Ventilation Strategies During Anesthesia

Ventilation Fundamentals - Breathing Blueprint

• Core Parameters:
  • Tidal Volume (VT): 6-8 mL/kg (Ideal Body Weight).
  • Respiratory Rate (RR): 10-16 breaths/min.
  • Minute Ventilation (MV): $MV = VT \times RR$.
  • PEEP (Positive End-Expiratory Pressure): 3-5 cmH2O (initial); prevents atelectasis.
  • FiO2 (Fraction of Inspired Oxygen): Titrate to SpO2 > 92%.
  • I:E Ratio: Typically 1:2 to 1:3.
• Basic Modes:
  • VCV (Volume Control): Set VT; pressure varies. Risk: barotrauma.
  • PCV (Pressure Control): Set pressure; VT varies. Use: ARDS, poor compliance.
  • SIMV (Synchronized Intermittent Mandatory): Weaning; mandatory breaths + spontaneous.
  • PSV (Pressure Support): Augments spontaneous breaths; set pressure support.
• Lung Mechanics:
  • Compliance ($C$): $C = \Delta V / \Delta P$. (Lung distensibility)
  • Resistance ($R$): $R = \Delta P / \text{flow}$. (Airway caliber)

⭐ Plateau pressure (Pplat), reflecting alveolar pressure, should be kept < 30 cmH2O to minimize Ventilator-Induced Lung Injury (VILI).

Lung Protective Ventilation - Gentle Air Waves

Aims to minimize Ventilator-Induced Lung Injury (VILI) via gentle mechanical support.

• 📌 Core Principles (The "LUNG" approach):
  • Low Tidal Volume ($V_T$): <6-8 mL/kg IBW
  • Use PEEP: Optimal, typically 5-10 cmH₂O
  • No high pressures:
    • Plateau Pressure ($P_{plat}$): <30 cmH₂O
    • Driving Pressure ($%Delta P = P_{plat} - PEEP$): <15 cmH₂O
  • Gentle handling: Avoids repetitive alveolar collapse/reopening.
• Benefits: ↓VILI (baro-, volu-, atelc-, bio-trauma), improved outcomes.
• Standard for: ARDS; increasingly for all ventilated patients.

⭐ Driving pressure ($%Delta P$) is independently associated with survival in ARDS patients, making it a critical target.

One-Lung Ventilation - Solo Lung Symphony

*Isolates one lung for thoracic surgery (lobectomy) or unilateral pathology (hemoptysis). Physiology: Creates intentional shunt. Relies on Hypoxic Pulmonary Vasoconstriction (HPV) to divert blood to ventilated lung.

• HPV impaired by: Volatiles >1 MAC, vasodilators, severe hypocapnia/alkalosis. Techniques:
• Double-Lumen Tube (DLT): Gold standard; Left DLT preferred.
• Bronchial Blocker (BB): For difficult airway/existing ETT. Ventilation (Ventilated Lung):
• VT: 4-6 mL/kg (PBW).
• PEEP: 5-10 cm H₂O.
• FiO₂: Start 1.0, titrate. Permissive hypercapnia is accepted. Managing OLV Hypoxemia (Confirm DLT/BB position first!):

⭐ After confirming tube position & FiO₂ 1.0, PEEP (ventilated lung) & CPAP (non-ventilated lung) are key to improve OLV oxygenation.

Special Populations & Monitoring - Tailored Breaths & Vital Signs

• Obese Patients (BMI >30 kg/m²):
  • Challenges: ↓FRC, atelectasis, hypoxemia.
  • Strategy: RSI, PEEP 5-10 cmH2O, recruitment maneuvers. Vt 6-8 mL/kg IBW.
  • Positioning: Reverse Trendelenburg.
• Pediatrics:
  • Anatomy: ↑O2 consumption, ↓FRC, compliant chest.
  • Strategy: Vt 6-8 mL/kg, cuffed ETT preferred. PCV often used.
• COPD/Asthma:
  • Risk: Air trapping, dynamic hyperinflation, bronchospasm.
  • Strategy: ↑Expiratory time (I:E 1:3-1:4), ↓RR, controlled permissive hypercapnia.
• Laparoscopy:
  • Physiology: CO2 insufflation → ↑PaCO2, ↓Compliance.
  • Strategy: PCV or PCV-VG, PEEP 5-8 cmH2O, monitor EtCO2.
• Essential Monitoring:
  • Capnography (EtCO2): 35-45 mmHg.
  • Pulse Oximetry (SpO2): >94%.
  • Airway Pressures: Pplat <30 cmH2O, Driving Pressure (Pplat - PEEP) <15 cmH2O.
  • TOF for NMB.

⭐ Plateau pressure (Pplat) should be maintained <30 cmH2O to minimize ventilator-induced lung injury (VILI).

High-Yield Points - ⚡ Biggest Takeaways

• Lung-Protective Ventilation (LPV): Low Vt (4-6 mL/kg IBW), PEEP, low driving pressure (<15 cmH₂O).
• PEEP: Routinely 5-10 cmH₂O prevents atelectasis; ↑ in obesity/laparoscopy.
• PCV: Limits PIP; preferred for high airway pressure risk (laparoscopy, OLV).
• VCV: Delivers set Vt; monitor PIP to prevent barotrauma.
• Permissive Hypercapnia: PaCO₂ 45-55 mmHg (pH >7.25) in LPV/asthma.
• Driving Pressure (ΔP): Pplateau - PEEP; aim <14-15 cmH₂O.
• Recruitment Maneuvers: For atelectasis, then apply optimal PEEP.