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Mechanical Ventilation - Types and Modes

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What is Mechanical Ventilation?

1. Ventilatory failure - impaired CO2 elimination
2. Hypoxemia - inadequate oxygenation

Indications (criteria for initiating mechanical ventilation)

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TYPES OF MECHANICAL VENTILATION

1. Negative-Pressure Ventilation (Iron Lung)

• Creates a negative pressure around the thorax, causing the chest wall to expand and air to flow in
• Does NOT require tracheal intubation
• Limitations: Cannot overcome substantial increases in airway resistance or decreases in pulmonary compliance; limits access to the patient
• Rarely used today; historical significance in polio epidemics

2. Positive-Pressure Ventilation (PPV)

• The dominant form used in modern clinical practice
• Lung inflation is achieved by periodically applying positive pressure to the upper airway via:
  • Tight-fitting mask - Non-invasive mechanical ventilation (NIV/BiPAP/CPAP)
  • Endotracheal tube (ETT) or tracheostomy tube - Invasive mechanical ventilation

Advantages:

• Can overcome increased airway resistance and decreased compliance
• Allows manipulation of inspiratory gas flow and pressure

Disadvantages:

• Altered ventilation-to-perfusion (V/Q) relationships
• Adverse circulatory effects (impaired venous return, reduced cardiac output)
• Risk of barotrauma and volutrauma
• Increases physiological dead space (gas flow preferentially goes to non-dependent, more compliant areas while blood flow favors dependent areas)

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Positive-Pressure Ventilation - Subtypes

A. Non-Invasive Ventilation (NIV)

• CPAP (Continuous Positive Airway Pressure): Provides a single constant positive pressure throughout both inspiration and expiration. No mandatory breaths. Used for obstructive sleep apnea, mild hypoxemic failure.
• BiPAP (Bilevel Positive Airway Pressure): Provides two pressure levels - a higher inspiratory positive airway pressure (IPAP) and a lower expiratory positive airway pressure (EPAP). Supports spontaneous breathing. Useful in COPD exacerbations, hypercapnic failure, cardiogenic pulmonary edema.

B. Invasive Ventilation

• Volume-targeted (Volume-Controlled): A preset tidal volume is delivered
• Pressure-targeted (Pressure-Controlled): A preset inspiratory pressure is delivered; tidal volume varies with compliance and resistance

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MODES OF MECHANICAL VENTILATION

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STANDARD / BASIC MODES

1. Controlled Mechanical Ventilation (CMV)

• The ventilator delivers all breaths at a preset rate
• Patient cannot trigger the machine
• Fixed tidal volume and fixed rate regardless of patient effort
• Indication: Patients with little or no ventilatory effort (e.g., paralyzed, sedated, apneic from brain damage)
• Disadvantage: Awake patients require heavy sedation +/- neuromuscular blockade
• Can be:
  • VC-CMV (Volume-Controlled CMV): preset tidal volume delivered at a fixed flow rate
  • PC-CMV (Pressure-Controlled CMV): preset pressure delivered for a fixed inspiratory time

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2. Assist-Control Ventilation (AC / ACV)

• Most commonly used mode in clinical practice
• A pressure sensor in the breathing circuit allows the patient's inspiratory effort to trigger inspiration
• If the patient does not trigger within a set time, the ventilator delivers a mandatory breath
• Every triggered breath (whether patient- or machine-initiated) delivers a full, preset breath
• VC-AC (Volume Control AC): Set tidal volume delivered each breath - most common because it controls minute ventilation
• PC-AC (Pressure Control AC): Set inspiratory pressure delivered - used when barotrauma is a concern

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3. Synchronized Intermittent Mandatory Ventilation (SIMV)

• Guarantees a set minimum number of mandatory positive-pressure breaths per minute
• If the patient's respiratory rate exceeds the set rate, the ventilator allows spontaneous breaths - these are either unassisted or receive pressure support (SIMV + PS)
• Breaths are synchronized with patient effort within a trigger window to avoid breath stacking
• Key difference from AC: In SIMV, spontaneous breaths above the set rate are NOT fully supported (patient does their own work); in AC every breath is fully supported
• SIMV + PS: Pressure support added for spontaneous breaths to reduce work of breathing
• Disadvantage: Evidence suggests SIMV delays liberation (weaning) from mechanical ventilation compared to other modes; has largely fallen out of favor for this reason

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4. Pressure Support Ventilation (PSV)

• Most widely used mode of mechanical ventilation globally (Murray & Nadel's)
• All breaths are patient-triggered - no mandatory breaths
• Ventilator delivers flow to maintain a set inspiratory pressure once triggered
• The ventilator cycles off (ends inspiration) once inspiratory flow declines to a threshold (usually 10-35% of peak inspiratory flow) - this is flow-cycling
• Patient controls their own respiratory rate, tidal volume, and inspiratory time
• Analogous to BiPAP in spontaneously breathing patients
• Advantages: Excellent synchrony, less work of breathing, patient comfort, ideal for weaning
• Disadvantage: No backup rate - if patient becomes apneic, no breath is delivered (must have apnea backup alarm)
• Clinical use: Weaning from mechanical ventilation, cooperative spontaneously-breathing patients

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5. Continuous Positive Airway Pressure (CPAP) on the Ventilator

