Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

Highlighted lines and questions below provide links to the pertinent description of criteria in The EBM User's Guide, now available at the Canadian Centres for Health Evidence

Article Reviewed:

Please visit the new Evidence Based Journal Club Reviews

The cuff leak test to predict failure of tracheal extubation for laryngeal edema.

De Bast Y, De Backer D, Moraine JJ et al.

Intensive Care Med 2002; 28: 1267-1272. [abstract]

Reviewed by Angela T. Wratney MD, Daniel Kelly Benjamin Jr. MD MPH PhD, and Ira M. Cheifetz MD FAARC FCCM, Duke University, Durham, NC

Review posted August 2, 2004

I. What is being studied?

Study objective:

Evaluate the usefulness of a quantitative assessment of air leak around the endotracheal tube as a screening tool for patients at risk for early reintubation secondary to laryngeal edema.

Study design

Prospective, cohort study.

Patient Population:

Adult patients extubated in a general medical-surgical intensive care unit.

Methods:

Patients were ventilated in a volume, assist-controlled mode. Expiratory tidal volumes (ETV) with the cuff inflated (ETV BI) and then deflated (ETV BD) were recorded. ETV data were recorded from display cells on the ventilators (Siemens SV300 and 900; Drager Evita 2 and Evita 4). Six measurements of ETV BI and ETV BD, varying < 30%, were obtained and averaged. The leak was calculated as follows: 100 x (ETV BI - ETV BD)/ ETV BI.

Outcomes assessed:

1. laryngeal edema requiring re-intubation
2. signs of respiratory distress: supraclavicular, suprasternal, and subcostal retractions or stridor

II. Are the results of the study valid?

Note: These questions follow from Randolph AG et al. Understanding articles describing clinical prediction tools. Crit Care Med 1998;26:1603-1612. [abstract]

1. Was a representative group of patients completely followed up? Was follow-up sufficiently long and complete?

Was a representative group of patients studied?: Maybe. Adult patients, > 18 years of age, with an endotracheal tube for more than 12 hours undergoing their first extubation attempt were eligible for enrollment. Prior to extubation, patients successfully completed a 30-minute T-piece trial. Enrollment occurred during a 3-month period (Feb-April, 1999). Due to the short study period, there is a concern that certain seasonal illnesses (e.g., influenza) were missed resulting in a sample that is not truly representative of the general medical-surgical population.

The sample size was small: 76/77 eligible patients were enrolled, 1 denied consent. Of the 76 subjects enrolled, 38 subjects were intubated for elective surgery, 8 subjects for emergency surgery and 30 subjects were intubated for medical etiologies. The authors presented sparse baseline characteristics of the study population (age, SAPS II score, and diagnosis), thus, making it difficult to determine how representative this population may be of other medical-surgical ICUs.

Post-extubation stridor is uncommon in the adult patient, and only rarely does it require treatment (1-4). Of 12 participants who were re-intubated, 8 subjects were re-intubated for laryngeal edema.

Was complete follow-up achieved?: Yes. All subjects were followed for the development of stridor and re-intubation for 24 hours. This length of follow up was appropriate because subjects failing extubation due to airway obstruction will generally fail within the first 24 hours and many will fail within 12 hours (5-7).

2. Were all potential predictors included?

No. The only variable they truly assessed for its predictive value was the leak test. Other variables recorded were: endotracheal tube size, duration of intubation, indication for intubation, PEEP, tidal volume and SAPS II score. The authors do not indicate how predictors were selected or if other predictors were considered. Potential predictors not considered include: recent exposure to steroids, underlying airway anomalies, previous airway surgery, and history of multiple intubations.

3. Did the investigators test the independent contribution of each predictor variable?

No. A univariable analysis of the predictors was not provided. The authors compared the variables noted above, but only to demonstrate they did not differ between the high and low leak subjects. The reader is not able to evaluate the ability of each independent variable to predict extubation failure. The authors performed subgroup analysis based upon duration of intubation (< or > 48 hrs) and the % cuff leak (demonstrating improved discriminatory capacity of the cuff leak test in those subjects intubated > 48 hrs), but information about the predictive power of either variable when assessed independently is not provided.

