Introduction
Global longitudinal strain from two-dimensional (2D) echocardiography is considered to be an accurate and sensitive parameter for the assessment of global left ventricular (LV) dysfunction [
1]. Longitudinal strain (LS) obtained from 3 standard apical views is used for the calculation of global longitudinal strain, but it can also be used to visualize regional dysfunction, such as in myocardial infarction or various types of cardiomyopathy on a bull’s eye map [
2]. The LV contraction in left bundle branch block (LBBB) is characterized as an early systolic contraction in the septum followed by delayed contraction in the lateral wall. The mid septum is usually a site of early contraction, while the early contraction is observed in a more apical location during right ventricular apical (RVA) pacing [
3]. Such differences in LV contraction are thought to reflect differences in the LV activation sequence between LBBB and RVA pacing [
4,
5]. In patients with LBBB or continuous RV pacing, electrical and mechanical dyssynchrony persist; these patients frequently develop heart failure. If the LV activation sequence can be estimated using echocardiography in these patients, additional information relating to the cardiac resynchronization therapy (CRT) response would be available [
6,
7]. In the present study, we evaluated the LS during the systolic period and time-to-peak strain using 2D speckle tracking echocardiography and examined the LV contraction process in heart failure patients with LBBB, RVA pacing, and LV pacing.
Discussion
The contraction sequence of the entire LV has been investigated using tagged MRI [
15,
16], SPECT [
17], electro-anatomical mapping (NOGA) [
18], and 3D echocardiography [
19]. 2D speckle tracking echocardiography has been used to evaluate the LV contraction abnormalities, but to our knowledge, this is the first report to evaluate the LV contraction process by constructing bull’s eye maps.
The LV contraction process in LBBB is thought to be caused by abnormalities in LV activation [
20,
21]. Based on electro-anatomical mapping using unipolar electrodes, a U-shaped activation wave front was observed moving from the LV breakthrough site to the lateral wall turning around the apical regions [
20]. In our study, LBBB patients showed the majority of the ESC in the septal and anterior-septal walls (Table
1). The mid-apical-opposite wall sequence was observed in both 4- and 3-chamber views in 74% of the patients with LBBB. These results are thought to reflect the LV activation abnormalities in LBBB.
The LV activation sequence in RVA pacing has been reported to differ from that in LBBB [
4,
5]. Using electro-anatomical mapping, Mafi Rad et al. evaluated the LV activation sequence in CRT candidates [
4]. At the time of RVA pacing, the LV was activated in the apical-to-basal direction and the regions of late activation shifted more toward the basal regions compared to the time of LBBB. In our study, ESC was observed most frequently in the apical segments in patients with RVA pacing (Table
1), and the LV contraction tended to proceed from apical to basal directions (Fig.
4). Such LV contraction processes observed in RVA pacing are thought to be due to the LV activation initiated from the apical regions.
Miranda et al. evaluated the timing of RV activation at the time of LV pacing [
22]. Electrical separation from the LV activation to the RV mid septum was 161.2 ± 23.7 ms, which was larger when compared to the RV outflow tract (154.1 ± 20.8 ms) or RV apex (148.0 ± 25.5 ms). The largest electrical separation was most frequently observed in the mid septum (40 of 50 patients). In our study, late contraction sites in patients with LV pacing were mainly observed in the mid segments in the septum (septal or anterior-septal wall; Fig.
7). This result did not contradict the reports of Miranda et al. [
22]. The apical-to-basal difference in delayed contracted wall was smaller for LV pacing compared to LBBB and RVA pacing (Fig.
6). Assuming that the Q-LNpeak in a segment reflects timing of LV activation in that segment [
13], our results indirectly indicate that the timing of LV activation between the apical and basal segments in the septum did not differ significantly at the time of LV pacing. In several reports that compared the response to CRT between patients with RVA and non-apical pacing, clear differences were not observed [
23‐
26]. This result may be related to the activation delay feature in the RV septum at the time of LV pacing.
There are few reports evaluating the beginning of contraction in a segment to estimate the LV activation sequence [
15]. Echocardiography has not been used extensively for this purpose due to the difficulty in evaluating subtle changes in myocardial shortening during early systole. Seo et al. [
19] evaluated regional deformation (area change ratio) using three-dimensional (3D) echocardiography, and analyzed the timing of area change ratio, which indicated 25% of the maximum value. Seo et al. observed a U-shaped propagation of the LV contraction in patients with typical LBBB. In our study, the Q-EPpeak had similar sequences to the Q-LNpeak in patients with LBBB, RVA pacing, and LV pacing. Therefore, the Q-EPpeak also has the ability to reflect the LV activation sequence. We think strain analysis is a sensitive and useful tool for the assessment of ventricular contraction process, and future studies are expected to investigate its relevance to the electrophysiological findings. Such studies could potentially enable non-invasive prediction of CRT effects and selection of optimal lead location using echocardiography.
Study limitations
First, evaluation using the apical images in 2D echocardiography did not provide information on circumferential directions. There was a large spatial gap, especially in the basal regions, and this might have reduced the number of segments with ESC in LV pacing (Table
1). Three-dimensional echocardiography obtains information from the entire LV and evaluates deformation in a local segment [
27]. Although there is a need to improve temporal and spatial resolutions, 3D echocardiography is thought to be a useful tool that improves on 2D echocardiography. For time-to-peak strain analysis, it depends on temporal resolution. It was difficult to identify the earliest contraction site using Q-EPpeak, as in the identification of the earliest excitation site in the electrophysiological testing. Second, the present study did not examine patient prognosis or the relationship to CRT efficacy. Although there were reports of latest contraction site and CRT efficacy assessed using radial strain [
6,
7], the clinical usefulness of assessing longitudinal strain from the three apical views is unclear. Third, since the patients in each QRS group were different, differences in the results between the QRS groups cannot be considered as a change. Mafi Rad et al. [
4] and Jackson et al. [
5] examined differences during LBBB and RVA pacing within the same patient, allowing the change to be evaluated. In order to perform an evaluation such as change in time-to-peak strain in a segment when switching from LBBB to RVA pacing, it is necessary to study this within the same patient. Finally, the time-to-peak strain may be influenced by factors other than electrical activation, such as scar formation or load to the myocardium [
15,
28]. These factors were not clear in this study.
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