Weaning from mechanical ventilation

Abstract

Mechanical ventilation has gone through a dramatic evolution over a relatively short space of time. After the Copenhagen polio epidemic in 1952, negative pressure ‘iron lungs’ were replaced by intermittent positive pressure ventilation. This was originally delivered at set volumes and rates. The next step forward was the introduction of intermittent mandatory ventilation, and shortly thereafter this was synchronized to the patient's respiratory effort. More recently, pressure support ventilation and bi-level positive airway pressure modes have become available. Modern ventilators are increasingly sensitive, allowing easy patient triggering of supported breaths, modes such as tube compensation, and measurement of numerous respiratory parameters. Developments in weaning techniques have paralleled these improvements in ventilator functionality.

Key points

Weaning may be hastened by spontaneous breathing trials and daily screening of respiratory function.

Respiratory rate/tidal ventilation ratio is a good predictor of successful weaning.

Synchronized intermittent mandatory ventilation is the least efficient method of weaning.

Use of non-invasive ventilation may improve outcome for some patients who develop respiratory failure after extubation.

Conventional invasive ventilation is associated with a number of complications such as pneumonia, tracheal stenosis and baro/volutrauma. Many of the complications increase in likelihood with duration of ventilation. It is therefore important to wean patients from mechanical ventilation as quickly as possible.

Weaning from mechanical ventilation is the process of reducing ventilatory support, ultimately resulting in a patient breathing spontaneously and being extubated. This process can be achieved rapidly in ∼80% of patients when the original cause of the respiratory failure has improved. The remaining cases will require a more gradual method of withdrawing ventilation.

Factors associated with successful weaning

To enable weaning to be successful, thought has to be given to the following areas:

• has the underlying condition improved?

• is the patient's general condition optimal?

• have potential airway problems been identified and remedied?

• is breathing adequate?

Cause of respiratory failure

In order for a patient to wean successfully, the cause of their respiratory failure has to have been resolved to a reasonable level. Thought has to be given to the patient's state before the current exacerbation to gauge what it is possible to achieve, and allow setting of realistic aims.

General optimization

Careful preparation before potential weaning can make the difference in the numerous borderline weanable cases encountered in the intensive care. This is very important because those patients who are re-intubated in general have worse outcomes. Common causes of weaning failure are listed in Table 1. Table 2 illustrates the usual preconditions that must be met before any consideration can be given to the institution of a weaning programme.

Table 1

Causes of weaning difficulty

Central drive

Drive to breathe reduced by:

Sedatives

Direct insults to the respiratory centre

Hyperventilation to abnormally low \(P\mathrm{a}_{\mbox{\textsc{\mathrm{co}}}_{2}}\) for a particular patient

Metabolic alkalosis (commonly exacerbated by hypokalaemia)

Loss of hypoxic drive (COPD)

Clinically patients may fail to demonstrate respiratory distress and will in time develop Type II respiratory failure

Neuromuscular

Primary neurological disorders

Guillain–Barré syndrome

Myasthenia Gravis

Botulism

Critical illness polyneuropathy (more common with steroids and neuromuscular blocking agents)

Critical care myopathy/malnutrition

Electrolyte abnormalities

Hypokalaemia

Hypophosphataemia

Hypomagnesaemia

Hypocalcaemia

Hypothyroidism

Increased respiratory load

Increased resistance

Bronchospasm

Increased or thick secretions

Reduced compliance

Pneumonia

Pulmonary oedema

Intrinsic PEEP

Pleural effusions

Pneumothoraces

Paralytic ileus or abdominal distension

Increased ventilation

Hypermetabolism (sepsis is a common cause)

Overfeeding

Metabolic acidosis

Shock

Pulmonary embolism

Table 2

General preconditions for commencement of weaning

Reversal of primary problem causing need for ventilation

Patient awake and responsive

Good analgesia, ability to cough

Reducing or minimal doses of inotropic support

Ideally—functioning bowels, absence of abdominal distension

Normalizing metabolic status

Adequate haemoglobin concentration

Airway problems

To successfully wean a patient the artificial airway needs to be removed. For this to happen, good upper airway reflexes are needed, including an adequate cough and minimal secretions. An adequate conscious level is required for airway maintenance after extubation.

Airway (particularly laryngeal) oedema may be under-recognized as a cause of difficulty in breathing after extubation, occurring in 10–15% of patients. The risk factors for post-extubation airway oedema include a medical reason for admission, a traumatic or difficult intubation, a history of self extubation, an overinflated tracheal tube cuff at admission, and intubation for extended periods. The ability to breathe around a deflated endotracheal tube cuff, or the presence of a cuff leak >130 ml during volume cycled ventilation, has been used to predict an adequate airway diameter.¹ In those patients at risk, corticosteroids are commonly used, but there is little evidence to support this practice.² Post-extubation stridor may be ameliorated by epinephrine nebulizers or inhalation of a helium/oxygen mixture. Continuous positive airway pressure administered after extubation may also help.

