Introduction: Diaphragm thickening fraction (TFdi) measured by ultrasound is widely used to evaluate diaphragm function to optimize ventilator support and weaning. TFdi cut-off values that could help in predicting weaning outcome have been reported. However, data regarding the relationship between TFdi and the changes in transdiaphragmatic pressure (ΔPdi) are lacking.Aims: We explored ΔPdi-TFdi relationship in healthy and mechanically ventilated (MV) subjects.Methods: Pdi was monitored and the right hemidiaphragm was imaged. Healthy subjects breathed against an external inspiratory load at 0-50% of maximal inspiratory pressure. MV patients were tested under several ventilator assistance levels and during a spontaneous breathing trial. A breath-by-breath analysis was performed. Pearson correlation coefficients (r) were used to determine within-individual ΔPdi-TFdi relationship.Results: Fifteen healthy volunteers and 22 MV patients were studied. Significant ΔPdi-TFdi correlation (r=0.32, p<0.01) was observed in 1 healthy subject only and in 3 MV patients (r=0.67, 95% CIs [0.41,0.83] in patients with significant ΔPdi-TFdi correlation, all p<0.05). Overall, relationship between ΔPdi and TFdi was absent in healthy subjects (R= -0.08, 95% CIs [-0.33,0.18], p=0.53) and weak in MV patients (R=0.17, 95% CIs [0.04,0.29], p<0.05).Conclusions: Unlike previous works investigating between-subject correlation, this study used breath-by-breath and within-subject correlation analyses to investigate ΔPdi-TFdi relationships. Our findings suggest that TFdi may not be considered as a surrogate of Pdi.FootnotesCite this article as: ERJ Open Research 2020; 6: Suppl. 4, 14.This is an ERS Respiratory Failure and Mechanical Ventilation Conference abstract. No full-text version is available. Further material to accompany this abstract may be available at www.ers-education.org (ERS member access only).