S/SR analysis
For LA and LV S/SR, a single reader (M.K.) analyzed the 2D apical four- and two-chamber and apical long-axis (APLAX) views with speckle tracking using the Q-analysis function of EchoPAC (v.203, GE-Vingmed AS, Horten, Norway). The LV S/SR from APLAX views was analyzed in 176 Tromsø7 participants and the global S/SR was derived from the four- and two- chamber views only.
Cycles for LV strain analysis were set to start at peak R, whereas the cycle for atrial contraction was defined as the end of the P-wave. In AF, peak R was defined as the onset of the cardiac cycle. Aortic valve closure was defined by using a transaortic CW Doppler signal. For atrial and ventricular measurements, the region of interest (ROI) was manually traced at the subendocardial border, with consecutive adjustments in the ROI width. Automated tracking was visually controlled and suboptimal tracking results were repeated a maximum of three times. For LV and LA strains, segmental values were extracted from longitudinal mid-myocardial strain curves averaged over the number of segments. The LV strain was measured at the time of ES. As shown in Figure 2, the LA S/SR was measured from the global strain curves derived from the strain and SR curves of all six segments of the four-chamber view. As the Q-analysis function did not provide all peak atrial S/SR measurements for the defined time periods, the results were automatically extracted from the strain curves using customized software. Although atrial strains reflect longitudinal shortening and should be expressed as negative values, there appears to be a general consensus on reporting atrial longitudinal strain values as positive numbers. LA strain was measured in three defined time periods: peak atrial contraction strain (PACS) between the onset of atrial contraction and peak negative strain; ATS, defined as the difference between the negative and positive peaks; and ACS, defined as the difference between the positive peak and the onset of atrial contraction. In the LA and LV, diastolic SR E was measured at peak SR after AVC and before the onset of the atrial contraction, SR during atrial contraction (A) was the SR peak after the onset of the atrial contraction, and systolic SR during systole (S) was the peak in the opposite direction between the start of the cycle and AVC. Tissue Doppler velocities from the septum and lateral wall were derived from basal segmental speckle tracking analyses, which were chosen to overcome the differences between the two study populations in terms of tissue Doppler acquisitions with different ultrasound systems and machine settings. Mitral E/e´ was calculated from the MV E and the average basal and lateral four-chamber e’ was calculated from the basal speckle tracking-derived velocities. LASI was calculated as the ratio of the E/e´ and ATS, where e´ was derived from the basal velocities of the septum and lateral wall in the four-chamber view.
Statistical analyses
Statistical analyses were performed using IBM SPSS version 28.0. (IBM Corp: Armonk, NY, US). The two groups were compared using either the t-test or χ2 test for continuous and categorical variables, respectively. Unless otherwise stated, continuous variables are presented as means ± standard deviations (SD). Variables with skewed distributions are presented as medians with quartiles (Q1/Q3). Categorical characteristics are presented as absolute numbers and proportions (%). For comparison between the three groups with EF more than or equal to 50% (EF≥50%), EF less than 50% (EF<50%), and AF, group differences in continuous variables were tested using one-way analysis of variance (ANOVA) with Bonferroni post-hoc tests.
For the independent and dependent correlations of echocardiographic indices with the presence of elevated NT-proBNP levels, univariate and multivariate logistic regression analyses were performed. Variables with a p-value of less than or equal to 0.20 in the univariate analysis were selected and tested by forward and backward multivariate logistic regression analysis. For final inclusion in the multiple regression model, a p-value of less than or equal to 0.05 was considered statistically significant. Independent predictors of the multivariate analysis were combined as weighted predictors and a receiver operating characteristic (ROC) curve analysis was performed.
Intra- and interobserver variability
For intra-and inter-observer variability in atrial S/SR measurements, the same observer repeatedly analyzed 45 randomly selected echocardiographic records. The same data was reanalyzed by a second experienced observer. Intra- and inter-observer values were calculated as intraclass correlations and 95% confidence intervals (CIs).