Mechanical Ventilators: Settings, Modes, and Transitioning

Mechanical ventilators are essential devices that provide respiratory support to patients who are unable to breathe adequately on their own. Understanding ventilator settings, modes, and how to transition patients on and off ventilator support is crucial for effective respiratory management.

I. Basic Ventilator Settings

These settings are adjusted to control the mechanical breaths delivered to the patient.

• Tidal Volume (Vt): The amount of air delivered with each breath (ml). Typical range: 6-8 ml/kg of ideal body weight.
• Respiratory Rate (RR): The number of breaths delivered per minute. Typical range: 12-20 breaths/min.
• Fraction of Inspired Oxygen (FiO2): The concentration of oxygen delivered (21-100%). Start with 100% and titrate down.
• Positive End-Expiratory Pressure (PEEP): Pressure applied at the end of expiration to prevent alveolar collapse. Typical range: 5-20 cm H2O.
• Inspiratory Time (I-Time): The duration of the inspiratory phase.
• Flow Rate: The speed at which the tidal volume is delivered (liters/minute).

II. Ventilator Modes

Ventilator modes determine how the ventilator supports the patient’s breathing. Modes can be categorized as assist-control or spontaneous.

• Assist-Control (AC) Modes: The ventilator delivers a set tidal volume at a set rate. Patient can trigger additional breaths, and the ventilator will deliver the full set tidal volume.
  • Volume Assist-Control (VC-AC): Ventilator delivers a pre-set volume.
    • Standard settings: Vt 6-8 ml/kg IBW, RR 12-20 breaths/min, FiO2 100% initially, PEEP 5 cm H2O.
    • Adjustments:
      • Vt: Adjust based on plateau pressure (target < 30 cm H2O) and ABGs.
      • RR: Adjust to achieve target PaCO2.
      • FiO2: Titrate to maintain SpO2.
      • PEEP: Increase to improve oxygenation, decrease to avoid overdistension.
  • Pressure Assist-Control (PC-AC): Ventilator delivers a pre-set pressure.
    • Standard settings: PIP 20-25 cm H2O (adjust to achieve target Vt), RR 12-20 breaths/min, FiO2 100% initially, PEEP 5 cm H2O, I-Time 0.8-1.2 seconds.
    • Adjustments:
      • PIP: Adjust to achieve target Vt (4-8 ml/kg IBW)
      • RR: Adjust to achieve target PaCO2.
      • FiO2: Titrate to maintain SpO2.
      • PEEP: Increase to improve oxygenation, decrease to avoid overdistension.
      • I-Time: Adjust to improve oxygenation or ventilation.
• Spontaneous Modes:
  • Pressure Support Ventilation (PSV): The ventilator provides pressure support during the patient’s spontaneous inspiratory effort. No set rate or volume.
    • Standard settings: PS 10-20 cm H2O, PEEP 5 cm H2O, FiO2 100% initially.
    • Adjustments:
      • PS: Increase to improve Vt, decrease to encourage spontaneous breathing.
      • PEEP: Increase to improve oxygenation, decrease to avoid overdistension.
      • FiO2: Titrate to maintain SpO2.
  • Continuous Positive Airway Pressure (CPAP): Provides constant pressure during both inspiration and expiration. Patient breathes spontaneously.
    • Standard settings: CPAP 5-10 cm H2O, FiO2 100% initially.
    • Adjustments:
      • CPAP: Increase to improve oxygenation, decrease if patient has difficulty exhaling.
      • FiO2: Titrate to maintain SpO2.
  • Synchronized Intermittent Mandatory Ventilation (SIMV): Delivers a set number of mandatory breaths, but also allows for spontaneous breaths. Ventilator synchronizes mandatory breaths with patient’s spontaneous efforts.
    • SIMV with Pressure Support (SIMV + PS): Adds pressure support to spontaneous breaths.
      • Vt: 6-8ml/kg IBW, RR 12-20 breaths/min, FiO2 100% initially, PEEP 5, PS 10-20
      • Vt and RR are for mandatory breaths. PS is for spontaneous breaths

III. Transitioning a Patient Off Mechanical Ventilation

The process of weaning a patient from mechanical ventilation should be gradual and carefully monitored.

• Readiness to Wean: Assess the patient’s readiness using specific criteria:
  • Underlying condition improving.
  • Hemodynamically stable.
  • Adequate oxygenation (PaO2/FiO2 > 200, PEEP ≤ 5-8 cm H2O, FiO2 ≤ 0.4-0.5).
  • Spontaneous breathing capability.
  • Adequate mental status.
• Weaning Techniques:
  • Spontaneous Awakening Trials (SATs): Daily interruption of sedation.
  • Spontaneous Breathing Trials (SBTs):
    • T-piece trials: Patient breathes through a T-piece with supplemental oxygen.
    • Low-level PSV: Gradually reduce pressure support.
  • SIMV Weaning: Gradually decrease the number of mandatory breaths.
• Monitoring During Weaning:
  • Closely monitor vital signs, respiratory rate, oxygen saturation, and work of breathing.
  • Assess for signs of weaning failure (e.g., tachypnea, tachycardia, increased work of breathing, hypoxemia, changes in mental status).
• Extubation: Removal of the endotracheal tube.
  • Ensure the patient is awake and able to protect their airway.
  • Suction the airway.
  • Deflate the ETT cuff and remove the tube.
  • Apply supplemental oxygen.
  • Monitor the patient closely post-extubation.