Mechanical Ventilation Principles

Indications & Goals - Why We Ventilate

Indications (Why):

• Acute Respiratory Failure (ARF)
  • Hypoxemic: PaO2 < 60 mmHg despite FiO2 > 0.6 (e.g., ARDS, pneumonia, pulmonary edema)
  • Hypercapnic: PaCO2 > 50 mmHg with pH < 7.30 (e.g., COPD exacerbation, neuromuscular disease)
• Airway Protection
  • ↓ Glasgow Coma Scale (GCS) ≤ 8
  • Risk of aspiration (e.g., bulbar palsy)
• Apnea or Inadequate Respiratory Effort
• Control of Ventilation (e.g., head injury to manage ICP)
• Prophylactic (e.g., major surgery, trauma)

Goals (What we aim for):

• Improve Gas Exchange
  • Correct hypoxemia (↑PaO2)
  • Correct respiratory acidosis (↓PaCO2, ↑pH)
• Reduce Work of Breathing (WOB)
• Reverse Respiratory Muscle Fatigue
• Permit Sedation/Neuromuscular Blockade
• Stabilize Chest Wall

⭐ Acute respiratory acidosis (pH < 7.25, PaCO2 > 50 mmHg) despite non-invasive measures is a strong indication for intubation and mechanical ventilation.

Key Terms & Physiology - Talking the Talk

• Tidal Volume ($V_t$): Air per breath (target 6-8 mL/kg IBW).
• PEEP (Positive End-Expiratory Pressure): Pressure at end-exhalation; prevents alveolar collapse.
• PIP (Peak Inspiratory Pressure): Max pressure during inspiration; reflects airway resistance + elastic recoil.
• $P_{plat}$ (Plateau Pressure): Inspiratory hold pressure (no flow); reflects alveolar pressure. Aim < 30 cmH₂O.
• Compliance ($C$): Lung/chest wall distensibility; $C = \Delta V / \Delta P$.
  • Static Compliance is given by the formula: $C_{stat} = V_t / (P_{plat} - PEEP)$.

  ⭐ Static Compliance reflects the elastic properties of the lung and chest wall. It is characteristically decreased in conditions like ARDS and pneumonia.

• Resistance ($R$): Airway opposition to gas flow; $R = (PIP - P_{plat}) / \text{flow}$. ↑ in asthma, COPD.
• Minute Ventilation ($V_E$): Total air exhaled per minute ($V_t \times \text{RR}$).
• I:E Ratio: Ratio of inspiratory time to expiratory time (e.g., 1:2).
• FiO₂: Fraction of Inspired Oxygen delivered to the patient.

Modes & Settings - Choosing Wisely

• Common Modes:
  • Volume Control (VC/ACVC): Set $V_t$; pressure varies. Risk: Volutrauma.

    ⭐ In Assist-Control Volume Cycled (ACVC) mode, every breath (patient-triggered or time-triggered) delivers the preset tidal volume, risking volutrauma if not monitored.

  • Pressure Control (PCV): Set PIP; $V_t$ varies. Risk: Hypoventilation (if compliance ↓ / resistance ↑).
  • Pressure Support (PSV): Patient-triggered, pressure-limited, flow-cycled. Aids weaning.
  • SIMV: Mandatory breaths (VC/PC) + spontaneous breaths (PSV).
• Initial Settings (Adult):
  • $V_t$: 6-8 mL/kg IBW (ARDS: 4-6 mL/kg).
  • RR: 12-16/min (adj. for $PaCO_2$, pH).
  • PEEP: Start 5 cm H₂O (titrate for $O_2$, lung protection).
  • $FiO_2$: Start 1.0, titrate to $SpO_2$ >92% ($PaO_2$ 60-80 mmHg).
  • I:E Ratio: 1:2-1:3 (obstructive: prolong E-time, e.g., 1:4).

Complications & Weaning - The Escape Plan

• Complications:
  • VAP: Prevent (HOB 30-45°, oral care, cuff 20-30 cmH₂O).
  • Barotrauma/Volutrauma (e.g., pneumothorax).
  • O₂ Toxicity: FiO₂ > 0.6 for > 24-48h.
  • Cardio: ↓CO (high PEEP).
  • VIDD (Ventilator-Induced Diaphragmatic Dysfunction).
• Weaning:
  • Readiness: Cause improving, stable, PaO₂/FiO₂ > 150-200, PEEP ≤ 5-8, FiO₂ ≤ 0.4-0.5.
  • SBT: 30-120 min (T-piece, PSV 5-7 cmH₂O, CPAP 5 cmH₂O).

    ⭐ A Rapid Shallow Breathing Index (RSBI = RR/Vt in Liters) < 105 breaths/min/L is a good predictor of successful weaning.

  • Key Parameters (📌):
    • RSBI: $RR/V_T$ < 105.
    • MIP/NIF: < -20 cmH₂O.
    • RR < 35/min, $V_T$ > 5 mL/kg.
  • SBT Failure: RR > 35, SpO₂ < 90%, HR > 140, distress. Extubate if SBT success + airway protected.

High‑Yield Points - ⚡ Biggest Takeaways

• ARDS: Use low tidal volume (6 mL/kg IBW) to prevent VILI.
• PEEP: Improves oxygenation, recruits alveoli, prevents atelectasis.
• PCV: Decelerating flow aids lung protection in non-compliant lungs.
• VCV: Guarantees minute volume; risk of barotrauma with changing compliance.
• Weaning: RSBI < 105 is a key predictor for extubation success.
• Complications: Monitor for VAP, barotrauma, volutrauma, oxygen toxicity.
• Oxygenation: Target SpO2 88-95% (PaO2 55-80 mmHg) with lowest FiO2.