Predicting successful weaning

Numerous numerical indices have been used to predict the outcome of weaning, some of which are listed in Table 3. The sensitivities and specificities of each vary depending upon the cut-off used. Many of the indices have good sensitivities but most have low specificities. When looking at these indices, it is not only important to look at the cut-off used but also the timing as to when the test was undertaken.

Table 3

Numerical indices used to predict successful weaning

Minute ventilation	<10 litre min−1
Vital capacity/weight	>10 ml kg−1
Respiratory frequency	<35 bpm
Tidal volume/weight	>5ml kg−1
Maximum inspiratory pressure	<−25 cm H2O
\(P\mathrm{a}_{\mbox{\textsc{\mathrm{o}}}_{2}}/P\mbox{\textsc{\mathrm{a}}}_{\mbox{\textsc{\mathrm{o}}}_{2}}\)	>0.35
Respiratory rate/tidal volume	<100 litre−1
\(P\mathrm{a}_{\mbox{\textsc{\mathrm{o}}}_{2}}/F\mbox{\textsc{\mathrm{i}}}_{\mbox{\textsc{\mathrm{o}}}_{2}}\)	>200 mm Hg (26.3 kPa)

A number of guidelines favour the use of the ratio of respiratory rate/tidal volume undertaken 1 min into a spontaneous breathing trial (SBT).³ In addition, a reasonable level of oxygenation should be demonstrated, often assessed by the

\(P\mathrm{a}_{\mbox{\textsc{\mathrm{o}}}_{2}}/F\mbox{\textsc{\mathrm{i}}}_{\mbox{\textsc{\mathrm{o}}}_{2}}\)

ratio at a positive end-expiratory pressure (PEEP) <5 cm H₂O. It is clear that despite their limitations, the systematic use of predictors produces better outcomes than clinical judgement by physicians alone.⁴

Assessing adequacy of breathing

The SBT is the traditional approach to weaning patients from mechanical ventilation. This originally involved disconnecting the patient from the ventilator and connecting a device such as a T-piece. Other variants of SBTs include continuous positive airway pressure (CPAP), which may maintain the functional residual capacity, and low level variable pressure support ventilation (PSV) to overcome the resistance to breathing through an endotracheal tube (often called tube compensation).

As well as assessing whether a patient is ready for extubation, SBTs of increasing duration can be used to aid the weaning process and can be performed without disconnecting the patient from the ventilator.

When patients are considered ready to wean, the best way to assess whether they will breathe on their own is by undertaking an SBT. It has been demonstrated that by doing this the weaning process may be hastened.

Trials comparing CPAP (5 cm H₂O), PSV (7 cm H₂O) and T-piece methods to ascertain readiness for extubation do not demonstrate any great superiority of one method relative to another. It has also been shown that SBTs for 30 and 120 min are equivalent.⁴ Evidence-based criteria for terminating weaning trials do not exist, so subjective clinical judgement is used backed up by arterial blood gases. The criteria used in some clinical trials are shown in Table 4.

Table 4

Criteria used in some trials to terminate (fail) SBTs

Respiratory rate	>35 bpm
\(S\mathrm{p}_{\mbox{\textsc{\mathrm{o}}}_{2}}\)	<90%
Heart rate	>140 beats min−1 or change by >20%
Systolic blood pressure	>180 or <90 mm Hg
Agitation
Sweating
Anxiety or signs of increased work of breathing (paradoxical breathing, intercostal retraction, nasal flaring)

Patients successfully completing an SBT may proceed to extubation. Those who fail SBTs may require a slower form of weaning involving SBTs of a gradually increasing duration. Consideration may also be given to the formation of a tracheostomy.

Patients failing the spontaneous breathing trial

Many patients will not pass a spontaneous breathing trial on their first attempt (those with numerous comorbidities, the elderly and patients who have been ventilated for long period of time often fall into this category).

The best trials looking at the weaning of patients that fail their initial spontaneous breathing trial have given conflicting results. The ventilatory choices for these patients include the following:

• T-piece trials;

• synchronized intermittent mandatory ventilation (SIMV); or

• pressure support ventilation (PSV).

T-piece trials involve periods of supported ventilation being gradually broken up by SBTs of increasing duration (most trials increase these durations twice per day). There is some evidence that once-daily breathing trials may be just as effective.⁶ Once the patient can manage 2 h without problems they are extubated (see the criteria to terminate SBTs).

SIMV is undertaken by gradually reducing the mandatory rate (most trials have done this by 2–4 bpm on a twice-daily basis, or more regularly if tolerated). The end-point for these SIMV patients is a rate of 4–5 min⁻¹, for varying periods of time depending upon the trial. Patients who meet preset criteria are then extubated.

PSV involves gradually reducing the pressure to assist spontaneous breaths (most trials have done this by reducing the pressure support by 2–4 cm H₂O twice a day and more often if tolerated). The end point is PSV at around 5–8 cm H₂O for a duration that varies from 2 to 24 h. Again, patients who have reached this stage successfully are then extubated.

