Update on Weaning from Mechanical Ventilation
Update on Weaning from Mechanical Ventilation
By David J. Pierson, MD
What is the best way to tell when a patient is ready for weaning from mechanical ventilation? How should weaning be carried out in order to maximize the likelihood of success? What should the clinician do when the patient fails a trial of weaning? For many clinicians, the answers to these questions may be different now, thanks to an exhaustive, federally funded examination of the world’s weaning literature, and a new set of evidence-based guidelines developed as a result of this process.
Genesis of the New Weaning Guidelines
The new guidelines1 are the culmination of an ambitious undertaking by the US Agency for Healthcare Research and Quality (AHRQ, formerly the AHCPR), which sponsored a structured, comprehensive review of previous studies by the McMaster University Evidence-Based Practice Center and the American College of Chest Physicians Expert Panel on Weaning from Mechanical Ventilation. The final document resulting from this review2 was peer reviewed by a top-level multidisciplinary international group of experts in the area of weaning, and was then used as the basis for guideline development in collaboration with the American Association for Respiratory Care and the American College of Critical Care Medicine. Published in a supplement to Chest,3 the guidelines are accompanied by a series of evidence-based systematic reviews by members of the Expert Panel and others on the various aspects of weaning. The guidelines themselves have also been published in Respiratory Care.4 In providing and elaborating on 12 specific recommendations, the guidelines also emphasize the quality of the evidence upon which each is based.
The Importance of Weaning Patients as Quickly as Possible
It has been estimated that 90% of all critically ill patients require mechanical ventilation, and that, while receiving ventilatory support, these patients are somewhere in the weaning process about 40% of the time.5 Discontinuing ventilatory support and removing artificial airways occupy a large proportion of the time and concern of clinicians working in the ICU. In addition, shortening the duration of ventilatory support—and especially of endotracheal intubation—is an extremely important goal for all patients, considering the high incidence, associated morbidity, and costs of ventilator-associated pneumonia6 and other adverse consequences of the intubated, ventilated state.7,8 That weaning and extubation be carried out as expeditiously and as safely as possible is thus an extremely high-priority item for everyone participating in the management of critically ill patients.
The Rise and Fall of Weaning Parameters
For 30 years, clinicians approached the problem of getting patients off ventilators by using various clinical predictors—commonly known by the misnomer "weaning parameters"—to tell them when the process of weaning should begin. Once patients achieved some set of threshold values for various measurements of mechanics and gas exchange intended to predict the ability to breathe without assistance, a trial of spontaneous breathing or reduced ventilatory support (using intermittent mandatory ventilation [IMV] or, later, pressure support) was carried out to determine if the prediction was correct. If this trial was successful, the patient was extubated. Most clinicians used the original "weaning parameters" of Sahn and Lakshminarayan published in 1973 (especially the vital capacity, minute ventilation, and maximum inspiratory pressure),9 plus the rapid shallow breathing index of Yang and Tobin (spontaneous respiratory rate divided by average spontaneous tidal volume).10 Although a variety of other putative predictors were introduced, all of them were more complex and more difficult to use than those just mentioned, and none became widely used.
The fact is that the various "weaning parameters" have not acquitted themselves very well over the years. Predicting when patients are ready to come off the ventilator has never worked as well in everyday practice as the results of the studies imply that it should. With the passage of time, the literature on weaning predictors has become more and more of a mishmash, with little real progress despite the publication of dozens of new studies. In their comprehensive review of 65 studies on predicting weaning, the Expert Panel11 concluded that no predictor was sufficiently accurate or applicable in different clinical settings to be used to reliably predict that a given patient was not yet ready for weaning.
In the last several years, there has been a shift in the thinking of many intensivists about how to approach the question of readiness for weaning. This shift has been from "predicting" to "checking." That is, having little confidence in the ability to know in advance whether weaning would be successful, clinicians have turned to simply performing a spontaneous breathing trial (SBT) and assessing the results. An important early study by Ely and associates12 demonstrated that this approach identified patients who could be weaned earlier and led to substantial shortening of the duration of ventilatory support. Studies comparing various strategies for weaning difficult-to-wean patients have varied in their conclusions about the best weaning mode, but these studies have all found that about three-quarters of all ventilated patients could simply be taken off the ventilator, without any dedicated weaning strategy, following an SBT. The new guidelines recommend this approach, and these guidelines provide simple criteria for identifying patients in whom an SBT should be done.
