For assessment of interventricular dyssynchrony, the electrocardiogram (ECG) and systolic flow through the pulmonary artery and aorta (assessed with Doppler echocardiography) are typically used. Both the right ventricular (RV) and left ventricular (LV) electromechanical delay are measured from the onset of the QRS complex (dashed line). The RV electromechanical delay is the time from the onset of QRS interval to the onset of pulmonary systolic flow (blue arrow). The LV electromechanical delay is the time from the onset of QRS complex to the onset of aortic systolic flow (red arrow). Subsequently, the interventricular dyssynchrony can be calculated as the difference between the RV and the LV electromechanical delays (black arrow).

Intraventricular dyssynchrony is represented by the delay in mechanical activation between different segments within the LV. In this example, longitudinal strain curves of the septum and the posterior or lateral wall are demonstrated. The time from onset of the QRS complex to peak systolic strain for the septum (green arrow) and the posterior or lateral wall (red arrow) is indicated. The difference in time-to-peak strain for the various segments is the delay in mechanical activation, or LV intraventricular dyssynchrony (indicated by the black arrow). Abbreviations as in Figure 1.

Echocardiographic analysis of LV dyssynchrony during intrinsic rhythm (A) and immediately after onset of RV apical pacing (B). Speckle-tracking strain analysis enables the evaluation of the timing of systolic strain. The color-coded curves represent the time-strain curves of 6 midventricular segments of the LV. During intrinsic rhythm (A), a synchronous contraction of all LV segments is present. In contrast, during RV apical pacing, significant LV dyssynchrony is present: there is a significant delay (130 ms) between the time-to-peak strain of the anteroseptum (yellow arrow) and the posterolateral segment (purple arrow). Abbreviations as in Figure 1.

In 153 patients undergoing pacemaker implantation, LV dyssynchrony was assessed during RV apical pacing. Patients were classified according to baseline left ventricular ejection fraction (LVEF): normal (LVEF >55%), moderately depressed (LVEF 35% to 55%), or severely depressed (LVEF <35%). The extent of LV dyssynchrony was strongly related to baseline LVEF. In patients with normal LVEF, 45% of the patients developed LV dyssynchrony (40 of 89), whereas 39 of the 42 patients (93%) with moderately depressed LVEF developed LV dyssynchrony. In patients with severely depressed LVEF (n = 22), all patients exhibited LV dyssynchrony during RV apical pacing (49). ANOVA = analysis of variance; other abbreviations as in Figure 1.

In 44 patients with conventional pacemaker indications, an upgrade to CRT was performed after a mean of 49 ± 34 months of RV apical pacing. After 6 months of CRT, New York Heart Association (NYHA) functional class improved from 2.5 ± 0.7 to 2.1 ± 0.4 (A) and the distance walked during the 6-min walk test increased from 324 ± 20 m to 386 ± 99 m (B). Data derived from Leclerq et al. (57). CRT = cardiac resynchronization therapy; other abbreviations as in Figure 1.

A meta-analysis of 5 randomized clinical trials including more than 7,000 patients compared atrial-based with ventricular-based pacing. This figure demonstrates the effect of the pacing modes on the different outcome parameters (mortality, stroke or cardiovascular death, stroke, heart failure hospitalization, atrial fibrillation), expressed as the hazard ratio and 95% confidence interval (CI). A significant reduction in the incidence of stroke and atrial fibrillation was observed, in favor of atrial-based pacing. The remaining outcome parameters did not show a significant difference between the 2 pacing modes. Data derived from Healey et al. (75).