Diaphragm ultrasound as a new index of discontinuation from mechanical ventilation

Predictive indexes of weaning from mechanical ventilation are often inaccurate. Among the many indexes used in clinical practice, the rapid shallow breathing index is one of the most accurate. We evaluated a new weaning index consisting in the diaphragm thickening fraction (DTF) assessed by ultrasound.

Forty-six patients were prospectively enrolled. All patients were ventilated in pressure support through a tracheostomy tube. Patients underwent a spontaneous breathing trial (SBT) when they met all the following criteria: FiO₂ < 0.5, PEEP ≤5 cmH₂O, PaO₂/FiO₂ > 200, respiratory rate <30 breaths per minute, absence of fever, alert and cooperative, and hemodynamic stability without vaso-active therapy support. During the trial, the right hemi-diaphragm was visualized in the zone of apposition using a 10-MHz linear ultrasound probe. The patient was then instructed to perform breathing to total lung capacity (TLC) and then exhaling to residual volume (RV). Diaphragm thickness was recorded at TLC and RV, and the DTF was calculated as percentage from the following formula: Thickness at end inspiration - Thickness at end expiration / Thickness at end expiration. Also, the rapid shallow breathing index (RSBI) was calculated. Weaning failure was defined as the inability to maintain spontaneous breathing for at least 48 h, without any form of ventilatory support.

A significant difference between diaphragm thickness at TLC and RV was observed both in patients who succeeded SBT and patients who failed. DTF was significantly different between patients who failed and patients who succeeded SBT. A cutoff value of a DTF >36% was associated with a successful SBT with a sensitivity of 0.82, a specificity of 0.88, a positive predictive value (PPV) of 0.92, and a negative predictive value (NPV) of 0.75. By comparison, RSBI <105 had a sensitivity of 0.93, a specificity of 0.88, a PPV of 0.93, and a NPV of 0.88 for determining SBT success.

This study shows that in our cohort of patients, the assessment of DTF by diaphragm ultrasound may perform similarly to other weaning indexes. If validated by other studies, this method may be used in clinical practice.