Question 1d

A 72 year old woman (55kg), Mrs X, with a history of severe emphysema and chronic bronchitis is intubated in the Emergency Department (ED) because of drowsiness associated with hypercarbia after her initial arterial blood gas analysis revealed:

  • pH 7.219        
  • PC02 98mmHg     
  • PO2 48mmHg    
  • HC03 39mmol/l        
  • lactate 2.5 mmol/l

You are called to the ED to assess and admit this woman to ICU. The history from her daughter reveals that Mrs X lives independently but is limited by severe breathlessness with exercise. 

At one week she remains ventilator dependent.

(d) What may interfere with her weaning and what may be done to facilitate weaning?

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College Answer

A list may be best here:
-     breathing system; demand valve resistance, humidifier, turbulence
-     ETT; too small
-     Airway; untreated asthma, secretions
-     Lung interstitium; oedema, collapse, infection
-     Musculoskeletal; weakness, hyperinflation, kyphoscoliosis
-     Cardiovascular; low cardiac output state, ischaemia
-     CNS drive; dugs, stroke,

The aim is to minimise the external work of breathing against inefficient ventilators and tubing, consider a tracheostomy which allows staged separation from the ventilator and improve all aspects of the patients general condition including nutrition, K/PO4/Mg, lung function and cardiovascular status.


A systematic approach is in order.

  • Airway:
    • increased work of breathing agaisnt ventilator tubing
  • Breathing:
    • Ongoing bronchospasm
    • ventilator-associated pneumonia
    • mechanical disadvantages of having a hyperinflated chest cavity and kyphosis
  • Circulation:
    • Cardiac failure, resulting in poor respiratory exercise tolerance and pulmonary oedema
  • Disability
    • Delirium, resulting in increased sedation requirements
    • Deconditioning of respiratory muscles
    • critical illness polyneuropathy
    • malnutrition resulting in weakness

In generic terms, this table lists the usual suspects:

Causes of Difficulty Weaning from Mechanical Ventilation

Respiratory load

Increased work of breathing

  • Inappropriate ventilator settings
  • Reduced compliance
  • Increased airway resistance
  • Dynamic hyperinflation
  • Endotracheal tube diameter
  • Increased airway secretions or sputum retention
Cardiac load
  • Heart failure
  • Increased cardiac workload (eg. increased metabolic demand)
  • Decreased oxygen-carrying capacity of blood, eg. anaemia or some sort of dyshaemoglobinaemia
Neurological causes
  • Depressed central drive, eg. due to drugs
  • Delirium, avolition
  • Peripheral neurological dysfunction, eg. ICU-acquired weakness
  • Pain, eg. due to a laparotomy wound
Musculoskeletal causes
  • Muscular problems (eg. steroid myopathy) or NMJ problems (eg, myasthenia)
  • Mechanical problems, eg. scolisosis-associated restrictive lung disease or a massive distended abdomen in ileus
  • Skeletal problems, eg. chest trauma, flail segments
Metabolic disturbances
  • Increased metabolic demand, eg. trauma, burns, sepsis
  • Extremes of nutrtion, eg. obesity or cachexia
  • Metabolic acidosis


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