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Modes of Mechanical Ventilation

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Basic / Standard Modes

1. Controlled Mechanical Ventilation (CMV)

• Volume-Controlled Ventilation (VCV): preset tidal volume, airway pressure is variable — used when CO₂ clearance and minute ventilation must be tightly controlled.
• Pressure-Controlled Ventilation (PCV): preset inspiratory pressure, tidal volume is variable — preferred when compliance is very poor and barotrauma risk is high.

In CMV, prolonged use promotes diaphragmatic atrophy and contractility dysfunction.

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2. Assist-Control Ventilation (AC)

• AC-VC (Volume Control): tidal volume set at 6–8 mL/kg predicted body weight; plateau pressure must be kept < 30 cm H₂O.
• AC-PC (Pressure Control): inspiratory pressure set; minute ventilation must be closely monitored because changes in compliance alter tidal volume.

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3. Synchronized Intermittent Mandatory Ventilation (SIMV)

• If sedation is deep or rate set high → functionally equivalent to VC
• If rate set low → functionally equivalent to PSV with occasional "sigh" breaths
• Useful for gradual weaning by reducing mandatory rate over time

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4. Pressure Support Ventilation (PSV)

• Requires the patient to have a reliable respiratory drive (apnea alarms mandatory)
• Reduces work of breathing
• Primary use: ventilator weaning and liberation
• Can be used stand-alone or embedded within SIMV

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5. Continuous Positive Airway Pressure (CPAP)

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Breath Types Within Modes

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The Pressure-Volume Curve and Safe Window

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Newer / Advanced Modes

6. Airway Pressure Release Ventilation (APRV)

• Tidal volume generated = difference between P-high and P-low
• Achieves high mean airway pressure → improved alveolar recruitment
• Hemodynamically well tolerated with minimal sedation
• Used in severe hypoxemia and ARDS
• A recent meta-analysis (nearly 19 years of data) suggested a mortality benefit in acute hypoxemic respiratory failure vs. conventional modes

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7. Bilevel Positive Airway Pressure Ventilation (BiLevel / DUOPAP)

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8. High-Frequency Oscillatory Ventilation (HFOV)

• Used as a salvage mode or bridge to ECMO in severe ARDS
• Physiologically appealing for lung protection in ARDS
• Clinical trials (e.g., OSCILLATE, OSCAR) have not shown survival benefit in adults; may even increase mortality in some subgroups
• May have benefit in pediatric ARDS

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9. Proportional Assist Ventilation (PAV) / Proportional Pressure Support (PPS)

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Key Concepts: Ventilator-Induced Lung Injury (VILI)

• Barotrauma — excess airway pressure
• Volutrauma — alveolar overdistension from large tidal volumes
• Atelectrauma — cyclic recruitment/derecruitment
• Biotrauma — inflammatory mediator release from injured lung

• Tidal volume: 6–8 mL/kg predicted body weight
• Plateau pressure: < 30 cm H₂O
• Monitor for auto-PEEP (measured via expiratory hold)

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Summary Comparison

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Mechanical Ventilation — Comprehensive Overview

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1. Objectives of Mechanical Ventilation

Increased work of breathing is the most common reason for intubation. It can be clinically assessed by palpating phasic sternomastoid contraction, tracheal tug, suprasternal recession, and intercostal recession — not by any single objective measurement threshold. — Fishman's Pulmonary Diseases

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2. Indications for Mechanical Ventilation

• Acute hypoxemic respiratory failure (PaO₂/FiO₂ < 200 in ARDS)
• Acute hypercapnic respiratory failure (respiratory acidosis with pH < 7.20–7.25)
• Apnea or impending respiratory arrest
• Airway protection (obtunded, GCS < 8, inability to protect airway from secretions/aspiration)
• Excessive work of breathing that is unsustainable
• Refractory status asthmaticus or status epilepticus

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3. Initial Ventilator Settings

Predicted body weight (not actual) drives VT settings because lung size correlates with height, not weight — critical in obese patients.

