Modes of Mechanical Ventilation and Weaning from the Ventilator

INTRODUCTION

CLASSIFICATION OF VENTILATOR MODES

A. Control Variables

1. Volume-Controlled (VC) - a preset tidal volume (VT) is delivered regardless of airway pressure; pressures vary with lung mechanics.
2. Pressure-Controlled (PC) - a preset inspiratory pressure is maintained; VT varies with lung compliance and resistance.

B. Breath Sequences

1. Continuous Mandatory Ventilation (CMV) - all breaths are mandatory (machine-controlled); patient effort triggers full machine breaths.
2. Intermittent Mandatory Ventilation (IMV/SIMV) - mandatory breaths at a set rate plus spontaneous breaths between them.
3. Continuous Spontaneous Ventilation (CSV) - all breaths are patient-initiated (e.g., PSV, CPAP).

SPECIFIC MODES OF VENTILATION

1. Controlled Mechanical Ventilation (CMV)

• All breaths delivered by the machine at a preset rate and volume.
• Patient has no role; requires deep sedation ± neuromuscular blockade.
• Used when the patient cannot breathe independently (e.g., during general anesthesia, flail chest, severe CNS depression).
• Disadvantage: respiratory muscle atrophy, haemodynamic compromise from high intrathoracic pressure, barotrauma.

2. Assist-Control Ventilation (A/C)

• A backup rate and VT are set; any spontaneous patient effort triggers a full-sized machine breath.
• Subtypes: AC-VC (clinician sets VT, inspiratory flow, RR, PEEP) and AC-PC (clinician sets target pressure, inspiratory time, RR, PEEP).
• Advantage: guaranteed minimum ventilation; reduces work of breathing.
• Disadvantage: patient-initiated breaths are not proportional to effort - a small effort gets a full breath, leading to hyperventilation, air trapping, auto-PEEP, hypotension, and poor synchrony. Requires adequate sedation.
• Indicated for: paralysed or deeply sedated patients; initial management of respiratory failure.

3. Synchronized Intermittent Mandatory Ventilation (SIMV)

• Delivers a preset number of mandatory breaths at set intervals, synchronized with the patient's spontaneous inspiratory effort.
• Between mandatory breaths, patient breathes spontaneously from the circuit.
• At high rates (10-12/min): essentially controls all ventilation. At low rates (1-2/min): provides minimal support, patient breathes almost independently.
• Advantage over unsynchronized IMV: prevents breath-stacking (superimposing a machine breath mid-spontaneous breath), improving patient comfort.
• Greatest use: weaning from ventilation (rate progressively decreased).
• Disadvantage: at low backup rates (4/min), may provide insufficient backup for weak patients; spontaneous breaths through ETT add work of breathing. Has the poorest weaning outcomes of all techniques when used alone.
• Parameters set: pressure or volume control, PEEP, backup RR.

4. Pressure Support Ventilation (PSV)

• CSV mode: patient-triggered, pressure-targeted, flow-cycled.
• Clinician sets a pressure level that augments every spontaneous effort.
• Patient controls respiratory rate, inspiratory time, and VT.
• VT is determined by the pressure setting, patient effort, and lung mechanics.
• Cycling to exhalation occurs when inspiratory flow falls to a preset threshold (usually 25% of peak inspiratory flow or 5 L/min).
• Clinical use: spontaneously breathing patients needing minimal support; weaning; in COPD, may prolong inspiration into neural expiration causing "fighting the ventilator" and expiratory muscle recruitment.
• Weaning: pressure progressively reduced by 2-3 cmH2O increments.

5. Continuous Positive Airway Pressure (CPAP)

• Maintains a constant positive pressure throughout both inspiration and expiration; no mandatory breaths delivered.
• Patient breathes entirely spontaneously; CPAP keeps alveoli open, prevents collapse, and increases FRC.
• Can be invasive (through ETT) or non-invasive (face mask - NIV-CPAP).
• Indications: OSA, cardiogenic pulmonary oedema, COPD exacerbations, weaning trials.
• Parameters set: CPAP level (cm H2O).

6. Bi-Level Positive Airway Pressure (BiPAP / IPAP-EPAP)

• Delivers two pressure levels: a higher IPAP (inspiratory positive airway pressure) and lower EPAP (expiratory positive airway pressure).
• Patient breathes spontaneously; IPAP augments inspiration like PSV; EPAP acts like PEEP/CPAP.
• Standard NIV modality for COPD exacerbations, hypercapnic respiratory failure, cardiogenic pulmonary oedema.
• Parameters set: IPAP, EPAP, backup rate, FiO2.

