Diaphragmatic ultrasound and its relationship to breathing effort and load: a prospective observational study

Failure to wean from invasive mechanical ventilation is multifactorial, with diaphragmatic dysfunction a significant contributing factor. Diaphragmatic function can be easily and non-invasively assessed by ultrasound. However, it remains unknown how ultrasound measurements of diaphragm function are affected by changes in apparent work of breathing.

In patients undergoing weaning from mechanical ventilation, we evaluated diaphragmatic ultrasound measurements [diaphragmatic excursion (Dex), diaphragmatic thickening fraction (Tfdi)] simultaneously with manometric indices of breathing effort and load [esophageal pressure swings (ΔPes), transdiaphragmatic pressure swings (ΔPdi), and the pressure-time product of esophageal pressure (PTPes)]. These assessments were performed during two distinct phases; during an unassisted spontaneous breathing trial (phase SBT) and during an inspiratory resistive loading with 30 cmH₂O/L/s (phase IRL), applied during the same SBT. Our primary aim was to evaluate the relationship between diaphragmatic ultrasound and breathing effort using the method of repeated measures correlation.

Forty-nine patients were enrolled. Dex correlated with ΔPes (r = 0.5, p < 0.001), ΔPdi (r = 0.55, p =  < 0.001) and PTPes (r = 0.32, p = 0.031). Tfdi did not correlate with ΔPes (r = 0.27, p = 0.052), ΔPdi (r = 0.2, p = 0.235) and PTPes (r = 0.24, p = 0.110). Dex and Tfdi increased during IRL compared to SBT [1.44(0.89-1.96) vs. 1.05(0.7-1.59), p = 0.002], [0.55(± 0.32) vs 0.46(± 0.2), p = 0.019] as did Pes, Pdi and PTPes [(11.87 (7.86, 18.32) vs. 6.8 (4.6-10.23), p < 0.001), (10.89 (± 6.42) vs. 7.94 (± 3.81), p < 0.001), and (181.10 (108.34, 311.7) vs. 97.52 (55.96-179.87), p < 0.001), respectively].

In critical care patients spontaneously breathing under resistive load, diaphragmatic excursion had a weak to moderate correlation with indices of breathing effort and differed between weaning success and failure.