The current literature on diaphragm ultrasound assumes the muscle thickness to be homogenous across its length. Most studies report normative values of thickness using standardized imaging protocols for insonation at a single anatomical landmark with high inter- and intra-rater reliability. Despite statistical accuracy, there is a paucity of literature investigating the inherent physiologic variability caused by the normal topography of an individual’s diaphragm within the same landmark.

The smallest difference between thickness measurements within the same image was 0 pixels and the largest difference was 55 pixels. Across all 44 images, the diaphragm thickness ranged from 13 pixels to 99 pixels. Despite a standardized protocol of acquisition, the average coefficient of variation was 0.272.

This retrospective analysis of diaphragm ultrasound images from 20 critically ill patients quantifies the physiologic variability of diaphragm thickness measurements. Many studies assessing changes in diaphragm thickness over time report cutoffs of 10-20% thickness correlating to mechanical ventilation outcomes.  Such studies need to account for this physiologic variability and the inherent coefficient of variation secondary to normal muscle topography when interpreting serial changes in thickness and reporting cutoffs for clinical decision-making.