To our knowledge, these distinctive features of cardiac motion in RBBB have not previously been systematically reported; they are qualitatively different from previously reported related motion findings in LBBB. While there is a range in the relative magnitude of these findings, and they are not present in all patients, they can be quite striking in some patients. Short-axis cine images were not included in our image analysis, due to potential confounding effects on the appearance of the images from through-plane motion of the curved heart wall, although these motion features can be seen in them, as well. While these motion patterns can also be seen in normal subjects, they are not usually found in them. Thus, while the sensitivity of these finding for RBBB is only moderate, they are quite specific for RBBB, when present. In particular, the basal bulge, a previously unreported finding in RBBB, can be qualitatively quite striking in some cases. The reverse septal flash in the 3-chamber view, which can be relatively subtle, is the least reliable finding for RBBB.
While one of the readers had distinctively poorer agreement with the other three readers in the scoring of the images, this is likely due to the relative unfamiliarity of these new findings, with some associated uncertainty as to how to score them.
As the mechanical events of the cardiac cycle are not directly tied to the corresponding voltages recorded on the ECG, the CMR motion patterns seen in patients with RBBB (read on a preceding ECG) are not expected to be fully correlated with the ECG findings. While ECGs acquired at the time of the CMR were generally not available, it is likely that RBBB present on a prior ECG was still present at the time of the CMR examination; that is why we used that as a selection criterion for cases to be analyzed.
The mechanisms of the motion patterns reported here are all likely related to the delayed mechanical activation of the RV free wall, including a relatively slow propagation of the wave of activation (via myocyte-to-myocyte excitation) from the apex to the base of the RV:
1) The delay of the motion of the RV base toward the apex likely reflects a delayed development of significant apex-ward traction on the base of RV, due to the delayed activation and contraction of the base of the RV wall, with an associated delayed development of tension in the basal RV wall.
2) The phenomenon of the reverse septal flash likely reflects a delayed increase in RV cavity pressure, with associated changing trans-septal pressure differences, due to delayed activation of the RV free wall. The rising pressure in the LV will initially push the interventricular septum to the right; this effect will then be partially balanced as the pressure increases in the RV cavity. A delay in the activation of the full thickness of the interventricular septum, related to the block in conduction in the right bundle, with an associated delay in the development of the full tension in the septal wall, would also contribute to this phenomenon.
3) The phenomenon of the basal bulge likely reflects delayed contraction of the base of the RV free wall, with associated initially persistent low tension in the basal RV wall, while pressure is already starting to rise in the RV cavity due to contraction of the more apical portions of the RV wall. This initial low tension in the basal RV wall can lead to an initial locally poor resistance to the expansile force from the pressurized RV cavity blood at the base, and a resulting initial outward bulge of the basal RV free wall. When the basal RV wall is then subsequently activated and starts to contract, the initial bulge will go away as the tension increases in the basal RV wall.
While the delayed apex-ward motion of the RV base described here is related to TAPSE, the tricuspid annulus systolic excursion, which can be used as a measure of RV function, TAPSE is only calculated at end-systole, while the delayed apex-ward motion of the RV base is a dynamic feature of early systole.
While the characteristic motion findings of RBBB were seen only in either the 3-chamber or 4-chamber views in some of the patients with RBBB, this may reflect actual heterogeneity of the phenomena, which may not be uniformly distributed over the base of the RV.
The shape of the LV tends to become more spherical in early systole, as would expected for a hollow organ with increasing tension in the wall. This can result in an appearance of early systolic “rounding” motion of the initially relatively flattened septum, that may look similar to the reverse septal flash in some normal subjects, as the LV base “rounds up” during its initial contraction; this can be particularly so if the IVS is initially relatively more flattened at end-diastole, as was seen in some of the normal subjects. However, this effect is usually associated with a relatively small amount of motion, and a clearly distinct reverse septal flash pattern of motion is unlikely to be produced by this phenomenon alone.
In practice, there is often some variability in the specific anatomic location and orientation of nominal 3-chamber and 4-chamber image orientations. The resulting variability of the specific portions of the RV wall imaged, and of the degree of obliquity of the wall with the image plane, can result in some variability of the appearance of the characteristic motion features of RBBB studied here, and may account for some of the variability of our results. However, the qualitative features of the motion seem to be fairly robust to minor variations in the image orientation, as long as the image plane is close to lying along the left ventricular axis, and the image includes the base of the RV (and not the proximal pulmonary artery, for example, where the sinuses may normally bulge during early systole).
CMR is obviously not a practical way to look for the presence of RBBB, in itself, as the standard diagnosis of RBBB is based on the common ECG findings associated with it. However, recognition of the distinctive motion patterns associated with RBBB in a CMR examination may indicate the presence of RBBB; if there has not been a recent ECG recorded, this finding may provide the first indication of RBBB. In addition, as the motion effects of RBBB are not directly related to the ECG findings, the presence of these characteristic dyssynchronous motion patterns could help to independently confirm the presence of significant RBBB when the ECG findings are ambiguous.
RBBB is found fairly commonly in congenital heart disease, as well as in many other conditions; it may contribute to decreased cardiac function, if there is underlying myocardial disease. Cardiac resynchronization therapy with placement of a basal RV lead (rather than just the usual apical lead) may potentially be beneficial, in such cases, if the mechanical effects of the RBBB on cardiac function are clinically significant [9].
While this report has focused on CMR findings of some distinctive motion patterns seen in RBBB, it is likely that similar motion findings will also be seen in RBBB when looking at the heart with other cardiac imaging modalities, such as echocardiography and X-ray computed tomography.
Study limitations: This study is retrospective, with possible associated selection biases. As these distinctive motion findings have not been previously described, and are thus unfamiliar, there may have been some confusion on the part of the readers as to how to interpret and score them, as discussed above. The average age of the RBBB patients was greater than that of the normal subjects, although that should not significantly alter the findings.