Rick Schreurs

24 Chapter 2 250ms 200ms 150ms 100ms 50ms QRSd 214ms ED 37ms QRSd 248ms ED 38ms ED 24ms QRSd 150ms QRSd 250ms ED 38ms RV pacing Ant Ant Ant Ant Inf Inf Inf Inf Intrinsic conduction LV pacing CRT Figure 1. Electrocardiographic images of an LBBB patient recorded during intrinsic conduction, RV pacing, LV pacing and CRT. Each pair of images shows the anterior (Ant) and inferior (Inf) views. The black lines depict isochronal lines. Crowded isochronal lines indicate slow conduction. Thick black lines indicate conduction block. An asterisk indicates the pacing site. The dotted lines in the images of intrinsic conduction show the septal aspect of the epicardium. The QRS duration (QRSd) and the electrical dyssynchrony index (ED; calculated as the standard deviation of activation times at 500 sites in the LV) are indicated in the figure for each pacing modality. LA: left atrium, RA: right atrium. Reproduced with permission from the Heart Rhythm Society [13]. Electro-mechanics of dyssynchrony In LBBB the normal activation pattern is disturbed because the LV is no longer activated via the left bundle branch and Purkinje fibers. Instead, the electrical activation spreads from the normally activated RV through the septum towards the LV. Since activation moving from myocyte to myocyte is much slower, the LV free wall – which is the site most remote from the RV – is activated latest. Several clinical [14, 15] and pre-clinical [16] invasive electrocardiac mapping studies have shown that the activation in LBBB hearts follows a specific pattern. LV depolarization moves from the septum in a circumferential and longitudinal direction. However, because conduction often appears slow at the RV-LV junctions an important contribution of activation comes from the wavefront passing over the apex towards the LV lateral wall (referred to as U-shaped activation pattern). Another characteristic feature is the slow transseptal conduction in LBBB, possibly caused by the vertical orientation of the laminar sheets of myocytes in the septum [17, 18]. Epicardial activation maps generated with non-invasive electrocardiographic imaging show comparable electrical patterns, as illustrated by the white arrow in the upper left panel of figure 1 [13]. The prolonged activation of the LV results in an increased total activation time