Highlighting Critical Care

Discontinuation of mechanical ventilation

Melissa F. Perkal, M.D., F.A.C.S. Assistant Professor, Director, Veterans Affairs Hospital Surgical Intensive Care Unit, and Director, Surgical Preceptor Program, New Haven, USA http://yalesurgery.med.yale.edu/

The article also available in PDF: 133KB

Published by permission of GASNet Inc. © 2004-5

Scope of problem

Discontinuation from mechanical ventilation is often the singular critical event hallmarking progression to recovery in the intensive care unit. Prolonged and unnecessary delays in tracheal extubation result in increased complication rates for pa-tients receiving mechanical ventilation, including: pneumonia, airway trauma, death, and increased hospital costs [1]. It has been estimated that as much as 42% of the time that a patient spends dependent on mechanical ventilation is during the discontinuation (or weaning) process [2]. However, premature discontinuation carries its own set of risks to the patient includ-ing difficulty in reestablishing an artificial airway, compromised gas exchange, an eight-fold higher odds ratio for nosocomial pneumonia [3] and a 6 to 12-fold increased mortality risk [4]. Reported reintubation rates range from 2 to 25% for different ICU populations [5,6].

Timing extubation

Discontinuation from mechanical ventilation is usually started only after the underlying disease process that necessitated mechanical ventilation has significantly improved or is resolved. A simple daily screening can identify patients who may be considered for successful tracheal extubation. Clinical assessment is based on adequacy of gas exchange, hemodynamic stability and appropriate neurological and muscular status. A simple daily assessment screen can identify patients who may be considered for successful tracheal extubation. This includes:

• Adequate gas exchange (e.g. PaO2/FiO2 > 150 to 200; level of positive end expiratory pressure (PEEP) < 5 to 8 cm H2O; FiO2 < 0.4 to 0.5; and pH > 7.25);
• Hemodynamic stability as defined by the absence of active myocardial ischemia and the absence of clinically significant hypotension (i.e., a condition requiring no vasopressor therapy or therapy with only low-dose vasopressors such as dopamine or dobutamine, < 5 mcg/kg/min); and
• Appropriate neurological and muscular status allowing the capability to initiate an inspiratory effort [7].

Daily screening can reduce the number of patients receiving mechanical ventilation for more than 21 days and has been associated with reduced in-hospital mortality [8].

Weaning parameters

In the last decade, there have been more clinical trials involved in ventilator discontinuation than in almost any other critical care field. Although there has been much written about the assessment for extubation readiness, the studies are highly variable and difficult to compare.

Therefore, in 1999, two large comprehensive reviews of world literature were undertaken and used to produce a set of evidence-based clinical practice guidelines for managing ventilator-dependent patients during the discontinuation process [7,9]. This meta-analysis found evidence in the literature supporting a possible role for 66 specific measurements as predictors [10]. From these, the report identified eight parameters that were consistently better at predicting success. Some are simple bedside spirometric measurements of lung function, while others combine multiple parameters (Table 1).

All of the above weaning criteria had a very high sensitivity (0.78-1.00) but a very low specificity (0.11-0.36) [11]. Weaning parameters with a low specificity, will result in the prevention of weaning in that percentage of patients who breathe independently. Therefore the report concluded that the clinical applicability of using these parameters alone for an individual patient was poor.

** Used primarily for pediatric patients

Spontaneous breathing trials

It appears that the best method to assess whether the patient is able to breathe on his or her own is to perform a trial of spontaneous ventilation. Multiple studies [12-16] have demonstrated that 60-80% of mechanically ventilated patients can be successfully extubated after passing a spontaneous breathing trial (SBT). The best technique for performing spontaneous breathing trials is not yet clear.

SBT comparing continuous positive airway pressure of 5 cm H2O to a T-piece for one hour, revealed no difference in the percentage of patients failing extubation [17]. In another study, the Spanish Lung Failure Collaborative Group compared weaning outcome after trials of spontaneous breathing with either T-tube or pressure sup-port of 7 cm H2O [12]. No difference was observed in the percentage of patients who remained extubated for 48 hours. There were 63% of them in the T-tube and 70% in the pressure support group.

Most studies set the duration of SBT at two hours. The Spanish Lung Failure Collaborative Group enrolled 526 patients to a randomized, prospective trial comparing SBT trials of 30 minutes with trials of 120 minutes. They found that in patients who tolerated the SBT, there was no difference in the rate of successful extubation (84% vs. 89%) between the two study groups. [13].

In reviewing the literature it appears that sometimes the criteria for SBT "tolerance" are not precisely defined, but the clinical judgment of the physician may have an impact. Rather, a combination of objective and subjective measurements indicating intolerance or failure has been advocated in several large trials (Table 2).

Removal of the artificial airway

Once it has been determined that a patient can breathe on his or her own, it must be decided if the artificial airway can be removed. Extubation failure can occur for reasons different than those for weaning failure. The upper airway must be patent and the patient must be able to protect the airway and clear secretions. The risk of upper airway obstruction increases with the duration of mechanical ventilation, female gender, trauma, and repeated or traumatic intubation [18].

Brain dysfunction can contribute to extubation failure, and depressed mental status has been shown to be a variable predictor of extubation outcome. While there are studies that show successful extubation in comatose patients [19], most practitioners would agree that the patient should have some capability to interact with the care team prior to removing the artifi-cial airway.

Assessment of airway patency before tube removal can be challenging. The absence of an audible air leak after deflation of the endotracheal balloon (qualitative cuff leak test) has been associated with an increased risk of post-extubation stridor, but the test is very subjective [20]. Another method, a quantitative cuff leak test has also been described which measures the difference between inspiratory and expiratory volume after cuff deflation.

Dividing the expiratory volume by the inspiratory volume and multiplying by 100, gives a "cuff leak percentage". Values of 12 to 16% have been shown to be predictive of extubation failure [21]. In assist-control ventilation, a cuff leak of less than 110 ml between inspiratory volume and expiratory volume has been shown to predict the development of postextubation stridor [22].

Many of the patients who develop post-extubation stridor can be treated with steroids and/or epinephrine. Other possibilities are non-invasive ventilation or use of helium in oxygen (Heliox) as a gas mixture. Typically, less than 50% of them require reintubation [6].

The capacity to protect the airway, and to expel secretions with an effective cough is vital for extubation success. Airway assessments should take into account the ability to cough during suction of the airway, the absence of excessive secretions, and the frequency of suction needed (ideally, suctioning less than every two hours) [23].

Peak cough flows of over 160 L/min, or the capability of propelling secretions onto a white index card held 1 to 2 cm from the end of the endotracheal tube may also predict extubation success. These methods can be useful especially in patients with neuromuscular weakness or spinal cord injuries [23].

Patients with poor cough and moderate to abundant secretions have been shown to have a high likelihood of unsuccessful extubation, despite successfully completing a SBT [23]. Similarly, patients with mild secretions and a good cough are likely to have successful tracheal extubation [23].

The process of weaning from mechanical ventilatory support and subsequent tracheal extubation will consume a significant portion of the patient's stay in the ICU. Although many variables for successful outcomes have been identified, specific and predictable criteria are not clearly defined. Guidelines combining objective and subjective endpoints offer strategies and direction for improved outcomes.

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Last updated: 21 March 2005Created
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