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Respiratory Failure: inability to maintain either the normal delivery of oxygen to tissues or normal removal of carbon dioxide from tissues

  • 3 types of respiratory failure:
    1. Acute hypoxemic respiratory failure (type I): pulmonary edema, pneumonia, pulmonary embolism, atelectasis

    2. Acute hypercapnic respiratory failure (type II): CNS depression, spinal cord injury, neuromuscular disease

    3. Chronic respiratory failure (hypoxemia & hypercapnia; type III): COPD, cystic fibrosis, bronchiectasis

  • Indications for mechanical ventilation: apnea/prophylactic ventilatory support; respiratory failure; impending respiratory failure, severe oxygenation problems

Impending Respiratory Failure: marginally able to maintain normal blood gases with ↑ work of breathing

  • Impending respiratory failure:
    • Inadequate alveolar ventilation: hypercapnia >55mmHg & pH <7.20 (changes in pH can alter drug potency)
    • Inadequate lung expansion: VT <5mL/kg IBW, VC <10mL/kg (inability to cough results in retained secretions, ↑ risk of pulmonary infections), RR >35breaths/min
    • Poor muscle strength: MIP or NIF ≤20cmH2O (weakness of inspiratory chest wall muscles, ↑ risk of failed extubation); VC <10mL/kg; MVV <2 × VE
    • Increased WOB: VE >10 LPM, VD/VT >60%
    • Hypoxemic respiratory failure: P(A-a)O2 on 100% >350mmHg, PaO2/FiO2 ratio <200 (estimate of normal P[A] = 7 × FiO2; P[a]O2 = 5 × FiO2)

Table F-1 Acronyms

  • Spontaneous breath: patient triggers & cycles the breath, controls VT (may be assisted by pressure support)
    • Partial-support mechanical ventilation: aka dual mode ventilation, patient assumes some WOB
  • Mandatory breath: triggered & cycled by machine; all mandatory breaths are assisted breaths
    • Full-support mechanical ventilation: to rest a patient with respiratory fatigue, MV assumes all the WOB; patient is typically sedated; manipulation of ventilation parameters results in specific, predictable changes in ABGs.


  • Continuous mandatory ventilation (CMV): control & assist control
  • Continuous spontaneous ventilation (CSV): pressure support & continuous positive airway pressure (CPAP)
  • Mandatory & spontaneous combined “dual mode ventilation”: synchronized intermittent mandatory ventilation (SIMV)

Table F-2 Modes of Ventilation

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