Weaning Failure

It is very important to recognise respiratory distress in patients undergoing mechanical ventilation.  This often presents as ventilator dyssynchrony, agitation, and general activation of the sympathetic nervous system (seating, tachycardia, hyperventilation).  The following possible causes need to be considered.

  • Weakness
      • Critical illness weakness
      • Cord injury
      • Drug-induced weakness from steroids, stains, amino glycosides
      • Autoimmune e.g. myasthenia gravis,
      • Inherited muscular dystrophies
  • Low pulmonary or chest wall compliance
      • Stiff lungs from pulmonary oedema, inflammation, interstitial lung diseases
      • Pleural effusions, pneumothorax
      • Mechanical disruption of ribs, diaphragm
      • Intra-abdominal hypertension
  • Airflow obstruction
      • Mucous plugging
      • Acute  bronchospasm
      • Anaphylaxis
      • Blocked ETT or tracheostomy tube
  • Central nervous system conditions
      • Acute delirium (e.g. drug-induced, encephalopathy, encephalitis)
      • Psychiatric
      • Pain
      • Environmental

Unless the cause is immediately obvious, such as a visibly kinked ETT, the patient should be given increased ventilatory support  or re-ventilated using controlled ventilation modes while the underlying (usually multifactoral) causes are being considered.  In some cases this may require re-paralysing the patient to gain control and reduce metabolic rate (CO2 production).  Sometimes the ventilator settings may be slightly suboptimal and the patient quickly responds to an increase in pressure support.  However, prolonged periods of suboptimal support in the presence of diaphragmatic weakness may lead to progressive basal collapse with re-distribution of ventilation to more compliant anterior regions of the lung.

 

A note of caution:  if the patient's respiratory drive is excessive, additional ventilatory support can exacerbate their lung injury.  Specifically moderate diaphragramatic  efforts can generate very high (unmeasured) transpulmonary pressures, which contribute to ventilation-induced lung injury.  If there is doubt these patients should be re-paralysed and their pulmonary mechanics measured (Static compliance, auto-PEEP, and peak to plateau pressure differences to assess resistive work of breathing).

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Page last reviewed: 13 May 2014
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