How Weaning Should be Done in 2002
The essence of the new guidelines is summarized in Table 1. Weaning (or, more accurately for most patients, discontinuation of ventilatory support) should be considered in every patient who requires ventilatory support for 24 hours or more. Rather than serially measuring spontaneous respiratory rate, tidal volume, minute ventilation, and maximum inspiratory pressure in order to find out when the patient is "ready" for weaning, the guidelines tell us to simply do an SBT and find out whether they are ready. The criteria for performing an SBT are based not on ventilatory mechanics but rather on the patient’s overall medical status, how much support is being provided, and whether he or she can make an attempt to breathe (see Tables 1 and 2, below).
The guidelines discourage half-hearted SBTs. That is, since the purpose is to determine whether the patient can breathe unassisted, it makes little sense to partially support their breathing during the trial. I have always advocated an old-fashioned T-piece trial for weaning—with no IMV breaths, no pressure support, and no CPAP. Such a trial can be done without disconnecting the patient from the ventilator by simply switching to CPAP mode with all rate and pressure settings at zero. There is no need to provide pressure support to overcome the resistance added by the endotracheal tube as long as the latter is a size 7.0 or greater and the patient’s minute ventilation is not more than about 10 L/min.13
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Table 1 |
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Basic Principles of Weaning from Mechanical Ventilation |
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1. Assess every patient who requires ventilatory support for 24 hours or more for possible weaning and
extubation.
2. Perform a spontaneous breathing trial (SBT) as soon as criteria in Table 2 are met. 3. No more than minimal support during SBT (T-piece, CPAP, and/or pressure support £ 5 cm H2O). 4. Strongly consider discontinuing ventilatory support if patient can sustain spontaneous ventilation for 30-120 minutes with reasonable comfort and acceptable arterial blood gas values. 5. Consider extubation separately from weaning, taking into consideration the abilities to maintain an adequate airway and clear secretions. 6. Investigate the potential reasons for ventilator dependency if patient fails SBT. 7. Repeat the SBT daily. 8. Between SBTs, provide a stable, non-fatiguing, comfortable form of ventilatory support. Source: ACCP-AARC-SCCM Collective Weaning Task Force: Evidence-based guidelines for weaning and discontinuing ventilatory support. I: Guidelines. Chest. 2001;120(6 suppl):375s-395s. |
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Table 2 |
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Criteria for Performing a Spontaneous Breathing Trial |
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1. Evidence for some reversal of the underlying cause for acute respiratory failure 2. 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) 3. Acceptable acid-base balance (for example, arterial pH 7.25 or higher) 4. Hemodynamic stability (absence of active myocardial ischemia, and blood pressure supportable without requirement for significant vasopressor support) 5. Sufficient ventilatory drive and neuromuscular function to initiate an inspiratory effort Source: ACCP-AARC-SCCM Collective Weaning Task Force: Evidence-based guidelines for weaning and discontinuing ventilatory support. I: Guidelines. Chest. 2001;120(6 suppl):375s-395s. |
Patients should be observed closely during an SBT in order to assure safety and to provide encouragement and reassurance when needed. Criteria for judging success or failure of the trial should be the respiratory pattern, the adequacy of gas exchange, hemodynamic stability, and patient comfort. A sustained respiratory rate more than about 30-35 breaths/min in a patient recovering from acute respiratory failure usually (but not always) means that he or she is not yet ready to breathe without the ventilator. Because acid-base status as well as oxygenation needs to be assessed during an SBT, arterial blood gases should be drawn (in addition to monitoring with pulse oximetry), either at the conclusion of the trial if it is tolerated well or if the patient appears to fail the trial and ventilatory support is to be reinstituted after more than a few minutes. Although most patients can adequately demonstrate the ability to breathe without the ventilator in 30 minutes, extending the SBT to as long as 2 hours may be justifiable if they are considered marginal weaning candidates.
Manifestations of inability to breathe spontaneously include a change in mental status (obtundation, agitation, anxiety), the onset or worsening of respiratory distress or other discomfort, diaphoresis, or such signs of excessive breathing work as accessory muscle use or paradoxical abdominal motion. Patients whose SBT proves unsuccessful should be investigated to determine the cause, and here is where the traditional "weaning parameters" may be useful. Rapid shallow breathing suggests that the work of breathing exceeds the patient’s ability, and measurement of maximum inspiratory force may reveal ventilatory muscle weakness. A minute ventilation requirement substantially exceeding 10 L/min should prompt a consideration of whether the cause is excessive metabolic demand or increased dead space ventilation, and a search for ways to modify these things.
