Improving ultrasonic measurement of diaphragmatic excursion after cardiac surgery using the anatomical M-mode: a randomized crossover study

Purpose: Motion-mode (MM) echography allows precise measurement of diaphragmatic excursion when the ultrasound beam is parallel to the diaphragmatic displacement. However, proper alignment is difficult to obtain in patients after cardiac surgery; thus, measurements might be inaccurate. A new imaging modality named the anatomical motion-mode (AMM) allows free placement of the cursor through the numerical image reconstruction and perfect alignment with the diaphragmatic motion. Our goal was to compare MM and AMM measurements of diaphragmatic excursion in cardiac surgical patients. Methods: Cardiac surgical patients were studied after extubation. The excursions of the right and left hemidiaphragms were measured by two operators, an expert and a trainee, using MM and AMM successively, according to a blinded, randomized, crossover sequence. Values were averaged over three consecutive respiratory cycles. The angle between the MM and AMM cursors was quantified for each measurement. Results: Fifty patients were studied. The mean (±SD) angle between the MM and AMM cursors was 37° ± 16°. The diaphragmatic excursion as measured by experts was 1.8 ± 0.7 cm using MM and 1.5 ± 0.5 cm using AMM (p < 0.001). Overall, the diaphragmatic excursion as estimated by MM was larger than the value obtained with AMM in 75 % of the measurements. Bland-Altman analysis showed tighter limits of agreement between experts and trainees with AMM [bias: 0.0 cm; 95 % confidence interval (CI): 0.8 cm] than with MM (bias: 0.0 cm; 95 % CI: 1.4 cm). Conclusion: MM overestimates diaphragmatic excursion in comparison to AMM in cardiac surgical patients. Using MM may lead to a lack of recognition of diaphragmatic dysfunction.