Weaning from mechanical ventilation

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Weaning from mechanical ventilation has appeared in  Question 1c from the second paper of 2000,  Question 1d from the first paper of 2001 and Question 1d from the first paper of 2000. Though this is a frequently examined topic in the vivas and hot cases, which makes it more surprising that it has not come up more often in the written exam. A more recent Question 24 from the second paper of 2014 also touches upon the topic of the various indices of extubation readiness, and discussed elective extubation on to NIV.

The rapid shallow breathing index, the spontaneous breathing trial and the various bedside tests to assess readiness for extubation are explored in greater detail in the chapter on the assessment of extubation readiness. This chapter is more about the techniques one may use to bring the patient closer to this assessment, and to ensure they satisfy those criteria.

Some of the best resources for this broad topic include the following:

• LITFL: Weaning from Mechanical Ventilation.
• LITFL: Difficulty Weaning from Mechanical Ventilation.
• LITFL: Non-invasive ventilation for weaning.
• Boles (2007) - a definitive resource, covering some widely accepted definitions
• McConville (2012) - a NEJM review article recommended in David Tripp's notes
• Branson (2012) - a USA-centric article focusing on protocols and closed loop systems.
• UpToDate -Methods of weaning from mechanical ventilation -  expensive but comprehensive.

Definition of weaning from mechanical ventilation

• Weaning from mechanical ventilation can be defined as the process of abruptly or gradually withdrawing ventilatory support, allowing the patient to assume a greater proportion of the ventilatory effort.
• According to the first line of Bransons's 2012 article, "weaning comprises 40% of the duration of mechanical ventilation". This statistic is derived from a 2002 prospective cohort study by Esteban et al, which looked at data from 361 ICUs in 20 countries (which did not include Australia, but did include Bolivia Tunisia and Ecuador).
• The term arose in the early days of mechanical ventilation when patients were routinely subjected to prolonged (10-14 day) periods of brutally mandatory ventilation, with heavy sedation and paralysis. "After days of respiratory muscle inactivity, the slow resumption  of spontaneous breathing and weaning ventilatory support must have appeared a necessity".
• These days, "liberation" or "discontinuation" may be better terms than "weaning", because they suggest an abrupt cessation of ventilation. The previously favoured method of gradually withdrawing ventilator support may actually be harmful.

Weaning strategies

Options of gradually decreasing ventilator support:

• Gradually decreasing the rate of mandatory breaths on a mandatory mode (eg. SIMV)
• Gradually reducing the pressure support on a  patient-triggered mode (eg. PSV).
• Brochard et al (1994)  compared T-piece trials, SIMV and pressure support (PSV) among 109 intubated patients, and found PSV to be the mode least likely to result in weaning failure (23% failure rate vs. 43% and 42%).
• LITFL suggest to "wean patients to about 50% of their maximal support levels (ie, 50% of their initial inspiratory pressure settings)".

Options of abruptly decreasing ventilator support:

• T-piece trial of spontaneous breathing
• Spontaneous breathing trial (SBT) with ventilator on zero settings
• Spontaneous breathing trial on a patient-triggered mode and with minimal ventilator support, eg. 5 of PEEP and 5 of pressure support
• One may do one, or several, of these trials every day.
• Esteban et al (1995) compared four different  methods (SIMV, PSV, daily SBTs and twice-daily SBTs). Weaning time was 5 days on average for the SIMV group, 4 days for the PSV group and 3 days for the SBT groups.
• ABC trial (2008) - 336 patients randomised to standard care with SBT or to a daily interruption of sedation together with an SBT. The treatment group had more ventilator-free days and shorter ICU stay. It is not surpising that decreasing sedation improves SBT success.

Which is better, gradual reduction or abrupt reduction of support?

• The evidence seems to favour abruptness.  Delayed weaning may be harmful; a study by Coplin et al (2000) revealed an increased rate of pneumonia and a longer ICU stay among a population of brain-injured patients who were weaned gradually (over 3 days). In this group the only reason to be intubated was their brain injury, which in Australia would be viewed as an insufficient (most of this group would be extubated during the morning round after their admission).

Protocol-driven weaning

• A protocol for weaning usually consist of a checklist of extubation failure risk factors, which must be satisfied before a awakening trial and spontaneous breathing trial are carried out. The protocol then leads to a prescribed period of spontaneous breathing; if at the end of this period the patient does not meet failure criteria, they are safe to extubate.
• A review article by Girard et al (2008) lists (in Box 1) the following rationale for the use of weaning protocols:

  • Less limited and biased by human decision-making than physician-directed weaning
  • Developed based on best evidence; less influenced by personal or local opinion
  • Efficacy and safety supported by numerous clinical investigations
  • Can make up for limitations in local resources or staff availability
  • Free physicians to perform other duties in the ICU
  • Facilitate quality monitoring and improvement
  • Can be the basis of a systematic approach to learning
  • Enhance transparency and communication

• The following criticisms support physician-directed weaning:

  • Flexible and adaptive to the needs of individual patients
  • Can lead to innovative changes in practice
  • Equivalent to weaning protocols in some studies, especially in ICUs with high staffing levels
  • Promotes education and leads to highly skilled practitioners
  • Does not require the additional resources needed to design, implement, and sustain weaning protocol use
  • Avoids institution-wide acceptance of a treatment strategy before best evidence is available

• In the United States, respiratory therapists and nurses run the weaning using prescribed protocols, and the advantage of employing them is cost - it is much more expensive to pay a physician to run your ICU.
• A Cochrane review of the topic (Blackwood et al, 2014) has assessed the available evidence for protocol-driven weaning. 17 trials were found, with 2434 patients. There was a distinct improvement in terms of protocol-driven weaning: duration of ventilation was reduced by about 26%. Neurosurgical patients did not seem to benefit.

Automated feedback ventilator weaning systems

• ASV (adaptive support ventilation) is well covered by the LITFL entry on this topic, as well as the 2013 article by Fernandez et al.
• The advantages is a computerised algorithm which determines the optimum tidal volume and I:E ratio of a PRVC/PSV mode of ventilation. The closed loop is a feedback loop which constantly modifies the ventilator settings.
• Automated algorithms may not be better than experienced staff. A recent RCT (Taniguchi et al, 2015) had instead found that an experienced respiratory physiotherapist is better (faster) than a weaning algorithm, but the patients were not "difficult to wean" as the weaning process took only 1-2 hours.
• A recent (2014) Cochrane review of automated vs non-automated weaning found 21 eligible trials, comprising 1676 participants (mainly adult).  The automated closed loop systems reduced the duration of ventilation by 10%, but there was no other effect (mortality, reintubation rate, etc etc). Furthermore, post-operative surgical populations did not seem to benefit.

Difficulty weaning from ventilation, and "weaning failure"

Again, the LITFL entry on this topic does it justice. "Weaning failure" is defined as failure to pass a spontaneous breathing trial, or the need for reintubation within 48 hours.

There are some definitions for the degree of weaning difficulty:

These classifications were defined by the International Concensus Conference in 2005 (Boles et al, 2007) on the basis of  outcomes data. "Simple wean" patients represent 69% of the ventilated population, and have low mortality (ICU mortality 5%, hospital mortality 12%). The rest have 25% mortality.

Causes of difficult weaning weaning failure can be summarised as a table. The one below is based extensively on the article by Boles et al (2007). A super-keen exam candidate may attempt to come up with another hundred or so causes, but the list offered here may already be on the long side, and certainly beyond the needs of a ten minute CICM SAQ answer.

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

Predictors of weaning failure

Table 2 from the McConville paper lists the risk factors for unsuccessful extubation:

• Failure of two or more consecutive spontaneous-breathing trials
• Chronic heart failure
• Partial pressure of arterial carbon dioxide >45 mm Hg after extubation
• More than one coexisting condition other than heart failure
• Weak cough
• Upper-airway stridor at extubation
• Age ≥65 yr
• APACHE II score >12 on day of extubation*
• Patient in medical, pediatric, or multispecialty ICU
• Pneumonia as cause of respiratory failure

Strategies to prevent weaning failure

One may view these strategies as answers to the questions posed by the causes of difficult weaning. The same table again can be turned to this purpose:

Solutions for the Causes of Difficult Weaning

Respiratory load	Reduce the work of breathing Sit the patient up (or lie them down if they are a high spinal injury patient) Set appropriate patient-centered ventilator settings Use a PSV mode (Brochard et al, 1994) Manage the bronchospasm with bronchodilators Ensure a satisfactory endotracheal tube diameter Decrease dead space; consider a tracheostomy Focus on improving fluid balance (i.e. decreasing interstitial and alveolar lung water) to improve lung compliance Ensure humidification and chest phsyiotherapy to help clear secretions Consider bronchoscopy to forcibly evacuate tenacious sputum plugs
Cardiac load	Optimise the management of heart failure by traditional methods Ensure adequate oxygen carrying capacity of the blood (i.e. correct anaemia and dyshaemoglobinaemia)
Neurological causes	Minimise opioids and sedatives which decrease respiratory drive Ensure satisfactory pain management for surgical wounds and trauma which might impair respiratory function; consider regional and neuraxial analgesia Actively screen for and treat delirium, focusing on nonpharmacological treatments Take preventative steps to avoid neuropathy and myopathy: limit the use of steroids and neuromuscular junction blockers to the necessary minimum Engage physiotherapy in the active rehabilitation of muscle power
Musculoskeletal and mechanical causes	Consider operative fixation of flail segment rib fractures Decompress the distended abdomen
Metabolic disturbances	Manage the cause of increased metabolic demand, eg. trauma, burns, sepsis Actively manage fever and hyperactive agitation Address nutritional needs with appropriately protein-rich supplements Ensure caloric goals are met, but not exceeded (to prevent hypercapnea) Consider lipid-rich carbohydrate-poor feeds for hypercapneic patients Correct metabolic acidosis