7. Positive End-Expiratory Pressure (PEEP)

• Not a mode per se but an adjunct used with most modes.
• Maintains airway pressure above atmospheric at end-expiration.
• Physiological effects: increases FRC, recruits collapsed alveoli, improves V/Q matching, improves oxygenation, reduces shunting.
• Intrinsic (auto) PEEP: air-trapping in obstructive lung disease; matching extrinsic PEEP to intrinsic PEEP improves triggering sensitivity and reduces WOB in asthma/COPD.
• Hazards: decreased venous return, reduced cardiac output, barotrauma, increased dead space.

8. Inverse Ratio Ventilation (IRV)

• I:E ratio is reversed (> 1:1, e.g., 2:1 or 3:1).
• Prolongs inspiratory time, recruits atelectatic alveoli, increases mean airway pressure.
• Creates intrinsic PEEP, improving oxygenation in severe ARDS.
• Very uncomfortable; requires deep sedation ± paralysis.

9. Airway Pressure Release Ventilation (APRV)

• Maintains a high CPAP level (P-High) for a prolonged period, with brief releases to a low pressure (P-Low).
• Promotes spontaneous breathing throughout the cycle (mainly at P-High).
• Maintains alveolar recruitment, reduces peak pressures, and promotes CO2 clearance during releases.
• Used in ARDS as a lung-protective strategy.

10. High-Frequency Ventilation (HFV)

• HFPPV (High-Frequency Positive Pressure Ventilation): small conventional VT at 60-120 breaths/min.
• HFJV (High-Frequency Jet Ventilation): pulsed jet at 120-600/min (2-10 Hz) via small cannula; used for laryngeal/tracheal/bronchial surgery and bronchopleural fistula.
• HFO (High-Frequency Oscillation): piston creates bidirectional gas movement at 180-3000/min (3-50 Hz); used in neonates and severe ARDS.
• All produce VT at or below anatomic dead space. Gas exchange mechanism: bulk convection, asymmetric velocity profiles, Taylor dispersion, molecular diffusion, cardiogenic mixing.
• Initial HFJV settings: rate 120-240/min, inspiratory time 33%, drive pressure 15-30 psi.

11. Volume-Targeted Pressure Control (VTPC / PRVC)

• Combines features of both VC and PC modes.
• Sets a target VT; the machine adjusts the pressure breath-to-breath to deliver that VT.
• Provides lung protection (pressure-limited) while guaranteeing minute ventilation.

VENTILATOR SETTINGS SUMMARY TABLE

WEANING FROM MECHANICAL VENTILATION

A. Prerequisites for Weaning (Readiness Criteria)

1. Reversal / improvement of the underlying cause of respiratory failure.
2. Oxygenation: PaO2 > 60 mmHg on FiO2 ≤ 50% (or SpO2 ≥ 90%); PaO2/FiO2 > 150-200.
3. Respiratory mechanics: Vital capacity ≥ 10-15 mL/kg; MIF (Maximum Inspiratory Force) > -25 cmH2O.
4. Ventilation: Near-normal blood gases; PEEP ≤ 5-8 cmH2O.
5. Cardiovascular stability: No active ischaemia; on minimal/no vasopressors.
6. Neurological: Able to follow commands; GCS adequate to protect airway.
7. Secretion management: Able to cough and clear secretions.
8. Sedation/paralysis: Reversed or withdrawn.
9. Metabolic: No major acid-base disturbances; K+, Mg2+, PO4 corrected.

B. Weaning Predictor Indices

RSBI = f / VT (respiratory frequency / tidal volume in litres)

• Measured during unassisted spontaneous breathing (not during PSV or CPAP - this gives inaccurate results).
• RSBI < 105 breaths/min/L: likely successful extubation.
• RSBI < 80: high probability of success.
• RSBI > 120: likely to need continued ventilatory support.
• Sensitivity ~0.90 (excellent screening tool).

• Minute ventilation < 10 L/min
• Vital capacity > 10-15 mL/kg
• NIF (Negative Inspiratory Force) < -20 to -30 cmH2O
• Compliance > 25 mL/cmH2O

C. Spontaneous Breathing Trial (SBT)

1. T-piece trial: ETT/tracheostomy connected to humidified O2-air circuit; patient breathes spontaneously with NO ventilator support.
2. Low-level CPAP (5 cmH2O): maintains FRC, prevents atelectasis from laryngeal bypass.
3. Low PSV (5-8 cmH2O): compensates for ETT resistance ("automated tube compensation").

