphics / Loops

• Pressure-time, flow-time, and volume-time waveforms help detect:

  • Leaks, asynchrony, auto-PEEP, airway resistance, and compliance changes

• Pressure-volume loops:

  • Slope = compliance

  • Hysteresis (difference between insp/exp curve)

13. Sedation and Analgesia in Ventilated Patients

• Maintain comfort and synchrony

• Common agents: Propofol, Midazolam, Fentanyl, Dexmedetomidine

• Use sedation scales (e.g., RASS)

• Perform daily sedation interruptions

14. Summary Table

Disease	Problem	Ventilation Strategy
ARDS	↓ Compliance	Low Vt, high PEEP, FiO₂ titration
COPD / Asthma	Air trapping	Low RR, long expiratory time
Neuromuscular failure	Weak effort	Full support (AC-VC or PC)
Head injury	Prevent ↑ ICP	Avoid hypercapnia (target PaCO₂ 35–40)

15. Key Concept: Cellular Respiration & Ventilation Link

• Ventilation → Brings O₂ in, removes CO₂

• Perfusion → Delivers O₂ to tissues

• Cellular respiration → Uses O₂ to produce ATP and CO₂

• If ventilation fails → CO₂ builds up → Respiratory acidosis

• If perfusion fails → Tissues shift to anaerobic metabolism → Lactic acidosis

In Summary:

Mechanical ventilation is life-saving but must be individualized. Understanding physiology, settings, and patient responses prevents complications and improves outcomes.