• A circuit pressure above atmospheric is maintained throughout the respiratory cycle
• No mandatory breaths - entirely patient-driven
• Keeps alveoli open (recruits alveoli), improves FRC and oxygenation
• Used as a weaning mode and for patients with primarily oxygenation failure

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ADVANCED / SPECIAL MODES

6. Pressure-Regulated Volume Control (PRVC)

• A hybrid mode: uses pressure-controlled breaths but the pressure target is automatically adjusted by the ventilator as feedback to achieve a target tidal volume
• If respiratory mechanics improve (better compliance), the applied pressure is reduced automatically
• If mechanics worsen, pressure is increased
• Advantage: Combines the benefits of volume guarantee with the flow pattern of pressure control (better synchrony, lower peak pressures)

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7. Volume Support (VS)

• Patient-triggered, pressure-targeted, flow-cycled breaths
• The pressure target is automatically adjusted to achieve a set target volume (similar concept to PRVC but for spontaneous breaths)
• Like PSV with volume guarantee

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8. Adaptive Support Ventilation (ASV)

• An assist-control, pressure-targeted, time-cycled mode
• Utilizes respiratory system mechanics (compliance, resistance) to set the optimal tidal volume - respiratory rate pattern
• The clinician sets only the desired minute ventilation and patient height (for estimating dead space)
• The ventilator automatically adjusts settings based on lung mechanics

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9. Inverse Ratio Ventilation (IRV)

• Reverses the normal I:E ratio (normal = 1:3 or greater) to greater than 1:1 (e.g., 1.5:1 or 2:1)
• Achieved by adding an end-inspiratory pause, decreasing peak inspiratory flow (in VC-IRV), or setting prolonged inspiratory time (PC-IRV)
• Prolonged inspiration recruits collapsed/flooded alveoli and improves oxygenation
• Generates intrinsic (auto-) PEEP due to incomplete expiration
• Used in severe ARDS when conventional ventilation fails
• Disadvantage: Very uncomfortable - requires heavy sedation and neuromuscular blockade

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

• Provides two levels of CPAP: a high pressure (P-High) maintained for a long time (T-High), which is intermittently released to a low pressure (P-Low) for a brief time (T-Low)
• Patient is allowed to breathe spontaneously at both pressure levels throughout
• The brief release to P-Low augments CO2 elimination
• Typical initial settings: P-High 20-30 cmH2O, T-High ~4-6 sec; P-Low 0-5 cmH2O, T-Low 0.4-0.8 sec
• P-Low and T-Low are set to prevent derecruitment (usually T-Low allows only 50-70% of expiratory flow)

• Less circulatory depression
• Less pulmonary barotrauma
• Less sedation required (patient breathes spontaneously)
• Attractive alternative for patients with high peak inspiratory pressures and reduced lung compliance (ARDS)

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11. High-Frequency Oscillatory Ventilation (HFOV)

• Delivers extremely low tidal volumes (often less than anatomic dead space) at very high rates (3-15 Hz, i.e., 180-900 breaths/min)
• Gas exchange occurs by mechanisms beyond bulk flow (asymmetric velocity profiles, molecular diffusion, pendelluft)
• Maintains a high mean airway pressure to keep alveoli open (open lung strategy)
• Clinical use: Salvage mode for severe refractory ARDS; bridge to ECMO
• Evidence: Clinical trials (OSCILLATE, OSCAR) have NOT shown benefit over conventional lung-protective ventilation in adults; may actually increase mortality in some populations. May have benefit in pediatric patients.

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12. Extracorporeal Membrane Oxygenation (ECMO)

• Not strictly a ventilator mode but the ultimate support when all ventilator strategies fail
• VV-ECMO (Venovenous): Used for isolated lung pathology and refractory hypoxemia; blood taken from a central vein, oxygenated through a membrane, returned to a central vein
• VA-ECMO (Venoarterial): Used when there is concomitant cardiac dysfunction
• "Sweep gas" removes CO2 by diffusion through the membrane oxygenator

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PHASE VARIABLES - How Ventilators Work

• Time trigger: Machine-initiated at a preset frequency regardless of patient effort
• Pressure trigger: Patient's inspiratory effort drops circuit pressure below a threshold
• Flow trigger: Patient's inspiratory effort creates a detectable flow change

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VENTILATOR SETTINGS (Practical Overview)

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WEANING FROM MECHANICAL VENTILATION

• pH > 7.25
• Adequate SpO2 on FiO2 < 0.5
• Hemodynamically stable, no current myocardial ischemia
• Patient can breathe spontaneously
• Reversal/control of the underlying cause

• RSBI = Respiratory rate (f) / Tidal volume (VT in liters)
• RSBI < 105 predicts successful weaning
• Measured during a spontaneous breathing trial on T-piece

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Summary Table of Modes

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• Morgan & Mikhail's Clinical Anesthesiology 7e - Chapter 58: Mechanical Ventilation
• Murray & Nadel's Textbook of Respiratory Medicine - Chapter 135: Modes of Ventilation
• Fishman's Pulmonary Diseases and Disorders, 2-Volume Set - Chapter 147: Modes of Mechanical Ventilation & Ventilator Settings
• Sabiston Textbook of Surgery 21e - Chapter on Standard and Advanced Modes of Mechanical Ventilation
• Miller's Anesthesia 10e - Mechanical Ventilation in Asthma and Critical Care