Response

Abstract

We welcome Laffon et al's interest and reply to their concerns about the validity of comparing their pulmonary artery (PA) pulse wave velocity (PWV) data (1) with our own (2).Given the scarcity of literature on the subject, we considered it reasonable to discuss their work [particularly as it employed cardiovascular magnetic resonance (CMR)]. Indeed, we have identified just two other non-invasive studies (3,4) that used the transit-time approach, like ours. Only one, which employed fluorocardiography, had comparable results [conus to right hilum, 2 m/sec (3) vs. 2.33 m/sec for right PA and 2.09 m/sec for left PA (2)].The transit-time method records the time a pulse takes to cover a known distance and provides a "weighted" measure of PWV over this path length. By using a high-temporal resolution acquisition to analyze flow arrival, ours was the first study to prove this to be achievable in the PAs with CMR. Conversely, Laffon et al approached PWV "locally" by estimating the elastic modulus using the equation proposed by Frank, and Bramwell and Hill (5). "Local" PWV has also been estimated by CMR in work reported after ours was submitted where the change in flow was related to the change in diameter in the early systolic phases of an MPA velocity map (6). In the 8 healthy volunteers included in the study, a mean PWV of 1.84 m/sec was found.It is our assertion that as long as the differences between the methods (as well as their limitations) are acknowledged clearly, discussion of all available measurements in this scarcely reported field will remain warranted [see the chapter on wave propagation in Milnor's Hemodynamics (5)].Laffon et al's main concern centers on the inclusion of the main PA's branch point in the examined region and the effects this might have on transit-time PWV. Because of the limitations imposed by pulmonary anatomy and temporal resolution, this was the only way to make the delay in pulse wave arrival discernible. Although future improvements in velocity mapping could limit study to a single vessel only, it will not stop reflections originating outside the examined region "contaminating" the area of interest.Although it was not the intention of this work to examine the issue of reflections or make comment on the behavior of the pulse wave at any specific position during its transit, we considered this problem when choosing our "marker" of pulse wave arrival. In normal subjects, where one could speculate that the backward wave would be both small and slow moving, we believed a point halfway up the wavefront would likely be relatively unaffected during its transmission and therefore robust. However, in pulmonary hypertension, where the wave profile changes to a far greater degree during propagation, the same point would likely be unrecognizable by the end of the path length. So, just as Fleischner et al (3) and Ring and Kurbatov (4) did over 50 years ago, we will be focusing on the wavefront foot, which we anticipate will prove least contaminated and hence easiest to study.In conclusion, we belie...