Predicting extubation outcome is of significant clinical importance since both extubation delay and extubation failure are associated with increased patient morbidity and mortality (12
). Extubation criteria that accurately discriminate between those patients who will successfully extubate and those who will fail extubation may help modify clinical determinations of extubation readiness (33
). Despite its prevalent use, the accuracy of the ALT in predicting post-extubation upper airway compromise such as stridor, upper airway obstruction, or the need for reintubation is debated (8
) This study was the first prospective evaluation of the ALT as a predictor of extubation outcome in mechanically ventilated PICU patients.
Two important findings emerge from our study. First, our hypothesis that PICU patients may be intubated with larger than recommended ETT size was false. The absence of an ETT air leak prior to extubation in these study patients was not related to the placement of larger-sized ETTs. Second, the pre-extubation ALT is not an accurate discriminator of patients likely to extubate successfully or likely to have significant post-extubation airway compromise and, therefore, adds little data to modify clinical decision-making regarding extubation readiness in the mechanically ventilated PICU patient.
The predictive capacity of the ALT may depend on the patient cohort studied and the definition of extubation failure. For intubated patients with known upper airway pathology, including intubation after upper airway surgical reconstruction, burns, or laryngotracheobronchitis, the ALT may be an accurate predictor of extubation outcome (8
). Small, single-center retrospective studies measuring the ALT in pharmacologically aralyzed patients found that an ALT < 20 cm H2
O prior to extubation was associated with a 100% extubation success rate while an ALT > 30 H2
O was 100% predictive of post-extubation stridor and/or reintubation (8
). The authors concluded that the ALT should determine extubation timing, recommending extubation be delayed in patients with an ALT > 20 cm H2
O until airway swelling decreased. However, for the patient without known upper airway disease or the non-pharmacologically paralyzed patient, such as most intubated PICU patients, only one recent retrospective study and this prospective study have evaluated the predictive capacity of the ALT. Mhanna et al
performed a retrospective review of 105 PICU patients who had an ALT performed prior to extubation (10
). Post-extubation stridor occurred in 42 patients and four patients were reintubated within 48 hours of extubation. The authors found the ALT to be a more sensitive predictor of post-extubation stridor in older patients (≥ 7 yrs) than in younger patients (<7 years). In this study, we found an air leak ≥ 30 cm H2
O was no more predictive of extubation outcome in patients ≥ 7 years of age (NPV 0%) than in patients <7 years of age (NPV 20%).
The value of the ALT measurement will vary considerably based upon testing conditions. For a given patient, the ALT measurement will vary if midline head positioning is not maintained, if the patient is not pharmacologically paralyzed, and if testing is performed by more than one observer (9
). Finholt et al provided the original description of ALT measurements performed in the setting of complete pharmacologic paralysis with the patient's head supine and midline (17
). When these conditions were not maintained, the air leak pressures required to produce an audible air leak were generally higher and more interobserver variability was noted. Finholt et al noted, the air leak pressure increased progressively from 16.9 ± 1.3 cm H2
O with complete neuromuscular blockade to an average of 30.6 ± 1.4 cm H2
O following full recovery from neuromuscular blockade (17
). In a similar study on non-paralyzed patients, Schwartz et al found an average variance of 38% at both high and low air leak pressure measurements between two trained observers (36
). In this study cohort of non-paralyzed patients, we found 28 of 59 (47.4%) patients had an ALT ≥ 30 cm H2
O prior to extubation. Despite an absent air leak, 23 of 28 patients (82.1%) successfully extubated. We did not administer a neuromuscular blocking agent to patients in order to obtain the ALT, therefore higher ALT pressures may be associated with the lack of complete muscle paralysis (16
). This may explain why higher ALT pressures in this cohort did not correlate with a greater risk for post-extubation stridor, airway compromise or the need for reintubation.
