Mechanical ventilation provides artificial respiratory support in patients unable to maintain adequate gas exchange. Modern systems primarily use positive pressure ventilation (PPV) to inflate the lungs by delivering gas under pressure via an endotracheal or tracheostomy tube.
Indications for Mechanical Ventilation
Mechanical ventilation is indicated in both acute and chronic respiratory failures. Common clinical situations include:
Acute Respiratory Conditions
- Acute Respiratory Distress Syndrome (ARDS)
- Severe trauma
- Pneumonia with respiratory failure
- Apnea or respiratory arrest (e.g., due to drug overdose)
Chronic Respiratory Conditions (Acute Exacerbations)
- Chronic Obstructive Pulmonary Disease (COPD)
- Neuromuscular disorders (e.g., Guillain-Barré syndrome, Myasthenia Gravis)
- Spinal cord injuries
Types of Respiratory Failure
- Hypoxemic (Type I) : PaO₂ < 60 mmHg despite O₂ therapy. Often due to V/Q mismatch or intrapulmonary shunting.
- Hypercapnic (Type II) : PaCO₂ > 50 mmHg due to alveolar hypoventilation (e.g., CNS depression, neuromuscular failure, obstructive lung disease).
Other Indications
- Acute respiratory acidosis
- Increased work of breathing
- Shock or hypotension (septic, cardiogenic)
- Failure of non-invasive ventilation (NIV)
Methods of Mechanical Ventilation
1. Non-Invasive Ventilation (NIV)
- Delivered via mask (nasal/oronasal)
- Avoids intubation
- Used in mild-to-moderate respiratory failure
2. Invasive Ventilation
- Delivered via endotracheal or tracheostomy tube
- Used in severe respiratory failure or when NIV fails
- Provides full ventilatory support
Mechanical Ventilation Terminology
Trigger
- Patient-triggered : Based on patient's inspiratory effort (pressure or flow)
- Time-triggered : Initiated by ventilator if patient effort is absent
Cycle
- Determines the end of inspiration:
- Volume-cycled : Ends when preset volume is delivered
- Pressure-cycled : Ends when preset pressure is reached
- Time-cycled : Ends after a preset inspiratory time
Limit
- Restricts maximum volume, pressure, or flow during inspiration
Modes of Invasive Mechanical Ventilation
1. Controlled Modes
Used when spontaneous respiratory drive is absent or suppressed.
a. Assist-Control Ventilation (ACV / ACMV / VCV)
- Every breath is either time- or patient-triggered
- Delivers a preset tidal volume or pressure
- Risk: hyperventilation, auto-PEEP, barotrauma in tachypnea
b. Pressure-Control Ventilation (PCV)
- Delivers air at a preset inspiratory pressure
- Tidal volume varies depending on lung compliance and resistance
- Useful in ARDS or restrictive lung disease
2. Spontaneous/Support Modes
Used during weaning or when patients can initiate breaths.
a. Pressure Support Ventilation (PSV)
- Patient-triggered and flow-cycled
- Every breath is supported by a preset pressure
- Tidal volume varies
b. Continuous Positive Airway Pressure (CPAP)
- Continuous pressure throughout the respiratory cycle
- No mandatory breaths delivered
- Often used in sleep apnea or for weaning
3. Combined Modes
Blend mandatory and spontaneous ventilation—used for maintenance or weaning.
a. Synchronized Intermittent Mandatory Ventilation (SIMV)
- Delivers preset number of mandatory breaths
- Allows spontaneous breaths between cycles
- Spontaneous breaths can be supported (SIMV-VC + PS or SIMV-PC + PS)
Mode Summary
| Mode | Trigger | Cycle | Usage | Advantages | Disadvantages |
|---|---|---|---|---|---|
| ACV/VCV | Patient or time | Volume | ARDS, unconscious patients | Full control, consistent minute ventilation | Risk of barotrauma, auto-PEEP |
| PCV | Patient or time | Time | ARDS, high peak pressures | Limits pressure, lung-protective | Variable volumes |
| PSV | Patient | Flow | Weaning, mild respiratory failure | Comfortable, patient-driven | No guaranteed minute ventilation |
| CPAP | Patient | Flow | Sleep apnea, post-extubation | Simple, spontaneous breathing support | Fatigue risk in weak patients |
| SIMV | Patient or time | Volume or pressure | Transition from full support | Allows spontaneous effort | Risk of respiratory muscle fatigue |
Complications of Invasive Mechanical Ventilation
- Ventilator-associated pneumonia (VAP)
- Barotrauma (pneumothorax)
- Volutrauma
- Auto-PEEP/dynamic hyperinflation
- Hemodynamic compromise
- Ventilator-induced lung injury (VILI)
Clinical Pearls
- Always tailor mode to patient pathology and respiratory mechanics.
- Use lung-protective strategies (low tidal volume, ≤6 mL/kg) in ARDS.
- Regularly assess readiness for weaning (spontaneous breathing trials).
- Monitor for signs of ventilator asynchrony (dyssynchrony increases work of breathing).
