The evolution of diaphragm activity and function determined by ultrasound during spontaneous breathing trials

Rapid shallow breathing index (RSBI) is a commonly used index for predicting the outcome of spontaneous breathing trial (SBT). Ultrasound is a non-invasive technique for assessing diaphragm activity and function. This study aimed to investigate changes in diaphragm activity during SBT, and to compare diaphragm function between patients with and without SBT success.

Forty-five patients undergoing SBT were enrolled. Thickening fraction of the diaphragm was assessed during tidal breathing (TFdi_tidal), and RSBI was measured during 30 min of SBT. Diaphragm function measured by maximum TFdi (TFdi_max) and diaphragmatic excursion (DE_max) was also evaluated.

TFdi_tidal and RSBI significantly increased during SBT (TFdi_tidal0vs. TFdi_tidal30 = 29.8 ± 13.8 vs. 37.4 ± 13.0%; p < .001, and RSBI₀vs. RSBI₃₀ = 64.8 ± 25.9 vs.70.8 ± 29.1 breaths/min/L; p = .034). In SBT failure (n = 13), there was no significant difference in TFdi_tidal compared to SBT success, except at the beginning of the trial (p = .043); however, RSBI significantly increased throughout SBT. No differences in TFdi_max or DE_max were observed between groups.

Patient inspiratory efforts significantly increased during SBT. TFdi_tidal measured by diaphragm ultrasound could not distinguish between patients with SBT success and failure. RSBI was significantly higher during SBT in patients with SBT failure.