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Updated: Dec 29 2021

Mechanical Ventilation

  • Snapshot
    • A 55-year-old alcoholic man presents to the emergency department acutely altered. Upon initial workup it is noted that the patient has a Glasgow coma scale of 7 and is hypoxic and hypercapneic. He is subsequently intubated and mechanically ventilated. Secondary exam reveals a patient covered in vomit and urine who smells like alcohol. (Acute respiratory distress syndrome).
  • Introduction
    • Clinical definition
      • a ventilator that mechanically oxygenates/ventilates the patients with settings determined by the physician
    • Therapeutic indications for intubation and mechanical ventilation include
      • failure to oxygenate
      • failure to ventilate
      • inability to protect airway
        • GCS < 8 (less than 8, intubate)
      • suspected clinical course requiring intubation and ventilation
    • Main modes
      • assist-control (volume-cycled or pressure-targeted)
        • volume cycled
          • settings: tidal volume (TV), flow rate, flow waveform, fraction of inspired oxygent (FiO2), positive end-expiratory pressure (PEEP), and frequency
          • airway pressures and auto-PEEP are dependent on respiratory system
        • pressure-targeted
          • settings: pressure target, inspiratory time, inspiratory rise rate, FiO2, PEEP, and frequency
          • tidal volume and auto-PEEP are dependent upon respiratory system
      • pressure-support ventilation
        • usually with set continuous positive airway pressure (CPAP)
        • 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
    • Barotrauma (trauma from high pressures)
      • findings
        • subcutaneous emphysema
        • pneumomediastinum
        • pneumothorax
        • pneumoperitoneum
        • interstitial emphysema
      • etiologies
        • high mean alveolar pressure
        • high peak airway pressures
        • necrotizing lung pathology
        • high minute ventilation
        • longer duration of mechanical ventilation
    • Ventilator-induced lung injury (atelectrauma or trauma caused by atelectasis)
      • PEEP to prevent widespread alveolar unit closure and atelectasis
    • Acid-base status derangements
      • generally, changing the respiratory rate is preferred for responding to derangements (because TV is based on height)
        • increase respiratory rate
          • metabolic acidosis
          • respiratory acidosis
        • decrease respiratory rate (assuming adequate oxygenation)
          • respiratory alkalosis
    • Infections
      • ventilator-associated pneumonia
        • broader antibiotic coverage for Staphylococcus aureusPseudomonas aeruginosa, and other gram-negative bacilli  
          • P. aeruginosa: cefepime, piperacillin-tazobactam, and imipenem/meropenem
          • Expanded gram-negative coverage: aminoglycosides
          • MRSA: vancomycin, linezolid
            • daptomycin cannot be used because it is inactivated by surfactant in the respiratory tract
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