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Letter to the editor: "Assessment of flow-mediated dilation in humans: a methodological and physiological guideline".

TO THE EDITOR: We read with great interest the article “Assessment of flow-mediated dilation in humans: a methodological and physiological guideline” (1), published in the American Journal of Physiology-Heart and Circulatory Physiology. Specifically, Thijssen et al. (1) present a series of recommendations based on a review and critical appraisal of recent physiological studies pertaining to the most appropriate methods to assess flow-mediated dilation (FMD) in humans. Although general considerations for selecting an adequate ultrasound transducer were described, we believe it is important to emphasize some technical aspects. Axial resolution is the ability to resolve discrete structures along the beam axis. Quantitatively, it is represented as the minimum distance between two structures at which both can be identified as two separate points (2). As the frequency increases, axial resolution improves (Table 1). Although a 7.5-MHz probe as the minimum frequency is recommended, average brachial FMD diameter changes (expressed in millimeter) are often close to the minimum axial resolution limit of this frequency. Hence, small vessels or little FMD changes might exceed the axial resolution limit of transducers with a 7.5-MHz frequency (e.g., 3.5-mm vessel diameter with an FMD of 5%). We consider that the minimum probe frequency should be updated to the actual state of the art to achieve an optimal relationship between diameter changes and the axial resolution limit.