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Modes of Mechanical Ventilation and Weaning Protocol

INTRODUCTION

1. Size of breath — tidal volume (volume control) or inspiratory pressure (pressure control)
2. Frequency — set respiratory rate or patient-triggered
3. PEEP — baseline positive pressure maintained in the circuit
4. FiO₂ — fraction of inspired oxygen

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CLASSIFICATION OF VENTILATOR MODES

A. CONTROLLED MANDATORY VENTILATION (CMV)

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B. ASSIST CONTROL VENTILATION (AC / CMV with assist)

1. AC – Volume Control (AC-VC) — Most common initial mode

• Guarantees minimum VT and minute ventilation → ensures CO₂ clearance
• Plateau pressure (measured via inspiratory hold with zero flow) reflects alveolar pressure; keep ≤30 cmH₂O (lung-protective)
• Peak pressure = dynamic airway pressure (flow × resistance) + alveolar pressure; elevated peak with normal plateau → ↑ airway resistance (kinked tube, bronchospasm, auto-PEEP)
• Disadvantage: High plateau pressures → barotrauma; dyspneic patients may have flow starvation → patient-ventilator dyssynchrony and increased work of breathing

2. AC – Pressure Control (AC-PC / PCV)

• A set driving pressure is applied above PEEP each breath; inspiration ends when flow decreases below a threshold (~20% peak flow)
• VT varies depending on lung compliance → must monitor closely; no guaranteed minute ventilation
• Useful when limiting peak pressures (ARDS, post-thoracic surgery with fresh suture lines)
• Important hazard: Spontaneously breathing patients generate negative intrathoracic pressure; net transalveolar pressure = set driving pressure + patient's negative effort → can produce larger VT than expected → volume trauma despite apparently normal ventilator pressures

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C. PRESSURE-REGULATED VOLUME CONTROL (PRVC)

• Clinician sets a target VT (like volume control) but the ventilator delivers the lowest pressure necessary to achieve it (like pressure control)
• Senses patient inspiratory effort and allows variable inspiratory flow → better patient comfort and ventilator synchrony than classic AC-VC
• VT can vary breath-to-breath (ventilator adaptation is not instantaneous)
• Caution in ARDS: Strong patient efforts can generate VT exceeding the set target → risk of volume trauma; must monitor closely

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D. PRESSURE SUPPORT VENTILATION (PSV)

• VT determined by lung compliance + patient effort; not guaranteed
• RR, VT, and VE must be monitored closely → risk of apnea and hypoventilation
• Uses: Transitioning off MV (primary weaning mode), ventilation for airway support in less-sedated patients, analogous to BiPAP machine
• Gradually decrease PSV during weaning by 2–3 cmH₂O steps targeting VT 4–6 mL/kg, RR <30 breaths/min

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E. SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION (SIMV)

• If rate is set high + patient deeply sedated → essentially equivalent to volume control
• If rate set low → essentially equivalent to PSV with minimal mandatory sigh breaths
• Weaning with SIMV: Progressively decrease mandatory rate by 1–2 breaths/min; when IMV rate reaches 2–4 breaths/min with acceptable ABGs and RR <30, discontinue MV

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F. CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP)

• Constant positive pressure maintained throughout respiratory cycle (both inspiration and expiration)
• Patient breathes entirely spontaneously — no pressure support
• Recruits collapsed alveoli, reduces work of breathing, improves oxygenation
• Used as part of SBT (spontaneous breathing trial) and weaning
• Non-invasive CPAP via mask for obstructive sleep apnea, mild-moderate respiratory failure

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G. NONINVASIVE POSITIVE PRESSURE VENTILATION (NIV / BiPAP)

• Delivered via nasal or full-face mask at two pressure levels:
  • IPAP (inspiratory positive airway pressure) → assists inhalation
  • EPAP (expiratory positive airway pressure) → equivalent to PEEP
• Key indications:
  • COPD exacerbation with hypercapnia (pH 7.25–7.35) → reduces intubation need, shortens hospital stay; pH <7.2 → usually requires intubation
  • Acute cardiogenic pulmonary edema (diuresis + vasodilators rapidly improve mechanics)
  • Chronic respiratory failure (severe scoliosis, neuromuscular disease, COPD with chronic hypercapnia)
• Contraindications: inability to protect airway, high aspiration risk, inability to clear secretions, facial trauma, hemodynamic instability, upper airway obstruction

