Multimodal Ultrasound Assessment of the Diaphragm During the Maximal Inspiratory Pressure Maneuver

Diaphragm ultrasound (DU) is increasingly used to assess diaphragmatic function, but its relationship with maximal inspiratory pressure (MIP), particularly when both are acquired during the same maneuver, remains unclear. This exploratory cross-sectional study investigated associations between MIP and DU parameters, compared diaphragmatic strain obtained with two different probes and imaging planes, and explored the association between diaphragmatic strain and the diaphragm thickening fraction (TF). Eleven healthy participants (median age 21 years; median body mass index 22.1 kg/m²) with normal phrenic nerve conduction, transcranial magnetic stimulation, and occlusion pressure at 100 ms underwent DU during the MIP maneuver using phased-array and linear probes at the subcostal window and zone of apposition, respectively. Each ultrasound acquisition was paired with an MIP recorded simultaneously during that acquisition; the two probes were therefore studied within the same session and not during a single shared maneuver. No a priori sample size or power calculation was performed. MIP showed moderate positive associations with diaphragmatic excursion (DE) (r_s = 0.603, p = 0.050) and TF (r_s = 0.482, p = 0.133), and moderate negative associations with time to peak diaphragmatic excursion (TPDE) (r_s = -0.400, p = 0.223) and linear-probe strain (r_s = -0.469, p = 0.241). Linear-probe strain was strongly associated with TF (r_s = -0.710, p = 0.048). Every bootstrap 95% confidence interval included zero. In this exploratory sample, DE and TF showed the strongest associations with MIP, while zone-of-apposition strain was strongly associated with TF; they merit evaluation as part of a multimodal DU assessment, as candidate quantitative parameters rather than validated markers.