Heart Failure
Figure 1: 3D Echocardiography Demonstrates Mechanical Dyssynchrony (A) and Complete Normalisation of Dyssynchrony Pattern (B)
AB
a pure LBBB, there remains a high level of heterogeneity in the LV activation pattern, accompanied by a wide variance in the line of functional block.10
Recent work has suggested that within
Also noteworthy is that patients with an RBBB have a 3.5- fold higher risk of death compared with those with an LBBB. Several explanations for a worse outcome in RBBB patients have been touted and include ventricular dyssynchrony patterns not favourable for CRT, concomitant RV dysfunction and more extensive conduction disease.
patients with a non-LBBB morphology, a non-specific intra- ventricular conduction defect (IVCD) subset has a poorer outcome, while those with right bundle branch block (RBBB) fare even worse.11
Figure 2: Intraprocedural Integrated Computed Tomography and Fluoroscopy Image of the Coronary Sinus
From the activation sequence perspective, it can be speculated that the reduced response to CRT in this patient group is consequent to an unchanged lead implantation strategy (i.e. targeting the lateral wall) despite the altered depolarisation wavefront characteristics in this group. In fact, it is unclear whether an LV lead is really mandatory to accomplish CRT functional gain or if an adequately timed pacing impulse from an RV lead may be adequate. Recent work suggests that although only one-quarter of patients with RBBB may have LV conduction delays comparable to LBBB, nearly 50% have some delay, which may be amenable to resynchronisation.12
Imaging
Angioplasty wire LV lead in CS RV defibrillator lead
Simplistic approaches examining the QRS axis, morphology and duration from the surface ECG have not been able to forecast the electrical activation pattern of the ventricles. Hence, a variety of echocardiographic measures have surfaced to improve our understanding of the anatomical and functional aspects of the cardiac substrate. M-mode, 2DE, 3DE tissue Doppler imaging (TDI) provides a better understanding of the level of baseline dyssynchrony, acute response and evidence of favourable remodelling to CRT. Despite being recently maligned via a couple of negative studies (RethinQ, and Predictors of Response to CRT [PROSPECT]), ultrasound TDI still remains one of the most convenient and better understood tools to measure dyssynchrony.13,14
However, there remains a lack of
Right and left venticular leads and angioplasty wire are demonstrated.
(post-pacing) does not predict outcome from CRT.8 This perhaps
relatesto the fact that a summated signal of RV endocardial and LV epicardial pacing does not correlate with the extent of change in mechanical dyssynchrony or global systolic function. The value of the surface QRS signal is further reduced by the fact that there are patients with a wide QRS duration who have minimal mechanical dyssynchrony, while there are those with a narrow QRS and significant mechanical dyssynchrony.
To further complicate matters, we now know that there is considerable inconsistency in the clinical response between patients with left bundle branch block (LBBB) and non-LBBB morphology. Characteristically, LBBB is linked with a U-shaped activation pattern that courses the apex with delayed activation of the lateral and postero-lateral portion of the LV.9
electrical activity parallels the mechanical activation,
This spread of and
constitutes the basic reasoning behind the conventional LV lead implantation strategy of targeting the lateral wall. Even in
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standardisation in accepted measures, as well as several limitations to this technology. TDI is still predominantly a 2D technique and the need for high frame rates required by this technique limits resolution and image quality. Also, on account of its angle dependence, TDI allows only specific views of the cardiac anatomy. Strain-rate imaging constitutes one of the newer echocardiographic imaging strategies, and is being prospectively evaluated as part of several ongoing studies. Also, of late,
realtime 3D ultrasound, which allows for
simultaneous imaging of all of the cardiac segments in a cardiac cycle with new segmental wall volume techniques, may provide a better understanding of the extent of cardiac dyssynchrony during the same cardiac cycle (see Figure 1).
As most of the imaging modalities provide complementary information in terms of cardiac structure (e.g. fluoroscopy for coronary venous anatomy) and function (e.g. echocardiography for mechanical dyssynchrony), combining some of these technologies may help improve the diagnosis and facilitate the planning of treatment and delivery of pacing therapy in patients receiving CRT (see Figure 2).15,16
Data from small retrospective
studies have shown that pacing over the site with maximal discordance and avoiding a region of scar may result in a better outcome.17,18
Current use of intra-procedural echo to demonstrate
EUROPEAN CARDIOLOGY
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