Feasibility and reference values of left atrial longitudinal strain imaging by two-dimensional speckle tracking

Sixty consecutive adult healthy subjects, referring to our Echo Laboratory for a diagnostic examination, were included in the study group. All had unremarkable clinical history and normal findings at physical examination, ECG, and baseline echocardiography, and none of them was taking cardiac medications. All subjects gave their written informed consent for participation in the study.

Echocardiographic studies were performed using a high-quality echocardiograph (Vivid 7, GE, USA). Subjects were studied in the left lateral recumbent position. Measurements of left ventricular (LV) and LA dimensions were made in accordance with current American Society of Echocardiography recommendations[12]. LV ejection fraction was measured using the modified biplane Simpson's rule. The ratio between peak early (E) and late (A) diastolic LV filling velocities and E wave deceleration time were determined by standard Doppler imaging. The timings of mitral and aortic valve opening and closure were defined by pulsed wave Doppler tracings of mitral inflow and LV outflow.

For speckle tracking analysis, apical four- and two-chamber views images were obtained using conventional two-dimensional gray scale echocardiography, during breath hold with a stable ECG recording. Particular attention was given to obtain an adequate gray scale image, allowing reliable delineation of myocardial tissue and extracardiac structures. Three consecutive heart cycles were recorded and averaged. The frame rate was set between 60 and 80 frames per second. These settings are recommended to combine temporal resolution with adequate spatial definition, and to enhance the feasibility of the frame-to-frame tracking technique[13].

Recordings were processed using an acoustic-tracking software (Echo Pac, GE, USA), allowing off-line semi-automated analysis of speckle-based strain[14, 15] (Figure 1). Briefly, LA endocardial surface is manually traced in both four- and two-chamber views by a point-and-click approach. An epicardial surface tracing is then automatically generated by the system, thus creating a region of interest (ROI). After manual adjustment of ROI width and shape, the software divides the ROI into 6 segments, and the resulting tracking quality for each segment is automatically scored as either acceptable or non-acceptable, with the possibility of further manual correction. Segments in which no adequate image quality can be obtained are rejected by the software and excluded from the analysis. Lastly, the software generates strain curves for each atrial segment. In subjects with adequate image quality, a total of 12 segments were then analyzed. To trace the ROI in the discontinuity of LA wall corresponding to pulmonary veins and LA appendage, the direction of LA endocardial and epicardial surfaces at the junction with these structures was extrapolated. Peak atrial longitudinal strain (PALS) was calculated by averaging values observed in all LA segments (global PALS), and by separately averaging values observed in 4- and 2-chamber views (4- and 2-chamber average PALS) (Figure 2). The time to peak longitudinal strain (TPLS) was also measured as the average of all 12 segments (global TPLS) and by separately averaging values observed in the two apical views (4- and 2-chamber average TPLS). In patients in whom some segments were excluded because of the impossibility of achieving adequate tracking, PALS and TPLS were calculated by averaging values measured in the remaining segments.

Reproducibility of PALS and TPLS measurements was assessed in 20 randomly selected subjects. Intra and inter-observer variability coefficients were calculated using images independently recorded in two different occasions by the same investigator or by two different observers.

Data are shown as mean ± SD. Inter- and intra-observer reproducibility was assessed by calculating variability coefficients. Reference values were expressed as mean ± SD and 5–95° percentile ranges. Comparisons were performed using the Student's t test for paired data. A P value < 0.05 was considered statistically significant. Analyses were performed using the SPSS (Statistical Package for the Social Sciences, Chicago, Illinois) software Release 11.5.

Table 1 shows the clinical and echocardiographic characteristics of the study population. Among a total of 720 segments, the software was able to track 697 (96.9%) segments. Adequate tracking of all 12 LV segments was achieved in 50 (83.3%) subjects, and in no cases the number of segments adequately explored was < 8. Average post-processing time per patient was 2 ± 1 min. Inter-observer variability coefficients of global PALS and TPLS were 3.4%, and 4.8%, respectively. For intra-observer variability, the corresponding variability coefficients were 2.9% and 3.8%.

When the reproducibility was separately considered in the two apical views, inter-observer variability coefficients were 4.3% and 4.6% for 4- and 2-chamber average PALS, and 5.4% and 5.3% for TPLS, respectively. Corresponding intra-observer variability coefficients were 3.6%, 4.0%, 4.5%, and 4.8%.

Mean ± SD and 5–95° percentile ranges of global, 4-chamber, and 2-chamber PALS and TPLS observed in the study population are reported in Table 2. Notably, 2-chamber average PALS was significantly higher than 4-chamber average PALS (p < 0.0001), whereas there was no difference between 2- and 4-chamber average TPLS (p = 0.93).