Letter to the editor: “Left ventricular mechanical limitations to stroke volume in healthy humans during incremental exercise”

Abstract

TO THE EDITOR: We would like to congratulate Stohr et al. (3) on their interesting and timely examination of left ventricular (LV) rotational mechanics during submaximal exercise. We (2) have previously acknowledged the importance of evaluating LV rotation during incremental to maximal exercise. It is great to see novel techniques applied to answer such fundamental questions. Although we found this article interesting, we would like to highlight some issues that may deserve further consideration. First, we would like to comment on the technical aspects of imaging the LV during exercise. Quantification of LV twist heavily relies on apical rotation (2). Accurately imaging the short axis of the apex during exercise is extremely challenging because of the increased heart and respiration rates. To be accurate, the longitudinal location of the image must be exactly repeated over each exercise stage. We expect that as exercise intensity increased, so did the difficulty of recording motion at identical locations. Support for this contention is the finding of a marked and progressive increase in the variability in apical rotation velocity as exercise intensity increased, whereas the variability in basal rotation was relatively consistent (3). In our experience, as apical rotation rate directly increases through an increase in heart rate, images become less clear. Following this, it may be possible that the point perceived as the “luminal obliteration” progressively becomes a more basal location as exercise intensity increased (4). If this occurred, changes in twist may be undetectable at higher exercise intensities. Second, although we have used the Teichholz method for determining stroke volume (SV), questions remain about its validity during exercise (in particular, high-intensity exercise). It follows that a possible limitation of this technique may lie in the aforementioned technical considerations thought to occur at the apex. If this is the case, it may have influenced the accuracy of SV estimates, since they were derived from short-axis images at the level of the papillary muscle. It would add to the specific discussion regarding SV and LV twist changes if the relationship was separately plotted for above and below the point of 50% SV increase. Our visual interpretation of Fig. 5 (3) suggests that the relationship between values becomes less clear above 50% SV increase. Although this point is partially documented in the discussion, the presence of SV changes during exercise is highly dependent on training status and postural position. Therefore, we are not sure that this study either supports or refutes either side of the ongoing debate (1, 5). This is particularly salient since considerable recent research (using techniques validated for maximal exercise) has shown that there is often a slight plateau in SV, with a further increase at maximal exercise. The fact that maximal exercise was not evaluated is a limitation, and as such all statements regarding SV plateauing should be tempered accordingly. Moreover, the authors have not presented the individual data of the participants, limiting the ability to interpret the varied responses to exercise. It appears from the mean data that SV may have not reached a true plateau in many participants (a finding that is consistent with the literature) (5). We applaud Stohr and colleagues (3) for their interesting work. Although there were several important limitations that required mention, we feel the application of techniques used in their work is the natural progression of this ongoing discussion.