Surgical Critical Care: Mechanical Ventilation – Settings

Rate (f/RR)

• Definition: Number of Breaths Delivered per Minute
• Typical Initial Rate: 8-12 Breaths per Minute
  • May Start Higher for a Patient Being Intubated with Acidosis & Tachypnea
• Complications of Excessively High Rates:
  • Decreased Exhalation Time > Increased Mean Airway Pressure & Air Trapping
  • Uncomfortable for Patients

Tidal Volume (Vt)

• Used in Volume Controlled Modes
• Definition: Volume of Air Delivered with Each Breath
• Typical Initial Vt: 8 ml/kg x Ideal Body Weight
  • Lung Protective Strategy: 4-6 ml/kg x Ideal Body Weight
• Excessively High Volumes Increase Pressure Which Causes Barotrauma

Pressure Support (PS)

• Used in Pressure Controlled Modes
• Definitions: Additional Pressure Added to PEEP to Cause Inspiration
  • Also Known as Inspiratory Pressure (PI) or Change in Pressure (∆P)
• Different Ventilator Manufacturers May Use Peak Inspiratory Pressure (PIP) Instead
  • Peek Inspiratory Pressure (PIP): Highest Pressure Seen During Inspiration
  • PIP = PI + PEEP
• Typical Initial PS: 5-10 cm H2O
  • Generally Avoid Pressures > 20 cm H2O or PIP > 30-35
• Excessively High Pressure Causes Barotrauma

Positive End Expiratory Pressure (PEEP)

• Definition: Positive Pressure Remaining in Airways at the End of Expiration
• Typical Initial PEEP: 5-8 cm H2O
• Benefits:
  • Increases Recruitment of Collapsed Alveoli to Keep Open – Maximizes Oxygen Transfer
  • Increases Functional Residual Capacity & Compliance
• Complications:
  • Increased PCWP & Pulmonary Vascular Resistance > Increased Right Atrial Pressure
  • Increased Right Atrial Pressure > Decreased Venous Return/Preload > Decreased Cardiac Output
  • Decreased Cardiac Output > Decreased Mean Arterial Pressure
    • Compensatory Increase in SVR Can Avoid MAP Decrease
  • Decreased MAP > Decreased Renal Flow > Decreased UOP & Increased Renin
• Auto/Intrinsic PEEP (Air Trapping)
  • Complication of Mechanical Ventilation Seen in Asthma/COPD
  • *See Surgical Critical Care: Mechanical Ventilation – Complications

Fraction of Inspired Oxygen (FiO2)

• Definition: Percentage of Oxygen in Air Delivered to the Patient
• Typical Initial FiO2: 100% & Titrated Down Off ABG & Pulse Oximetry
• FiO2 of Room Air: 21% (Regardless of Altitude)
• Oxygen Toxicity:
  • High Oxygen Levels Produce Reactive Oxygen Species (ROS)
  • Lorraine Smith Effect – Lung Damage Occurs with High FiO2 (> 60%) for a Prolonged Period of Time (> 24 Hours)
  • Paul Bert Effect – CNS Toxicity from High FiO2 Over a Short Period of Time When Barometric Pressure is High (1.6-4.0 ATA)