Reply to Jonkman et al.: Tissue Doppler Imaging of the Diaphragm: A Novel Approach but Too Early for Clinical Implementation?

From the Authors:

We thank Dr. Jonkman and colleagues for their comments concerning our paper on diaphragmatic tissue Doppler imaging (TDI) (1).

We agree that the echo settings (angle correction, filters, and gains) may affect the TDI measurements; our ultrasound settings can be seen in the videos and images provided in our paper.

The comment that velocity–time integral (VTI) should match M-mode displacement is theoretically correct; however, because simultaneous recording of TDI and M-mode displacement is not feasible, this assumption is impossible to prove because VTI and displacement will be measured during different breaths. Moreover, to our knowledge, there is no study demonstrating that M-mode displacement is less sensitive to measurement errors compared with TDI. Furthermore, it is also an assumption that inspiratory and expiratory VTI should be similar; in normal individuals this seems correct over a large number of breaths. However, in ICU patients, the presence of various levels of intrinsic positive end-expiratory pressure during weaning can affect the end-expiratory position of the diaphragm from breath to breath, making this assumption also particularly false and misleading. Additionally, according to the authors’ suggestion, we looked into the relationship between the mean values of the peak relaxation velocity and transdiaphragmatic pressure (Pdi)-derived maximal relaxation rate (MRR); the relationship is indeed better compared with TDI-MRR and Pdi-MRR (Figure 1).

Figure 1.

Relationship between Pdi-MRR and PRV. Pdi-MRR = transdiaphragmatic pressure–derived maximal relaxation rate; PRV = peak relaxation velocity.

Finally, we share Dr. Jonkman’s concern about the early clinical implementation of TDI. However, we provide a large number of data in normal individuals and in ICU patients to stimulate further clinical investigation to assess diaphragmatic function. We believe that TDI is a fascinating, bedside, noninvasive, real-time tool in the hands of the intensivists and physiologists. We should not forget that so far, the contractile and relaxation properties of the diaphragm were investigated with invasive, cumbersome, and indirect methods.