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Overview
  • Therapeutic indications for mechanical ventilation include
    • Inadequate alveolar ventilation (clearance of CO2)
    • Inadequate Oxygenation
    • Excessive work of breathing
    • Circulatory shock
      • Septic, hemorrhagic, etc
    • congestive heart failure 
      • decrease oxygen consumption of respiratory system
      • Positive end-expiratory pressure (PEEP) reduces afterload on the left ventricle and may improve cardiac output in failing heart
  • There are several main modes
    • Assist-Control (Volume-cycled or Pressure-targeted)
      • Volume Cycled
        • Settings: tidal volume, flow rate, flow waveform, FiO2, PEEP, frequency
        • Airway pressures and auto-PEEP are dependent on respiratory system
      • Pressure-targeted
        • Settings: pressure target, inspiratory time, inspiratory rise rate, FiO2, PEEP, frequency
        • Tidal volume and auto-PEEP are dependent upon respiratory system
    • Pressure-Support Ventilation
      • Usually with set CPAP (PEEP)
      • Spontaneous mode where support can be adjusted; 
      • cannot set frequency or tidal volume, these are dependent upon patient effort, strength and compliance of respiratory system
      • Tolerated well because flow, depth and length are all patient controlled
    • Synchronized intermittent mechanical ventilation (SIMV)
      • combination of spontaneous breathing and set number of ventilator breaths that are fully supported but coincide with spontaneous efforts
Complications
  • Cardiovascular impairment
    • positive intrathoracic pressure can decrease venous return, decreasing cardiac output
      • most problematic in patients who are intravascularly volume deplete
  • Barotrauma
    • Subcutaneous emphysema, pneumomediastinum, pneumothorax, pneumoperitoneum and interstitial emphysema
    • Associated with
      • high mean alveolar pressure
      • high peak airway pressures
      • necrotizing lung pathology
      • high minute ventilation
      • longer duration of mechanical ventilation
  • Ventilator-induced lung injury (including volutrauma)
    • Low tidal volume strategy (6 mL/kg ideal body weight) reduces mortality in ARDS
      • lower driving pressure needed to achieve lower goal tidal volumes in damaged lungs
    • Tissues stress can be decreased by increased levels of PEEP when plateau pressures are elevated (goal plateau pressure < 30 cmH2O)
      • Use PEEP to prevent widespread alveolar unit closure at end-expiration to decrease tissue stress, which leads to shearing and inflammatory signaling
  • Infections
    • Ventilator-associated Pneumonia
    • Sinusitis
 
 

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