In the trial by Brochard and colleagues, PSV led to a significantly shorter duration of weaning relative to T-piece or SIMV methods.⁷ Esteban and colleagues, however, demonstrated T-piece trials to be superior to either PSV or SIMV.⁶

In the majority of trials, SIMV was found to be the least efficient method of weaning. It must be noted, however, that in these trials, there was no support for spontaneous breaths between triggered mandatory breaths. Modern ventilators often combine SIMV or bi-level positive airway pressure and a form of PSV. There is also evidence from both trials that protocols may hasten weaning.

A suggested algorithm for discontinuation of mechanical ventilation is shown in Figure 1.

Fig. 1

Algorithm for discontinuation of mechanical ventilation.

Non-invasive ventilation

Patients who are re-intubated have higher complication and mortality rates. Non-invasive ventilation could not only avoid intubation in some patients, but may also have a role in preventing re-intubation in patients who have failed extubation.⁸ Patients with chronic obstructive pulmonary disease (COPD), and those who are immunosuppressed with bilateral infiltrates, have been shown to have reduced intubation and mortality rates with the application of non-invasive ventilation. Benefit has also been demonstrated for patients with cardiogenic pulmonary oedema. Whether non-invasive ventilation has advantages over CPAP has yet to be proven for this group of patients (one study actually showed a higher rate of myocardial infarction with use of non-invasive ventilation).

Studies looking at heterogeneous populations with acute hypoxaemic respiratory failure have found no benefit in using non-invasive ventilation to facilitate the discontinuation of conventional ventilation or avoid re-intubation. Only trials looking at patients with COPD or cardiogenic pulmonary oedema, or with a predominance of such patients, have demonstrated improved survival, decreased pneumonia rates and decreased length of intensive care stays under these circumstances.^9–11

Tracheostomies

It is a generally held belief, despite the lack of evidence demonstrating direct benefit, that patients requiring long-term ventilatory support are better managed using a tracheostomy. Some of the advantages include easier mouth care, improved mobility of the patient, facilitation of oral nourishment; improved patient comfort allowing decreased sedation and better communication. Decreasing sedation use has been shown to reduce the length of intensive care stay. In view of these advantages, it would prove difficult to recruit patients for a trial comparing continued translaryngeal and tracheostomy routes of ventilatory support. Surgical and percutaneous tracheostomies have broadly been shown to carry equal risk. The forthcoming randomized controlled multi-centre ‘Tracman’ trial is aiming to elucidate whether there is an advantage to performing tracheostomies at a particular time. One single centre randomized trial by Rumbak and colleagues,¹² in a medical intensive care unit, demonstrated a reduced length of stay, reduced mortality and nosocomial pneumonia rate in the early tracheostomy group.

Patients proving difficult to wean

Equipment problems such as encrustation of the endotracheal tube or secretions in the filter can influence the ability to wean. The endotracheal tube may itself cause bronchospasm. Obese patients may require a higher level of PEEP while intubated to prevent atelectasis. Those with severe restrictive disease may normally breathe with a high respiratory rate. It may be appropriate in these sorts of patients to attempt an extubation.

Some patients will not wean quickly and may require the services of a specialist-weaning unit. These units take a much longer-term approach to weaning than most acute ICUs, and have a number of ventilatory modalities at their disposal. In the UK, they also have access to the resources required to arrange ongoing ventilation in the community, whether invasive or non-invasive.

An example of referral criteria used in a recent study included mechanical ventilation for more than 2 weeks, and having failed two spontaneous breathing trials. Of 403 patients studied, 68% were successfully weaned from the ventilator. The hospital mortality of those admitted was 25%. Only 50% of those admitted were alive at 1 yr, and 38% at 3 yr.¹³

Weaning protocols

It is has been shown in many studies that use of a weaning protocol reduces time on the ventilator and shortens ICU stay.¹⁴ Much of this work has been conducted in ‘open’ intensive care units in the USA, many of which are not run by specialist intensive care physicians. In these units, where a physician may only see patients once per day, these nurse-led protocols clearly work. A recent study, however, compared weaning by protocol with physician-directed weaning in a ‘closed’ ICU, staffed and directed by ICU-trained physicians. The results showed no difference between the two groups of patients with regard to duration of ventilation, ICU stay, hospital and ICU mortality, and re-intubation rate.¹⁵ The message from this study is that it is not the protocol that hastens weaning, but the constant vigilance and attention that the protocol necessitates.

Conclusion

All patients receiving ventilatory support should be assessed on a daily basis for their suitability for weaning. This may involve meeting several preconditions, and then an SBT. If unsuccessful, weaning should be attempted using either PSV, or daily spontaneous breathing periods of increasing duration. A tracheostomy may be helpful in patients who are difficult to wean. Over 95% of patients should be weanable in this way. A few patients per year may need referral to a long-term weaning unit.

See multiple choice questions 85–87.