Mechanical Ventilation

Aka: Mechanical Ventilation, Ventilator, Assist Control, Intermittent Mandatory Ventilation, Pressure Support Ventilation

1. See Also
   1. Ventilator Troubleshooting
   2. Ventilator Weaning
   3. Positive End-Expiratory Pressure (PEEP)
   4. Continuous Positive Airways Pressure (CPAP)
2. History
   1. First described by Andreas Vesalius in 1555
   2. Negative pressure Ventilators (IronLung) first used
   3. Positive pressure Ventilator first used in 1955
      1. Response to Polio epidemic of 1955
      2. Emerson company tested at Massachusetts General
3. Types
   1. Pressure Cycled Ventilators
      1. Initial Ventilator design
      2. Inflates lungs until preset pressure is reached
      3. Difficult to keep inflation volume constant
   2. Volume cycled Ventilators
      1. Newer devices
      2. Deliver constant volume independent of lung mechanics
4. Physiology
   1. Cardiac Output
      1. Enhanced by modest thoracic positive pressure
         1. Reduces left ventricular afterload
      2. Decreased by excessive intrathoracic pressure
         1. Reduces diastolic ventricular filling
5. Technique: Ventilation Modes
   1. Assist Control (AC)
      1. Assist: Patient initiates mechanical breath
      2. Control: Provides ventilations when patient unable
      3. Advantages over SIMV Mode
         1. Decreased work of breathing
         2. Decreased respiratory muscle Fatigue
         3. Better response to patient's Ventilatory needs
   2. Intermittent Mandatory Ventilation (IMV)
      1. Intersperses spontaneous breath with machine breath
      2. Machine breaths synchronized (SIMV)
         1. Given at start of spontaneous breaths
      3. Indications
         1. Introduced in 1971 for neonates with RDS
         2. Often used for Ventilator Weaning of adults
         3. May be used if Respiratory Rate is rapid
      4. Advantages over Assist Control Mode
         1. Less Respiratory Alkalosis
         2. Improves cardiac output
         3. Prevents respiratory muscle atrophy
   3. Pressure Support Ventilation (PSV)
      1. Augments spontaneous breathing (as with SIMV)
      2. Augments every breath (as with assist mode)
      3. Inspired gas to desired pressure (5-10 cmH2O)
      4. Advantages over IMV
         1. Increases Tidal Volume
         2. Decreases work of breathing
      5. Methods of Setting Pressure
         1. Method 1: Maximum Inspiratory Pressure
            1. Pressure = Maximum Inspiratory Pressure / 3
         2. Method 2: Proximal Airway Pressure
            1. Pressure = Peak Pressure - Plateau Pressure
      6. Often used for Ventilator Weaning
6. Technique: Parameters
   1. Tidal Volume
      1. Ventilator Tidal Volume: 6-8 ml/kg
         1. Prior levels of 10 to 15 ml/kg thought too high
         2. ARDS: Start at 6 ml/kg based on ideal body weight
      2. Reduce Tidal Volume: Lung Resection history
         1. Reduce Tidal Volume by percent loss in lung
      3. Increase Tidal Volume
         1. Stiff Lungs (e.g. Pulmonary edema)
            1. High Peak Inflation Pressure (>20-40 cm H2O)
            2. Results in large loss of Tidal Volume in tubing
   2. Respiratory Rate
      1. Set at 12 to 14 breaths per minute
      2. Ensures adequate carbon dioxide removal
      3. Keep to a minimum to avoid Respiratory Alkalosis
   3. Fraction of Inspired Oxygen (FIO2)
      1. Start: 80% or higher
      2. Titrate: decrease in 10-20% steps
      3. Goal: Keep FIO2 <60% (<50% if possible)
         1. Higher FIO2 is associated with Oxygen Toxicity
      4. Monitoring: Arterial Blood Gas
         1. Wait 20 minutes after each change in FIO2
         2. Keep PaO2 60 to 80 mm Hg (90-95% O2 Sat)
7. Technique: Adjunctive measures
   1. Paralytic Agents
      1. No longer routinely recommended due to Myopathy
      2. Advantages
         1. Reduced oxygen demands
         2. Improved Metabolic Acidosis
         3. Reduced barotrauma
      3. Indications
         1. Ventilator-patient desynchrony
         2. High peak airway pressure
         3. Failed response to Sedation
      4. Complications
         1. Myopathy (exacerbated by Corticosteroids)
         2. Increased Deep Vein Thrombosis risk
         3. Unable to assess mental status
      5. Pearls
         1. Define lowest effective dose with nerve stimulator
         2. Hold infusion every 4-6 hours (avoids accumulation)
         3. Concurrent Sedation is imperative (see below)
   2. Use adequate Sedation
      1. Paralytic Agents do not sedate
      2. Adult doses
         1. Midazolam 1-2 mg IV prn
         2. Propofol 60-80 mg IV or 50-100 ug/kg/min
         3. Lorazepam 1-2 mg IV prn
   3. References
      1. Cornwell (2003) UW New Therepeutics Lecture, Cable,WI
8. Adverse Effects
   1. Severe Respiratory Alkalosis
      1. Occurs with high Respiratory Rates
      2. Consider IMV ventilation mode or patient Sedation
   2. Barotrauma
9. References
   1. Marino (1991) ICU Book, Lea & Febiger, Philadelphia