4. Were outcome variables clearly and objectively defined?

Yes. One of the strengths of this paper is the clear and objective determination of outcome. The outcome was defined as re-intubation due to laryngeal edema within 24 hrs of extubation. Laryngeal edema was suspected in the presence of inspiratory stridor associated with any sign of respiratory distress requiring re-intubation within 24 hours of extubation. Each case was confirmed by fiberoptic examination before, or by direct view of the glottis during, re-intubation. Fiberoptic examination is the gold-standard method for upper airway evaluation (8-10), but we are not told if the investigators performing this examination were blinded to the results of the cuff leak test. Unblinding could bias outcome determination.

III. What are the results?

1. What is(are) the prediction tool(s)?

Using a receiver operating characteristic (ROC) curve, the authors chose a cuff leak difference of 15.5% as a predictive tool to discriminate among those subjects who will have a successful vs. failed extubation. Using this threshold, they found 6/25 subjects (24%) with a cuff leak test < 15.5% were re-intubated for laryngeal edema. In contrast, only 2/51 (4%) with a cuff leak test > 15.5% were re-intubated for laryngeal edema (p < 0.01).

The authors carefully detail their decision to choose 15.5% as a threshold value based upon their assumption of an equivalent impact of the risk of prolonging unnecessary intubation and the risk of re-intubation for laryngeal edema. A ROC curve was provided (Figure 3). The ROC curve is a plot of the sensitivity (y-axis) vs. 1-specificity (x-axis). The ROC curve may be used by the investigator to choose a value that minimizes false negatives and, thereby, maximizes sensitivity (i.e., minimizes the risk of extubation failure) vs. one that minimizes the false positives (i.e., maximizes specificity and minimizes the risk of prolonged unnecessary intubation).

The authors also reported that none of the 38 patients intubated < 48 hours required reintubation for laryngeal edema.

2. How well does the model categorize patients into different levels of risk?

The cut-off value of 15.5% for the percent cuff leak difference yielded a sensitivity of 75%; specificity of 72.1%; % positive predictive value (PPV) of 25%; and % negative predictive value (NPV) of 96.1%. The percent of correct classification was 72.4%. The area under the ROC curve describes the discriminating power of the test. The authors do not report the area under the ROC curve.

The high NPV indicates the patient with a high leak, > 15.5%, will have a very small risk of post-extubation laryngeal edema requiring reintubation. However, the low PPV indicates a low cuff leak test does not discriminate well those patients who will and will not need reintubation for post-extubation laryngeal edema.

3. How confident are you in the estimates of the risk?

Unknown. The authors do not provide confidence intervals. Given the small samples size of this study, the estimates are likely to be imprecise.

IV. Will the results help me in caring for my patients?

1. Does the tool maintain its prediction power in a new sample of patients?

Not assessed. The authors did not test the validity of the predictive tool in a new population. The cuff leak test has been shown to be effective at predicting extubation outcome in other patient populations (1,2,17,18). The cuff leak test is not predictive in select patient populations intubated < 48 hrs (3).

2. Are your patients similar to those patients used in deriving and validating the tool(s)?

This study evaluated the cuff leak test in adult subjects with cuffed endotracheal tubes. Extrapolating to pediatric intensive care unit populations is difficult since infants and children have a higher prevalence of post-extubation stridor, a higher incidence of extubation failure, and limited use of cuffed endotracheal tubes. Pediatric specific studies are required for three principle reasons:

1. Measuring the cuff leak test in infants and children based on expiratory tidal volumes determined at the expiratory valve of the ventilator overestimates the actual delivered tidal volume (as determined at the endotracheal tube) (12). Thus, the % air leak calculated will depend upon the location where the tidal volume is measured.
2. Post-extubation stridor is more common in the pediatric population than the adult population, and upper airway obstruction is cited as the primary reason for extubation failure in pediatric patients (6,13).
3. Cuffed endotracheal tubes are used in 50% of PICU patients (6); however, there is considerable center variance (6,19).

3. Does the tool improve your clinical decisions?

Yes, in adult patients. Utilizing clinical judgment alone to determine the likelihood for successful extubation, one survey of intensivists reported a sensitivity of only 35% and specificity of 79% (16). Other trials have demonstrated the tendency for clinical judgment alone to prolong unnecessary intubation (14,15). Thus, the tool theoretically improves clinical decision making in adults. The cuff leak test is a non-invasive, low risk, bedside procedure that can be performed on all subjects intubated with a cuffed endotracheal tube. However, the low PPV of this test indicates the cuff leak test should not be used to exclude a trial of extubation in these patients.