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H. ADVANCED MODES

1. Airway Pressure Release Ventilation (APRV) / Bilevel Ventilation

• Two levels of CPAP: PHigh for a longer time (THigh) + PLow for a shorter time (TLow)
• Allows spontaneous breathing at both pressure levels → minimizes sedation requirement
• PHigh ensures oxygenation; PLow/TLow ensures ventilation without derecruitment
• Best for ARDS with severely reduced compliance and large chest wall
• Recent meta-analysis (~19 years of data) showed mortality benefit of APRV vs. conventional modes in acute hypoxemic respiratory failure

2. High-Frequency Oscillatory Ventilation (HFOV)

• Extremely low VT (1–3 mL/kg) at very high frequency (3–15 Hz)
• Theoretical appeal in ARDS (ultralow barotrauma/volutrauma)
• Clinical trials have not shown benefit in adults; may have a role in pediatric ARDS
• Used as salvage therapy or bridge to ECMO in refractory ARDS

3. ECMO (Extracorporeal Membrane Oxygenation)

• For entirely refractory hypoxemia unresponsive to all ventilator strategies
• VV-ECMO (venovenous): isolated lung pathology
• VA-ECMO (venoarterial): combined cardiac + lung dysfunction

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MODES SUMMARY TABLE

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WEANING FROM MECHANICAL VENTILATION

Concept

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Step 1: Daily Assessment of Readiness to Extubate

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Step 2: Spontaneous Breathing Trial (SBT)

• Reduce ventilator support to minimal (pressure support 5–7 cmH₂O, PEEP 5 cmH₂O) to compensate for ETT resistance only
• Duration: 30–120 minutes
• Alternatively: T-piece trial (no positive pressure support)

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Weaning Indices (Physiologic Parameters)

Important: Over-reliance on these indices versus SBT outcome delays extubation unnecessarily. SBT remains the gold standard.

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Weaning Techniques (When Gradual Weaning Is Needed)

1. SIMV Weaning

• Decrease mandatory rate by 1–2 breaths/min as tolerated
• Monitor ABGs and RR at each step (minimum 15–30 min)
• Target: RR < 30, PaCO₂ < 45–50 mmHg (or pH > 7.35)
• Discontinue MV when rate reaches 2–4 breaths/min with acceptable ABGs

2. PSV Weaning (Preferred method)

• Decrease pressure support by 2–3 cmH₂O stepwise
• Monitor VT (target 4–6 mL/kg), RR (< 30), ABGs
• Extubate when PS = 5–8 cmH₂O (tube compensation level) with acceptable parameters

3. T-piece Trials / CPAP Sprints

• Patient breathes through T-piece (no ventilator support) or low-level CPAP
• Duration progressively increased over hours/days
• Assess for respiratory distress, haemodynamic changes, and desaturation

Evidence: Daily SBT protocols reduce ventilator days by 25% and ICU length of stay by 10% compared to traditional gradual weaning.

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Step 3: Extubation

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Weaning Algorithm (Harrison's 22E)

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COMPLICATIONS OF MECHANICAL VENTILATION

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KEY POINTS FOR MD EXAM

1. AC-VC is the most common initial mode; guarantees VT and minute ventilation but requires monitoring of plateau pressure (keep ≤30 cmH₂O)
2. PSV is the preferred weaning mode; every breath is patient-triggered
3. SIMV is a hybrid; useful for gradual weaning by reducing mandatory rate
4. RSBI = f/VT(L); <100–105 predicts successful extubation
5. SBT duration: 30–120 minutes; >70% success rate predicts extubation
6. SBT-based daily liberation protocols reduce ventilator days by 25%
7. Post-extubation NIV or HFNO in high-risk patients reduces reintubation
8. In ARDS: lung-protective strategy = VT 6 mL/kg IBW, plateau ≤30 cmH₂O, PEEP titrated to lower inflection point
9. Approaching MV removal as a "wean" (vs. active liberation) extends unnecessary MV by up to 40%

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