The guidelines call for daily SBTs in patients who have failed a weaning attempt, once the putative causes have been identified and are being treated or monitored. Between SBTs, the patient should be rested, not worked to the point of fatigue. To quote the guidelines, "patients. . .who fail an SBT should receive a stable, nonfatiguing, comfortable form of ventilatory support." In its simplest form, this means full ventilatory support with volume assist-control, which is what I personally prefer to use. Clinicians who do not wish to use something so old-fashioned may choose IMV at a mandatory rate sufficient to eliminate most or all patient efforts, or pressure support with sufficient inspiratory pressure to make the patient comfortable with a respiratory rate less than 20-25 breaths/min. There is no good clinical evidence to recommend any one of these approaches over any other. Whether the new dual-control modes (volume support, volume-assured pressure support, etc), promoted by ventilator manufacturers for facilitating weaning, offer any real advantage over older modes is entirely unknown at this point, and I do not use them.
Weaning and extubation are 2 separate things. Some patients can be weaned from ventilatory support but still require an artificial airway for secretion clearance or to prevent upper airway obstruction or aspiration. The guidelines urge that these factors be considered and that all patients not simply be extubated once a successful SBT has been performed.
Recommendation No. 7 of the guidelines states that "Anesthesia/sedation strategies and ventilator management aimed at early extubation should be used in postsurgical patients." This recommendation is based on Grade A (highest-level) clinical evidence. Studies have shown that concerted attempts to minimize sedation and anesthesia recovery pay off in shorter ventilation times and less cost. "Closed-loop" ventilation modes that seek to adapt to patients’changing needs (such as mandatory minute ventilation or adaptive support ventilation) might be advantageous here, but this has not been demonstrated.
Why Your Unit Needs a Weaning Protocol
It has now been convincingly demonstrated that non-physician healthcare providers such as respiratory therapists and nurses can successfully execute protocols that enhance clinical outcomes and reduce costs for critically ill patients.14,15 Having those members of the ICU team who are actually at the patient’s bedside manage the weaning process on an hour-by-hour basis, rather than relying on intermittent (and often infrequent) physician decision-making, makes a great deal of sense. And it works. Studies have consistently shown that weaning protocols ordered by physicians but carried out by therapists or nurses shorten weaning time without adverse effects.16-18 Various attitudinal and other barriers need to be overcome,19,20 but weaning protocols are now much more than just one available option—they are the way to go.
Which Patients Should Undergo Tracheotomy, and When?
Although a detailed discussion is beyond the scope of this essay, the new guidelines address this question as well (see Table 3, below). Tracheotomy facilitates weaning for many patients (see Critical Care Alert 2002;9[11]:121-123), and can decrease morbidity and discomfort for patients requiring prolonged ventilatory support.
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Table 3 |
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Circumstances in which Tracheotomy Should Be Considered in Patients Requiring Mechanical Ventilation, if Weaning and Extubation Cannot be Achieved Following Initial Evaluation |
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1. Permanent neurological injury or irreversible neuromuscular disease precluding the recovery of spontaneous breathing capability 2. Requirement for high levels of sedation to tolerate translaryngeal intubation 3. Tachypnea and other evidence of marginally inadequate ventilatory mechanics in patients for whom a reduction in airway resistance might reduce the risk of muscle overload 4. Likelihood of substantial psychological benefit to the patient from being able to eat, articulate speech, and achieve greater mobility 5. Facilitation of physical therapy through increased physical mobility Source: ACCP-AARC-SCCM Collective Weaning Task Force: Evidence-based guidelines for weaning and discontinuing ventilatory support. I: Guidelines. Chest. 2001;120(6 suppl):375s-395s. |
Special Considerations for the Patient Who Cannot be Weaned
Although they are only a minority of all ventilated patients, patients who cannot be weaned despite multiple attempts generate a disproportionate amount of concern and expense in the ICU. The new weaning guidelines emphasize that patients should not be considered "unweanable" until repeated, concerted attempts have been unsuccessful for 3 months. Weaning the long-term, marginal patient is a whole separate art and science from what applies to the majority of ICU patients, and special expertise is required.21 Fifteen years ago, patients in American ICUs who failed multiple weaning attempts were in a nearly impossible situation, as there were almost no options for them other than remaining in the ICU of an acute-care hospital, sometimes for months or even years. Now, however, institutions and expertise are available in the community for managing such patients. Although the evidence is not yet at the highest level, outcomes for such patients may be improved if they are transferred to special weaning and long-term ventilation units. It is incumbent upon intensivists to be aware of such resources in the community and to facilitate transfers when appropriate (see Table 4, below).