4. Are the results useful for reassuring or counseling patients?

No. The cuff leak test has not been studied similarly in the pediatric population. Mhanna and colleagues (6) demonstrated a greater incidence of post-extubation stridor in children > 7 years of age with an airleak at > 20 mm Hg. Without pediatric studies, the cuff leak test methods and threshold value for "low" and "high" leak (< and > 15.5%, respectively) cannot be applied to our patient population in the PICU. Therefore, this information is not useful for counseling our patients. The recently published study of extubation failure in children by Kurachek and colleagues (13) is recommended reading for the pediatric intensivist.

References:

1. Jaber S, Chanques G, Matecki S, et al. Post-extubation stridor in intensive care unit patients. Risk factors evaluation and importance of the cuff-leak test. Intensive Care Medicine 2003; 29(1):69-74. [abstract]
2. Miller RL, Cole RP. Association between reduced cuff leak volume and postextubation stridor. Chest 1996; 110(4):1035-40. [abstract]
3. Engoren M. Evaluation of the cuff-leak test in a cardiac surgery population. Chest 1999; 116(4):1029-31. [abstract]
4. Epstein SK. Decision to extubate. Intensive Care Medicine 2002; 28(5):535-46. [abstract]
5. Epstein SK, Ciubotaru RL. Independent effects of etiology of failure and time to reintubation on outcome for patients failing extubation. American Journal of Respiratory & Critical Care Medicine 1998; 158(2):489-93. [abstract]
6. Mhanna MJ, Zamel YB, Tichy CM, Super DM. The "air leak" test around the endotracheal tube, as a predictor of postextubation stridor, is age dependent in children. Critical Care Medicine 2002; 30(12):2639-43. [abstract]
7. Adderley RJ, Mullins GC. When to extubate the croup patient: the "leak" test. Canadian Journal of Anaesthesia 1987; 34:304-6. [abstract]
8. Vila J, Bosque MD, Garcia M, et al. Endoscopic evolution of laryngeal injuries caused by translaryngeal intubation. European Archives of Oto-Rhino-Laryngology 1997; 254 Suppl 1:S97-100. [abstract]
9. Liebler JM, Markin CJ. Fiberoptic bronchoscopy for diagnosis and treatment. Critical Care Clinics 2000; 16(1):83-100. [abstract]
10. Walker P. Forte V. Failed extubation in the neonatal intensive care unit. Annals of Otology, Rhinology & Laryngology 1993; 102(7):489-95. [abstract]
11. Randolph AG et al. Understanding articles describing clinical prediction tools. Crit Care Med 1998; 26:1603-1612. [abstract]
12. Hubble CL, Gentile MA, Tripp DS, et al. Deadspace to tidal volume ratio predicts successful extubation in infants and children. Critical Care Medicine 2000; 28(6):2034-40. [abstract] [PedsCCM EB Journal Club Review]
13. Kurachek SC, Newth CJ, Quasney MW, et al. Extubation failure in pediatric intensive care: a multiple-center study of risk factors and outcomes. Critical Care Medicine 2003; 31(11):2657-64.[abstract]
14. Esteban A, Frutos F, Tobin MJ, et al. A comparison of four methods of weaning patients from mechanical ventilation: the Spanish Lung Failure Collaborative Group. New England Journal of Medicine 1995; 332:345-350. [abstract]
15. Brochard L, Rauss A, Benito S, et al. Comparison of three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. Am J Respir Crit Care Med 1994; 150:896-903. [abstract]
16. Stroetz RW, Hubmayr RD. Tidal volume maintenance during weaning with pressure support. American Journal of Respiratory and Critical Care Medicine 1995; 152:1034-1040. [abstract]
17. Fisher MM, Raper RF. The 'cuff-leak' test for extubation. Anesthesia 1992; 47:10-12. [abstract]
18. Sandu RS, Pasquale MD, Miller K, Wasser TE. Measurement of endotracheal tube cuff leak to predict postextubation stridor and need for reintubation. Journal of the American College of Surgeons 2000; 190: 682-687. [abstract]
19. Deakers TW, Reynolds GE, Stretton M, et al. Cuffed endotracheal tubes in pediatric intensive care. Journal of Pediatrics 1994; 125: 57-62. [abstract]

 

Comments

Back to the EB Journal Club Index