No single test is likely to predict extubation outcome for an individual patient with absolute certainty. However, a useful predictive tool must be able to accurately discriminate between patients who will extubate successfully and those who will require reintubation (41
). The discriminatory power of a diagnostic test is expressed in terms of its sensitivity, specificity, PPV, and NPV. We found the ALT did not accurately predict extubation outcome. Low air leak pressures (ALT < 20 cm H2
O) may be a reassuring pre-extubation result (PPV 88.3%), but high air leak pressures (ALT ≥ 30 cm H2
O) do not predict extubation failure (NPV 12.9%). Likelihood ratios (LR) are calculated to express how significantly a predictive tool may modify clinical judgment (43
). LR > 10 or < 0.1 indicate predictive tools which significantly modify clinical assessment. Experienced clinicians estimate the probability of extubation success or failure for each patient (i.e., pre-test probability) using an assimilation of laboratory and radiologic data plus a subjective interpretation of the patients' ability to resume effective gas exchange and airway control once mechanical ventilation is discontinued. The LR for the predictive tool adds objective data to change the direction and magnitude of the pre- to the post-test probability of extubation outcome. We found the LR+ and LR− for all ALT results were between 0.8 and 1.4 indicating the ALT does not enhance clinical judgment of experienced providers to determine extubation readiness. Therefore, the ALT measured prior to extubation cannot be used as the sole criterion to determine extubation timing. Instead, the clinician must weigh clinical determination of extubation readiness along with an objective assessment of the likelihood for upper airway compromise (known upper airway disease or surgical condition or acquired airway edema) to determine the optimal extubation management and timing.
The ideal extubation failure rate for the PICU population is unknown. Thus, extubation timing is often a balance between the unknown risks of reintubation vs. prolonging the length of mechanical ventilation. The extubation failure rate in our cohort was 15.3%. Previous authors have reported similar rates of reintubation, ranging from 2.7%-22%, when extubation readiness is determined by clinical judgment of experienced clinicians without standardized assessments of spontaneous breathing capacity (2
). The extubation failure rate among PICU patients is much higher when the patient has had prior airway surgery (29%) or a longer length of ventilation (failure rate of 8% if LOV > 48 hrs and of 17.5% if LOV > 10 days) (4
). The extubation failure rate in our cohort may reflect the relative prevalence of these risk factors associated with extubation failure risk in our study population.
A limitation of this study is the small sample size which may have contributed to the inability to find a true association between the ALT and extubation outcome (type II error). A sample size calculation using a two-sample proportion (assuming α=0.05 and β=0.2) indicated that 141 patients in each group were needed to detect a 10% reduction in post-extubation airway compromise (given an adverse event rate of 5% in patients with an ALT <30 cm H2O and of 15% in patients with an air leak ALT ≥ 30 cm H2O). Thus, a larger study would be required to thoroughly assess the ALT in the context of other systematic evaluations of extubation readiness. However, the prospective, blinded study design maximized our potential to capture the population at risk for extubation failure and to accurately record patient, intubation, and mechanical ventilation characteristics to characterize the mechanically ventilated PICU population.
It is important to state the ALT when performed at intubation
still remains a valuable tool to select an appropriately sized ETT, to monitor ETT cuff inflation pressure, and to serve as an indicator of the potential for tracheal wall injury. When measured at the time of intubation, an ALT ≥ 30 cm H2
O has been associated with compromised mucosal capillary blood flow and a higher incidence of postextubation adverse events (24
). Thus, some pediatric institutions may elect to maintain the patient's mean arterial pressure higher than the air leak pressure, to reintubate these patients with a smaller ETT size, or to place a cuffed ETT and adjust the ETT cuff to maintain a suitable leak pressure (24
). Unfortunately, reintubations and multiple intubation attempts to place a correctly sized ETT can also lead to tracheal injury and a greater risk for postextubation compromise (14
). The benefits of a cuffed ETT for children undergoing general anesthesia has been documented, but has not yet been validated for the PICU patient with a longer length of ventilation (24
). We measured the ALT within 12 hrs of intubation which may have allowed time for post-intubation airway edema to occur and thus may account for the high prevalence of ALT ≥ 30 cm H2
O in our study cohort despite intubation with an appropriately sized ETT.
In the mechanically ventilated, critically ill pediatric patient it is likely that no single criterion can predict extubation failure. Extubation failure in the PICU population may be multifactorial and not isolated to a single etiology such as upper airway edema. An endotracheal tube air leak ≥ 30 cm H2O was common in this population prior to extubation and was not associated with a greater likelihood for post-extubation stridor, airway compromise, or need for reintubation. When the pre-extubation ALT is measured in the non-pharmacologically paralyzed patient, ALT ≥ 30 cm H2O may reflect a recovery of laryngeal and hypopharyngeal muscle tone rather than the presence of laryngotracheal edema. Therefore, the pre-extubation ALT should not be used as a sole criterion of extubation timing. Measurement of the ALT at intubation remains an important tool to select an appropriately sized ETT and ETT cuff inflation pressure which minimize the potential risk for tracheal wall injury. Further research is needed to determine accurate predictors of extubation failure in mechanically ventilated pediatric intensive care unit patients.