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4. Ventilator-Induced Lung Injury (VILI)

• Plateau pressure (measured by inspiratory hold): target < 30 cmH₂O
• Driving pressure = Plateau pressure − PEEP: target < 15 cmH₂O (emerging evidence)
• Auto-PEEP (measured by expiratory hold): reflects breath stacking/air trapping

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5. PEEP — Titration and Rationale

• Increasing mean airway pressure
• Recruiting collapsed alveoli
• Reducing intrapulmonary shunt

1. ARDS Network FiO₂–PEEP table — empirical combinations of FiO₂ and PEEP levels
2. Pressure-volume curve — set PEEP just above the lower inflection point
3. Esophageal pressure monitoring — estimates transpulmonary pressure to individualize PEEP; however, a recent phase 2 trial showed no benefit over empirical high PEEP-FiO₂ titration

High PEEP must be balanced against hemodynamic compromise: elevated intrathoracic pressure reduces venous return and cardiac output.

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6. MV in ARDS — Special Strategies

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7. MV in Obstructive Disease (COPD / Asthma)

• Permissive hypercapnia — accept higher PaCO₂ to allow longer expiratory time
• Low respiratory rate (10–14/min) and long expiratory time (I:E ≥ 1:3 or 1:4)
• Minimize PEEP in asthma (air trapping already generates intrinsic PEEP)
• Extrinsic PEEP set at ~75–85% of measured auto-PEEP in COPD to offset triggering effort
• Deep sedation ± paralysis during most severe phase of asthma
• Non-invasive positive pressure ventilation (NIPPV/BiPAP) is the preferred initial approach in COPD exacerbations — reduces need for intubation and mortality

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8. Non-Invasive Ventilation (NIV) vs. Invasive MV

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9. Liberation from Mechanical Ventilation (Weaning)

Step 1: Daily Readiness Assessment

Step 2: Weaning Predictor — Rapid Shallow Breathing Index (RSBI)

• Measured during unaided spontaneous breathing (not with PS or CPAP, which falsely lower it)
• RSBI < 100 → likely to tolerate extubation (high sensitivity ≥ 0.90)
• RSBI > 100 → rapid shallow breathing pattern predicts failure

RSBI is a screening test (high sensitivity), not a confirmatory test. A positive screen should prompt an SBT, not immediate extubation. — Fishman's Pulmonary Diseases

Step 3: Spontaneous Breathing Trial (SBT)

• Comfortable, no marked anxiety or dyspnea
• RR < 35/min
• SaO₂ > 90%
• SBP 90–180 mmHg
• HR change < 20% from baseline

Step 4: Extubation Decision

• Ability to follow commands
• Adequate cough and ability to manage secretions
• No excessive secretion burden

• COPD patients — extubate to NIPPV → reduced mortality and HAP
• Chronic hypercapnic respiratory failure — NIPPV reduces reintubation rates
• General low-risk patients — HFNC reduces reintubation vs. conventional O₂ at 48–72h

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10. Failure to Wean — Causes

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11. Complications of Mechanical Ventilation

Pulmonary

• VILI (barotrauma, volutrauma, atelectrauma, biotrauma)
• Pneumothorax, pneumomediastinum
• Ventilator-Associated Pneumonia (VAP)

VAP — Key Facts

• Incidence: up to 15% of mechanically ventilated patients
• Mortality: ~50% when it occurs
• Defined as pneumonia ≥ 48h after intubation
• Common pathogens: S. aureus, P. aeruginosa, gram-negative enteric rods
• Empiric treatment: IV β-lactam (pip-tazo, cefepime, or ceftazidime) ± vancomycin/linezolid (for MRSA risk) or carbapenem (MDR gram-negative risk)
• Treatment duration: 7 days (shorter is non-inferior)

VAP Prevention Bundle

Extrapulmonary Complications

• GI stress ulcers/bleeding → PPI or H₂ blocker prophylaxis
• DVT/PE → pharmacological ± mechanical prophylaxis
• Delirium and sleep disruption → minimize sedation, early mobility, day-night cycle
• ICU-acquired weakness (diaphragm and peripheral muscle atrophy) → minimize NMB, early physiotherapy

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12. Hemodynamic Effects of Positive Pressure Ventilation

• Increases intrathoracic pressure → reduces venous return → reduces preload → may drop cardiac output
• High PEEP exaggerates this effect
• RV afterload may increase (alveolar overdistension compresses alveolar vessels)
• Clinically important in patients with hypovolemia or RV dysfunction — always assess fluid responsiveness when hemodynamic instability develops on the ventilator

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