Basics of Mechanical Ventilation

Mechanical ventilation is a life-support technique used to assist or replace spontaneous breathing in patients who are unable to maintain adequate ventilation or oxygenation. It is delivered via an endotracheal tube (ETT), tracheostomy, or noninvasive interface (e.g., mask).

Key Objectives of Mechanical Ventilation

1. Provide Adequate Oxygenation and Ventilation:
   • Maintain oxygen delivery (PaO₂ 60–100 mmHg or SpO₂ > 90%).
   • Remove carbon dioxide to maintain pH (PaCO₂ 35–45 mmHg for most patients).
2. Reduce Work of Breathing:
   • Alleviate respiratory muscle fatigue.
3. Protect the Lungs:
   • Minimize ventilator-induced lung injury (VILI) by using lung-protective strategies.
4. Treat or Prevent Respiratory Failure:
   • Address hypoxemic, hypercapnic, or mixed respiratory failure.

Components of Mechanical Ventilation

1. Modes of Ventilation:
   • Volume-Controlled Ventilation (VCV): Delivers a preset tidal volume (VT) regardless of pressure. Pressure varies based on lung compliance and airway resistance.
   • Pressure-Controlled Ventilation (PCV): Delivers breaths at a preset peak inspiratory pressure (PIP). Tidal volume depends on lung compliance and airway resistance.
   • Pressure-Support Ventilation (PSV): A spontaneous mode where the patient initiates breaths, supported by a preset pressure to assist ventilation.
   • Synchronized Intermittent Mandatory Ventilation (SIMV): Combines mandatory breaths with spontaneous breathing, providing synchronization between patient and ventilator.
   • Assist-Control Ventilation (AC): The ventilator provides full support for every breath, whether initiated by the patient or the machine.
2. Ventilator Settings:
   • Tidal Volume (VT): Volume of air delivered per breath (6–8 mL/kg of ideal body weight [IBW] for lung-protective strategies).
   • Respiratory Rate (RR): Number of breaths delivered per minute (e.g., 12–20 bpm in most patients).
   • FiO₂ (Fraction of Inspired Oxygen): Oxygen concentration in the delivered gas mixture (21%–100%).
   • Positive End-Expiratory Pressure (PEEP): Pressure maintained at the end of expiration to prevent alveolar collapse (5–10 cmH₂O in most settings).
   • Inspiratory Time (I-time) and Inspiratory:Expiratory (I:E) Ratio: Time spent in inspiration relative to expiration. Typical I:E ratios are 1:2 to 1:3.
3. Monitoring:
   • Airway Pressures:
     • Peak Inspiratory Pressure (PIP): Total airway pressure during inspiration.
     • Plateau Pressure (Pplat): Pressure in the alveoli during inspiration, measured during a breath-hold maneuver (<30 cmH₂O to minimize VILI).
   • Blood Gases (ABG): Used to assess oxygenation, ventilation, and acid-base status.
   • Lung Mechanics:
     • Compliance: Reflects lung and chest wall distensibility.
     • Resistance: Reflects airway obstruction.

Lung-Protective Ventilation

1. Low Tidal Volume:
   • Use 6–8 mL/kg of IBW to minimize barotrauma and volutrauma.
2. Plateau Pressure:
   • Keep <30 cmH₂O to reduce the risk of lung injury.
3. PEEP:
   • Optimize PEEP to prevent atelectrauma (repetitive alveolar collapse and reopening).
4. Driving Pressure:
   • Aim for a driving pressure (Pplat − PEEP) <15 cmH₂O.

Complications of Mechanical Ventilation