May 022014
Diaphragmatic Ultrasonography to Assess Readiness for Extubation

By Muhammad Adrish, MD

Weaning a patient from mechanical ventilation is a challenge that intensivists face routinely. Clinical examination and objective measurements like minute ventilation, respiratory rate, maximal inspiratory pressure, and ratio of respiratory rate to tidal volume have all been used with varying reported sensitivity and specificity for extubation success. However, none of these parameters have shown a clear independent ability to identify patients with adequate respiratory pump function -- i.e., ready for extubation -- with certainty.

Waiting too long to extubate poses risks of ventilator acquired pneumonia and neuromuscular complications; extubating patients who aren't ready exposes them to the risk of aspiration or emergent reintubation. Under ideal circumstances, up to 15-20% patients believed ready nonetheless fail extubation and require re-intubation. Re-intubation is associated with prolonged ICU stay, need for transfer to a long-term care facility as well as independently associated with mortality in up to 10-40% of these patients (although it's not clear that extubating too early, as opposed to underlying illness, causes these complications). There remains a strong need for more accurate tools to predict successful weaning from mechanical ventilation.

DiNino and colleagues addressed this dilemma by using direct ultrasonographic guided measurement of diaphragmatic function. In a study of 63 patients at two centers, identified by treating intensivists as ready to undergo a pressure support or spontaneous breathing trial, diaphragmatic thickness was measured at its apposition with the rib cage, using a linear ultrasound probe at both end inspiration and end expiration. This information was then used to calculate percent change in diaphragmatic thickness taken over 3-5 breaths. The diaphragm should increase in thickness during inspiration; in healthy spontaneously breathing patients, it may increase from 0.2 mm at rest to 1.4 mm.

Authors also calculated the rapid shallow breathing index during ultrasonographic evaluation for comparison. Patients who remained extubated for more than 48 hours were defined as successfully extubated. The primary intensivists' teams were unaware of the ultrasound results and made their own clinical judgment to decide timing of extubation.

A change in diaphragmatic thickness of > 30% had a positive predictive value of 91% at predicting successful extubation (43 of 49 successfully extubated patients had >30% thickness change). Conversely, of the other 14 patients who failed extubation, 10 had a percent change in diaphragm thickness of <30% resulting a negative predictive value of 63%. Change in diaphragmatic thickness predicted successful weaning with equal success for both pressure support and spontaneous breathing trial groups: sensitivity was 88% and specificity 71%. In 4 patients who failed extubation despite having a >30% change in diaphragmatic thickness, factors including CHF, altered mentation and mucus plugging led to re-intubation.

Ultrasonography has evolved into a versatile clinical tool for intensivists. In addition to valuable information regarding patients' fluid status, etiology of shock and respiratory failure, and evaluation for cardiac function, it is also used extensively in routine performance of many ICU procedures. Nonetheless, there is a learning curve to be able to perform these studies effectively requiring time and training (which may be costly). Of course, no single parameter (including change in diaphragmatic thickness) can independently predict weaning outcome. However, in this study, measurement of percent change in diaphragm thickness predicted weaning outcomes significantly better than rapid shallow breathing index, a tool frequently used in current decision-making. As authors note, another prospective trial testing diaphragmatic thickness change with inspiration immediately before extubation would be advisable to further test this diagnostic test. (Here, they tested many patients on the day before extubation.)

takeawayClinical Takeaway: In the hands of experienced operators, ultrasonographic assessment of diaphragmatic function might provide additional predictive value for assessment of the respiratory pump and could help prevent extubation failures. Further validation of this small, nonrandomized study is needed.

E. DiNino et al. Diaphragm ultrasound as a predictor of successful extubation from mechanical ventilation. Thorax 2014 May;69(5):431-5.

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Diaphragmatic ultrasound could predict extubation success