What is mechanical ventilation?

Mechanical Ventilation: Principles and Applications

Mechanical ventilation is a life-support technique that assists or replaces spontaneous breathing by delivering air into the lungs through an external device, most commonly used in patients with respiratory failure to ensure adequate gas exchange and reduce work of breathing. 1

Definition and Indications

Mechanical ventilation can be delivered through two primary approaches:

1. Invasive mechanical ventilation: Delivered through an endotracheal tube or tracheostomy
2. Non-invasive mechanical ventilation: Delivered through interfaces such as face masks or nasal masks without intubation 1

Common indications for mechanical ventilation include:

• Refractory hypoxemia (PaO₂ < 60 mmHg despite high-flow oxygen) 1
• Respiratory rate > 35 breaths/min 1
• Vital capacity < 15 ml/kg 1
• Inability to protect airway 1
• Severe work of breathing with risk of respiratory muscle fatigue 1

Modes of Mechanical Ventilation

Controlled Mechanical Ventilation (CMV)

• Full ventilatory support with no patient effort required
• Both inflation pressure/volume and respiratory frequency are set
• Requires deeper sedation and possibly neuromuscular blockade 2

Assist/Control Ventilation (ACV)

• Delivers a preset number of mandatory breaths but allows patient triggering
• Also called Spontaneous/Timed (S/T) mode on some NIV machines 1
• Each breath delivers identical volume or pressure regardless of whether machine- or patient-triggered

Pressure Support Ventilation (PSV)

• Patient's respiratory effort triggers the ventilator both on and off
• Respiratory frequency determined by the patient
• Most frequently used mode of assisted mechanical ventilation 2
• May include backup rate of 6-8 breaths/minute 1

Continuous Positive Airway Pressure (CPAP)

• Maintains positive pressure throughout respiratory cycle
• Improves oxygenation by recruiting underventilated lung areas
• Unloads inspiratory muscles 1
• Not a true ventilation mode as it doesn't assist with ventilation

Ventilator Settings and Lung Protection

Tidal Volume

• Set at 4-8 ml/kg of predicted body weight (not actual weight) 1
• Calculation formulas:
  • Men: 50 + 2.3 × (height in inches - 60)
  • Women: 45.5 + 2.3 × (height in inches - 60) 1

Plateau Pressure

• Should be maintained ≤ 30 cmH₂O to prevent ventilator-induced lung injury 1
• Monitored regularly during volume-controlled ventilation

Positive End-Expiratory Pressure (PEEP)

• Prevents alveolar collapse and improves oxygenation 1
• Higher PEEP (>10 cmH₂O) recommended for moderate to severe ARDS 1, 3
• Lower PEEP (5-10 cmH₂O) for mild hypoxemia 3

Driving Pressure

• Difference between plateau pressure and PEEP
• Should ideally be < 15 cmH₂O to minimize lung injury 3, 4

FiO₂ (Fraction of Inspired Oxygen)

• Titrated to maintain arterial oxygen saturation around 90% (PaO₂ ~60 mmHg) 1
• Target PaO₂ 70-90 mmHg in most cases 3

Advanced Strategies for Severe Respiratory Failure

Prone Positioning

• Strongly recommended for severe ARDS (PaO₂/FiO₂ < 100 mmHg) 1
• Should be applied for >12 hours per day 1, 3
• Improves ventilation-perfusion matching and mortality 3

Neuromuscular Blocking Agents

• Consider in early severe ARDS (first 48 hours) 3
• Prevents patient-ventilator dyssynchrony and excessive transpulmonary pressure

Recruitment Maneuvers

• Temporary increase in airway pressure to open collapsed alveoli
• Conditionally recommended in moderate to severe ARDS 1
• Must be performed carefully to avoid barotrauma

Extracorporeal Membrane Oxygenation (ECMO)

• Consider for severe ARDS failing conventional therapy 3, 5
• Allows for "lung rest" with ultra-protective ventilation strategies
• Types include:
  • VA-ECMO: Provides circulatory support and gas exchange 1
  • VV-ECMO: Provides gas exchange only 1, 5

Complications of Mechanical Ventilation

Ventilator-Induced Lung Injury (VILI)

• Caused by excessive pressure (barotrauma), volume (volutrauma), or cyclic opening/closing of alveoli (atelectrauma) 4
• Minimized through lung-protective ventilation strategies

Hemodynamic Effects

• Positive pressure ventilation can reduce venous return and cardiac output 4
• May affect cerebral perfusion pressure and renal function

Ventilator-Associated Pneumonia (VAP)

• Occurs in 9-27% of intubated patients 3
• Preventive measures include head elevation, oral care, and minimizing sedation

Patient-Ventilator Dyssynchrony

• Mismatch between patient effort and ventilator delivery
• Can increase work of breathing and patient discomfort 6
• May require adjustment of trigger sensitivity, rise time, or ventilator mode

Weaning from Mechanical Ventilation

• Daily assessment for weaning readiness is essential 3
• Spontaneous breathing trials should be conducted when patients meet criteria
• A structured weaning protocol reduces the risk of weaning failure 3
• Extubation should be considered when patients can maintain adequate gas exchange with minimal support

Special Considerations

Non-Invasive Positive Pressure Ventilation (NIPPV)

• Contraindicated in patients with:
  • Respiratory arrest
  • Cardiovascular instability
  • Impaired mental status
  • Copious secretions
  • Recent facial surgery or trauma 1

Mechanical Ventilation during Pregnancy

• Requires special attention due to:
  • Decreased functional residual capacity
  • Increased oxygen consumption
  • Risk of rapid hypoxemia development
  • Need for left lateral tilt after 20 weeks gestation 3

By implementing evidence-based mechanical ventilation strategies tailored to the patient's specific condition, clinicians can optimize gas exchange while minimizing complications and improving outcomes.