Weaning and Extubation

The clinician faces multiplicity of options when reducing or discontinuing ventilatory support, which has resulted in considerable controversy regarding the ‘best’ approach.  In the last decade there has been a fundamental change in the approach to discontinuing invasive mechanical ventilation--from "predicting" to "checking." 

For the last 30 years of the Twentieth Century clinicians used a variety of measures of ventilatory mechanics in attempting to identify the point at which a patient was ready to switch from full ventilatory support to fully spontaneous breathing.  These predictive measures or "weaning parameters" included spontaneous respiratory rate, often divided by the patient's average spontaneous tidal volume during a brief disconnection from the ventilator (f/VT, or the "rapid shallow breathing index"), inspiratory vital capacity, minute ventilation, and the maximum inspiratory pressure the patient could generate during end-expiratory airway occlusion.  Most often, patients remained fully supported until threshold criteria were reached in one or more of these "weaning parameters," and then a trial of spontaneous breathing or progressively reduced inspiratory support was undertaken. 

A systematic review of existing evidence for the efficacy of the substantial number of weaning predictors, singly and in combination, was undertaken in conjunction with the development of international practice guidelines for weaning [ref-2].  Two important findings of this process were that no existing measurement, index, or combination of measurements could predict the ability to discontinue ventilatory support with sufficient accuracy, and that many patients considered by the clinicians managing them to be "unweanable" or in need of a prolonged weaning regimen could, in fact, breathe fine without assistance if simply taken off ventilatory support.   This led to the recommendation that, rather than attempting to predict when patients were ready for weaning, clinicians should simply "check," both early and repeatedly, to determine whether they were, in fact, ready.  

Assessment for Readiness to Wean

According to the new evidence-based guidelines, patients should be checked for readiness to wean when the following conditions are present:

• Evidence for some reversal of the underlying cause for acute respiratory failure;
• Adequate arterial oxygenation (for example, arterial PO₂ at least 60 mm Hg on 40% oxygen with positive end-expiratory pressure 5 cm H₂O or less);
• Acceptable acid-base balance (for example, arterial pH 7.25 or higher);
• Hemodynamic stability (absence of active myocardial ischemia, and blood pressure supportable without requirement for significant vasopressor support); and,
• Sufficient ventilatory drive and neuromuscular function to initiate a spontaneous inspiratory effort.

The Spontaneous Breathing Trial

Patients meeting the above criteria should have a spontaneous breathing trial.  Although the time-honored T-piece method remains the favorite of many clinicians, this requires disconnecting the patient from the ventilator and interposing a different breathing circuit.  A simpler alternative is to leave the patient connected to the ventilator circuit and switch to the CPAP mode, so that no mandatory or assisted breaths are provided.  Low-level CPAP (e.g., 5 cm H₂O) can be provided during the trial at the discretion of the clinician, although adding more than 5 cm H₂O of inspiratory pressure support tends to defeat the purpose of the spontaneous breathing trial and should be avoided.  Increasing the FIO₂ by 0.10 during the trial is recommended to avoid hypoxemia, as lung volumes and ventilation-perfusion matching may change during spontaneous breathing.

Patients should be observed closely during the trial for evidence of increasing respiratory distress (such as progressively increasing respiratory rate over several minutes to 35 breaths/min or more), or for increasing tachycardia or systemic arterial hypertension, indicating that ventilatory support should be reinstituted.  If spontaneous breathing is clinically tolerated for 30 minutes, an arterial blood specimen should be obtained for analysis to confirm that arterial oxygenation and acid-base status have not deteriorated unacceptably.   The trial may be extended for up to 120 minutes if it is not initially clear whether it will be successful.  However, there is little to gain by lengthening the period of spontaneous  breathing beyond this point,.  Prompt extubation should be considered for patients who tolerate the trial clinically and maintain acceptable arterial blood gas values, providing no separate indication for an endotracheal tube is present (see below).

Evaluation of the Patient Who Fails a Spontaneous Breathing Trial

An unsuccessful spontaneous breathing trial in a patient who fulfills the criteria for initiating it should be regarded as a diagnostic and therapeutic problem requiring deliberate attention.  Table 10 [58k PDF*] presents an algorithm for sorting out the reason or reasons for a failed trial.  According to best available evidence, patients should undergo a spontaneous breathing trial once each day.  Between trials, a level of support should be provided (such as assist-control ventilation, or pressure support sufficient to prevent tachypnia and maintain patient comfort) such that the patient is able to rest without development of ventilatory muscle fatigue.

For difficult cases, as with prolonged mechanical ventilation in acute respiratory failure or in patients who have a serious underlying illness in other organ systems, it is helpful to approach weaning in a systematic fashion, considering possible impediments to success in approximate order of their likelihood, as summarized in Table 9. In most cases of initial weaning failure, the reason becomes apparent on proceeding through the algorithm in a stepwise fashion. Indeed, perhaps the most common reason for inability to wean from mechanical ventilation among patients who have been critically ill for a week or more is that the primary illness has not improved sufficiently for weaning to be achievable.

When patients are unable to be weaned because of a high minute ventilation requirement, as is commonly seen in ARDS, the clinician may be helped by an analysis of the physiologic mechanism responsible. Only three basic mechanisms can account for a higher than normal minute ventilation: hyperventilation (i.e. respiratory alkalosis), increased carbon dioxide production, and increased dead space ventilation (Vd/Vt). Hyperventilation is readily identified by the presence of hypocapnia. Carbon dioxide production and Vd/Vt can be assessed either by collecting expired gas in a bag or using a metabolic cart (as used for determining nutritional requirements), in conjunction with an arterial blood sample. Making these simple measurements can aid in identifying the reason for a high minute ventilation:

• If increased carbon dioxide production is the cause, the clinical problem is a systemic rather than pulmonary one—perhaps the patient is receiving excessive nutritional support;
• If increased Vd/Vt is responsible, increased ventilation requirement results from inefficient ventilation, as seen with ARDS, dynamic hyperinflation, or pulmonary thromboembolism.

Weaning technique

The main techniques currently in use for gradual weaning from ventilatory support are the T-piece method and PSV. The time-honored T-piece method consists of repetitive periods of spontaneous ventilation to the patient’s tolerance, interspersed with periods of rest with full ventilatory support. When PSV is used in weaning, the patient is switched from full ventilatory support to PSV at an inspiratory pressure sufficient to provide the same Vt as before; inspiratory pressure is then gradually reduced, using the patient’s respiratory rate (usually maintained below 30 breaths/min) as a guide to the adequacy of support. Weaning with SIMV, which consists of gradually reducing the mandatory rate and thus progressively decreasing the ventilator’s contribution to the required minute ventilation, using the total respiratory rate (mandatory plus spontaneous breaths) and other clinical signs as indicators of progress, is no longer recommended because multicenter clinical trials have shown it to prolong the process when compared to T-piece trials and PSV.

Extubation

Discontinuation of ventilatory support and extubation are not the same thing. In most instances, both can be carried out together – as after general anesthetic. However, failure to recognize the important exceptions can lead to serious problems. In addition to the need for mechanical ventilation, indications for translaryngeal intubation include upper airway problems, inability to protect the lower airways from aspiration of oropharyngeal or gastric contents, and inability to clear secretions without suctioning or other measures.

Unfortunately, no specific tests or measurements are currently available to assess patients’ ability to protect their airway from aspiration and their ability to clear secretions. A trial of extubation is often the best way to determine whether the patient needs continued airway protection (with a tracheostomy, for example).

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