• Respiratory rate > 35/min
• SpO2 < 90% or PaO2 < 60 mmHg
• Tachycardia or arrhythmias
• Hypertension or hypotension
• Agitation, diaphoresis, altered consciousness
• Use of accessory muscles, paradoxical breathing

D. Methods of Gradual Weaning

1. T-Piece Weaning

• Ventilator disconnected; ETT/tracheostomy connected to T-piece with humidified O2.
• Oxygen enrichment slightly above the level required during ventilation.
• Patient observed continuously; ABGs at intervals.
• If inadequate: put back on ventilator (especially overnight).
• For tracheostomy: fenestrated tracheostomy tube allows patient to breathe via larynx and speak normally; a cork can occlude external end for extended trial while retaining suction access.
• Progressive trials: 10-30 min, increasing by 5-10 min per session.

2. SIMV Weaning

• Mandatory rate progressively decreased by 1-2 breaths/min at each step.
• Criteria to step down: PaCO2 < 45-50 mmHg; RR < 30/min; acceptable oxygenation.
• If PSV is used concomitantly, reduce PSV to 5-8 cmH2O before stopping SIMV.
• Check ABG after minimum 15-30 min at each setting.
• When SIMV reaches 2-4 breaths/min with acceptable oxygenation, MV can be discontinued.
• Note: SIMV has the poorest weaning outcomes compared to T-piece and PSV trials.

3. PSV Weaning (Preferred Method)

• PSV level progressively decreased by 2-3 cmH2O increments.
• Monitor: VT (target 4-6 mL/kg), PaCO2, RR (< 30/min), SpO2.
• When PSV is reduced to 5-8 cmH2O, patient is considered weaned.
• Often combined with low PEEP of 5 cmH2O ("5+5" protocol).

4. IMV (Intermittent Mandatory Ventilation)

• Spontaneous T-piece breathing between mandatory breaths.
• Mandatory rate gradually reduced to zero.
• Simplest and safest traditional technique; the historical precursor to SIMV.
• SIMV is preferred over IMV because synchronization prevents breath-stacking.

E. Extubation to NIV / HFNC

• Extubation to NPPV/BiPAP: particularly beneficial in COPD patients with acute respiratory failure - reduces mortality and hospital-acquired pneumonia.
• High-Flow Nasal Cannula (HFNC) post-extubation: delivers 40-60 L/min of heated, humidified high-FiO2 oxygen, generates low-level PEEP, reduces work of breathing. Shown to reduce reintubation within 48-72 hours; non-inferior to NIV for preventing reintubation in select patients.

F. Failure to Wean - Causes

COMPLICATIONS OF MECHANICAL VENTILATION

1. Barotrauma - pneumothorax, pneumomediastinum, subcutaneous emphysema; related to high peak inflation pressures.
2. Volutrauma - repetitive alveolar overdistension; related to large VT.
3. Atelectrauma - repetitive collapse/re-expansion; minimized by PEEP.
4. Altered V/Q matching - gas preferentially flows to non-dependent (high-compliance) areas; blood to dependent areas.
5. Haemodynamic compromise - elevated intrathoracic pressure impairs venous return, reduces cardiac output.
6. Ventilator-associated pneumonia (VAP) - major ICU infection complication.
7. Respiratory muscle atrophy - from prolonged controlled ventilation without spontaneous effort.
8. Oxygen toxicity - with prolonged high FiO2.

LUNG-PROTECTIVE VENTILATION STRATEGY (ARDSNet)

• VT 6 mL/kg ideal body weight (IBW)
• Plateau pressure < 30 cmH2O
• PEEP titrated to FiO2 requirements
• Permissive hypercapnia acceptable if pH > 7.20
• Prone positioning for severe ARDS (PaO2/FiO2 < 150)
• Driving pressure (plateau - PEEP) < 15 cmH2O

QUICK SUMMARY DIAGRAM

TYPES OF VENTILATION
├── Negative Pressure (Iron Lung) — obsolete
└── Positive Pressure (universal)
    ├── Non-Invasive (NIV)
    │   ├── CPAP
    │   └── BiPAP (IPAP + EPAP)
    └── Invasive (ETT/tracheostomy)
        ├── Control Variable
        │   ├── Volume-Controlled (VC)
        │   └── Pressure-Controlled (PC)
        └── Breath Sequence
            ├── CMV (all mandatory) → AC mode
            ├── IMV/SIMV (mix)
            └── CSV (all spontaneous) → PSV, CPAP

WEANING
├── Assess Readiness (RSBI, ABG, clinical)
├── Spontaneous Breathing Trial (T-piece / CPAP / PSV 5+5)
├── Methods: T-piece → SIMV → PSV (PSV preferred)
├── Extubation → HFNC or NIV bridge
└── Failure → seek cause (muscle, cardiac, metabolic, psych)

RECENT EVIDENCE