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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
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- 3 types of respiratory failure:
Acute hypoxemic respiratory failure (type I): pulmonary edema, pneumonia, pulmonary embolism, atelectasis
Acute hypercapnic respiratory failure (type II): CNS depression, spinal cord injury, neuromuscular disease
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
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Impending Respiratory Failure: marginally able to maintain normal blood gases with ↑ work of breathing
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- 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)
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- 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.
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- 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)
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