*2.3. Outcome Variables*

The key clinical end-points were the relationships between clinical response to CRT and LA function changes. The main outcome measures were LA function, peak atrial longitudinal strain (PALS), and the total EF.

## *2.4. Data Extraction*

Eligible studies were reviewed, and the following data were abstracted: (1) first author s name; (2) year of publication; (3) study design; (4) data on two arms: CRT responders and non-responders; (5) LA strain measured by echocardiography; (6) baseline characteristics of the patients; (7) baseline LA strain; (8) mean follow-up period; (9) age and gender of study participants; and (10) follow-up LA function.

Peak atrial longitudinal strain (PALS) was measured as the first peak between the QRS complex and the T wave during LV systole, guided by the superimposed electrocardiogram (ECG). When P–P gating was used, and the strain curve was taken as the negative deflection (that represents the peak LA contraction strain) followed by a positive deflection. The sum of the two parts of the curve (negative + positive) represented the peak LA wall strain (Figure 1) [11]. The two types of gating were extracted from the papers' available text or images.

**Figure 1.** Two types of gating, based on zero reference point (P–P and R–R gating).
