Mechanical dyssynchrony

Finally, there is variability of the treatment given and the underlying disease process for any individual patient. For example, a patient may have substantial ventricular mechanical dyssynchrony and have a successfully placed CRT device with appropriate lead locations. If very careful measures of ventricular performance are made, the patient s heart functions better with CRT on than off. However, if the patient suffers from a subsequent myocardial infarction or progressive ischemia, he may not improve clinically. Accordingly, at the end of a trial period, such a patient is termed a nonresponder even though he may be doing better with the device... 

Sade LE, Kanzaki H, Severyn D, Dohi K, Gorcsan J, III. Quantification of radial mechanical dyssynchrony in patients with left bundle branch block and idiopathic dilated cardiomyopathy without conduction delay by tissue displacement imaging. Am. J. Cardiol. 2004 94 514-8. 

Curry CW, Nelson GS, Wyman BT, et al. Mechanical dyssynchrony in dilated cardiomyopathy with intraventricular conduction delay as depicted by 3D tagged magnetic resonance imaging. Circulation 2000 101 4. 

RETHINQ (20) 2007 172 NYHA III, LVEF <35% QRS duration <130 but with echo evidence of mechanical dyssynchrony (TDI or M-mode) NSR Double blind, parallel No significant change in proportion of patients with an increase in V02max >1.0 ml/kg/ min after 6 months... 

First, it is important to consider circumstances that promote intrinsic activity. In sinus rhythm, it is necessary for the AV interval to be sufficiently short to permit capture of both ventricles. In left bundle branch block for example, if the AV interval is not sufficiently short, the right ventricle may still be activated first and it may not be possible to activate the left ventricle in time to avoid mechanical dyssynchrony. Since in sinus rhythm AV conduction is usually relatively constant from beat-to-beat it may be possible to pacing the left ventricle... 

Electrical dyssynchrony (wide QRS complex) is a surrogate marker of mechanical dyssynchrony, which is used to select patients for CRT. Mechanical dyssynchrony can occur between the atrium and the ventricle, between the RV and the LV (interventricular), and between the walls of a single ventricle (intraventricular). The baseline QRS dnration is a good marker of interventricular (RV-LV) dyssynchrony however snbsequent studies have shown that intraventricular (LV) dyssynchrony is a more accurate predictor of response to CRT (15) and this does not correlate with the baseline QRS dnration (25). 

Echocardiography is a cost-effective imaging modality that is portable, easily accessible at most medical centers, and has a high temporal resolution. It has been used as a screening tool to assess EF in patients with advanced heart failure. Most of the CRT cUnical trials have used 2D echo to assess the EF. Echo has also been used to assess mechanical dyssynchrony and to program optimal atrioventricular (AV) timing and interventricular (VV) timing after CRT implantation to achieve maximal hemodynamic benefit with biventricular... 

Echo has been used to assess aU levels of mechanical dyssynchrony dys-synchrony between the atrium and the ventricle, interventricular dyssynchrony, and intraventricular dyssynchrony. Echo Doppler imaging at the level of the mitral valve inflow shows the effects of a long AV interval (fused E and A waves with diastolic mitral regurgitation), short AV interval (truncation of A wave with loss of atrial kick), and optimal AV interval (aortic systolic flow starts at the end of A wave) (5,31). This is illustrated in Fig. 11.4. Pulsed-wave Doppler echocardiography has been used to evaluate interventricular dyssynchrony that is defined as the time difference between right and left ventricular pre-ejection intervals. This is usually measured from the onset of the QRS complex on the EKG (that correlates with the end of diastole) to the onset of the aortic and pulmonary ejection. Delayed aortic ejection time (>40-50ms) has been used as a marker of interventricular dyssynchrony that improves with CRT (32-34). 

Analysis of patients in many CRT studies has shown that 30-40% of the patients failed to respond to CRT as measured by clinical improvement (NYHA class, hospitalization from heart failure, improved exercise capacity) or more objective echocardiographic parameters (LV ejection fraction, LV volumes, mitral regurgitation) (1,11,13,36). The response to CRT cannot be reliably predicted by currently accepted EKG criteria for implantation (QRS width greater than 120 ms). In fact, many patients with widened QRS complexes do not respond while many who do respond do not show changes in their QRS complex (25). Eigure 11.7 shows the lack of correlation between the QRS duration on EKG and mechanical dyssynchrony evaluated with TDI. 

Kapetanakis S, Kearney MT, Siva A, Gall N, Cooklin M, Monaghan MJ. Real-time three-dimensional echocardiography a novel technique to quantify global left ventricular mechanical dyssynchrony. Circulation 2005 112 992-1000. 

Intraventricular conduction delay often leads to late activation of the left ventricular free wall with significant mechanical consequences. The mechanical consequences of abnormal electrical activation of the heart have long been recognized [58, 60, 86]. These include dyssynchrony between the atria. 

By shortening AV delay and reducing LV dyssynchrony, CRT can increase LV filling time, increase sttoke volume, and reduce mitral regurgitation (5,10,11). These are the underlying mechanism of acute improvement with CRT. In fact, patients frequently feel better almost immediately after implantation. Occasionally, we see patients with end-stage heart failure who cannot be weaned off inotropic support be successfully weaned 1-2 days after CRT implantation. 

QRS > 120ms with dyssynchrony (aortic preejection > 140ms, interventricular mechanical delay > 40ms, delayed activation of postlateral LV) QRS > 150ms (no dyssynchrony evidence needed)... 

Fig. 11.8 Stepwise algorithm for management of heart failure patients who are nonresponders to CRT. AV = atrioventricular CXR = chest X-ray EKG = electrocardiogram Htx = heart transplant LV = left ventricular LVAD = left ventricular assist device MR = mitral regurgitation RV = right ventricular VV = interventricular. Cardiac ischemia is evaluated in patients with ischemic cardiomyopathy. Evidence of dyssynchrony includes septal to posterior wall motion delay > 130ms, intraventricular mechanical delay >40ms, and tissue Doppler imaging > 65 ms. (Reproduced witih permission from Aranda JM, Woo GW, Schofield RS, et al. J Am Coll Cardiol 2005 46 2193-8.)...

Turner MS, Bleasdale RA, Vinereanu D, Mumford CE, Paul V, Fraser AG, Frenneaux MR Electrical and mechanical components of dyssynchrony in heart failure patients with normal QRS duration and left bundle-branch block impact of left and biventricular pacing. Circulation 2004 109 2544-9. 

Disordered electrical and mechanical ventricular activation can compromise cardiac function. Pacing technology has been nsed to att pt to correct the inter- and intraventricular conduction in an effort to optimize cardiac performance. The earliest attempts were performed during surgery when epicardial leads were placed over the lateral left ventricle free wall. Later, the coronary sinus was utilized to activate the left ventricle. Cardiac-resynchronization therapy (CRT) for treatment of patients with congestive heart failure and ventricular dyssynchrony can have a remarkable beneficial effect. Use of this technology continues to evolve. 

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