Diaphragm ultrasound as a new index of discontinuation from mechanical ventilation

Abstract

Background: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

Keywords: Diaphragm; Ultrasonography; Weaning.

Figures

Figure 1

Diaphragm thickness at TLC and RV in patients who failed and succeeded the spontaneous breathing trial. Boxplot of diaphragm thickness: the central line represents the median value, the box boundaries represent the 25th and 75th percentiles, the whiskers represent the lowest datum within 1.5 inter-quartile range (IQR) of the lower quartile and the highest datum within 1.5 IQR of the upper quartile, and the circles represent outlier values. Diaphragm thickness at total lung capacity (TLC) and residual volume (RV) for patients who failed the spontaneous breathing trial: 0.30 [0.20 to 0.40] and 0.24 [0.17 to 0.30], p < 0.09. Diaphragm thickness at TLC and RV for patients who succeeded the spontaneous breathing trial: 0.38 [0.29 to 0.45] and 0.25 [0.19 to 0.28], p < 0.0001.

Figure 2

Diaphragm thickening fraction (DTF) in patients who failed and succeeded the spontaneous breathing trial. Boxplot of diaphragm thickening fraction: the central line represents the median value, the box boundaries represent the 25th and 75th percentiles, and the whiskers represent the lowest datum within 1.5 IQR of the lower quartile and the highest datum within 1.5 IQR of the upper quartile. DTF was 0.26 [0.22 to 0.30] and 0.56 [0.38 to 0.64], respectively (p < 0.0001).

Figure 3

Receiver operating characteristic curve for the diaphragm thickening fraction (DTF). AUC 0.948 (95% CI 0.89 to 1.00).

Figure 4

Correlation between diaphragm thickening fraction (DTF) and maximum inspiratory pressure (PImax). The circles represent measure of PImax and DTF for each single patient; rho = 0.75, p < 0.001.