Surgical Critical Care: Mechanical Ventilation – Settings
Basic 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)
Advanced Settings
Inspiratory/Expiratory (I/E) Ratio
- Definition: Ratio of Inspiratory Time to Expiratory Time
- Normal Settings: 1/3
- Patients with Airflow Limitation (Asthma/COPD Exacerbation) Should Have Ratios ≥ 1/4 to Limit Auto-PEEP
- Either I/E Ratio or Inspiratory Flow Rate Can be Adjusted – But Not Both
Inspiratory Flow Rate
- Definition: Rate of Air Flow Delivered During Inspiration
- Normal Settings: 60 L/min
- Patients with Airflow Limitation (Asthma/COPD Exacerbation) Can Have Rate Increased Up to 120 L/min to Limit Auto-PEEP
- Either I/E Ratio or Inspiratory Flow Rate Can be Adjusted – But Not Both
Trigger Sensitivity
- Definition: Negative Pressure Required to Initiate a Patient-Triggered Breath
- Normal Setting: -2 cm H2O
- If Too High: Weak Patients are Unable to Trigger Breaths
- Patient with High Auto-PEEP May Have Difficulty Inhaling Enough to Activate
- If Too Low: Machine will Auto-Cycle Causing Overinflation