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Table 4 |
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Recommendations for ICU Patients Requiring Prolonged Mechanical Ventilation Who Have Failed Multiple Weaning Attempts* |
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1. Except in the case of irreversible neurological or neuromuscular conditions, patients should not be considered permanently ventilator-dependent until weaning efforts have continued for 3 months. 2. Clinicians caring for patients requiring prolonged mechanical ventilation should be aware of resources in the community for managing such patients and consider transfer when appropriate. 3. Weaning strategies for patients requiring prolonged mechanical ventilation are different from those for short-term ventilatory support and should be slow-paced with gradually lengthening self-breathing trials. Source: ACCP-AARC-SCCM Collective Weaning Task Force: Evidence-based guidelines for weaning and discontinuing ventilatory support. I: Guidelines. Chest. 2001;120(6 suppl):375s-395s. |
References
1. ACCP-AARC-SCCM Collective Weaning Task Force: Evidence-based guidelines for weaning and discontinuing ventilatory support. I: Guidelines. Chest. 2001;120(6 suppl):375s-395s.
2. Criteria for weaning from mechanical ventilation. Evidence Report/Technology Assessment No. 23 from the Agency for Healthcare Research and Quality: AHRQ Publication No. 01-E010.
3. MacIntyre NR, et al. Evidence-based guidelines for weaning and discontinuation of ventilatory support. Chest. 2001;120(6 suppl):375s-484s.
4. Evidence-based guidelines for weaning and discontinuing ventilatory support. Respir Care. 2002;47(1):69-90.
5. Esteban A, et al. Modes of mechanical ventilation and weaning: A national survey of Spanish hospitals; The Spanish Lung Failure Collaborative Group. Chest. 1994;106:1188-1193.
6. Cook DJ, et al. Ventilator-associated pneumonia: Incidence and risk factors. Ann Intern Med. 1998;129:433-440.
7. Pierson DJ. Complications associated with mechanical ventilation. Crit Care Clin. 1990;6(3):711-724.
8. Stauffer JL. Complications of endotracheal intubation and tracheotomy. Respir Care. 1999;44(7):828-843.
9. Sahn SA, Lakshminarayan S. Bedside criteria for discontinuation of mechanical ventilation. Chest. 1973; 63:1002-1005.
10. Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991;324:1145-1150.
11. Meade M, et al. Predicting success in weaning from mechanical ventilation. Chest. 2001;120(6 suppl):400s-424s.
12. Ely EW, et al. Effect on the duration of mechanical ventilation of identifying patients capable of breathing spontaneously. N Engl J Med. 1996;335:1864-1869.
13. Pierson DJ. Indications for mechanical ventilation in adults with acute respiratory failure. Respir Care. 2002;47(3):249-265.
14. Ely EW, et al. Mechanical ventilator weaning protocols driven by nonphysician health-care professionals: Evidence-based clinical practice guidelines. Chest. 2001;120(6 Suppl):454S-63S.
15. Stoller JK. Are respiratory therapists effective? Assessing the evidence. Respir Care. 2001;46(1):56-66.
16. Meade MO, et al. Weaning from mechanical ventilation: The evidence from clinical research. Respir Care. 2001;46(12):1408-1415.
17. Ely EW. The utility of weaning protocols to expedite liberation from mechanical ventilation. Respir Care Clin N Am. 2000;6(2):303-319,vi.
18. Keenan SP. Weaning protocols: Here to stay. Lancet. 2002;359(9302):186-187.
19. Ely EW, et al. Large-scale implementation of a respiratory therapist-driven protocol for ventilator weaning. Am J Respir Crit Care Med. 1999;159(2):439-446.
20. Ely EW. Challenges encountered in changing physicians’practice styles: the ventilator weaning experience. Intensive Care Med. 1998;24(6):539-541.
21. Scheinhorn DJ, et al. Outcomes in post-ICU mechanical ventilation: A therapist-implemented weaning protocol. Chest. 2001;119(1):236-242.
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