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Cardiac imaging plays a pivotal role in CRT

EuroEcho-Imaging 2017

Congress News provides an overview of the latest insights and technology developments in imaging for patients undergoing cardiac resynchronisation therapy (CRT).

Non-invasive Diagnostic Techniques

sade-leyla-2017.jpgKey studies in cardiac imaging and CRT in the past year have found that CRT use is lower than indicated and that response rate is below that ideally expected among those treated. Researchers are in the pursuit of expanding their identification of populations who could benefit from CRT, and refining patient selection, as well as exploring novel pacing approaches and technologies to augment the efficacy of resynchronisation.

Professor Leyla Elif Sade (Baskent University, Ankara, Turkey) explains: “Recent studies have focused on mechanical dyssynchrony by taking into account wasted myocardial work, indices of regional stretch, and constructive work with the use of strain analyses and haemodynamic data. These may better predict response than simple time difference measurements.”

The role of imaging in patient selection is to identify myocardium amenable to resynchronisation, which can also predict prognosis. Echocardiographic strain imaging can define and quantify myocardium with recruitable contractile reserve, and strain imaging and late gadolinium enhanced (LGE) magnetic resonance (MR) imaging accurately reflect ischaemia and scar burden, which significantly modulates response to CRT. Patient selection is complemented by an image-guided targeted approach to left ventricle (LV) lead placement to the site of latest mechanical activation, remote from scar, to optimise CRT outcome.

For example, EURO-CRT, a multicentre, multimodal prospective registry initiated in 2017 by the European Association of Cardiovascular Imaging (EACVI) echo section, uses speckle tracking echo and assessment of myocardial scar using late gadolinium hyperenhancement cardiac magnetic resonance (MR) imaging to predict response to CRT. “A multimodal approach can tremendously optimise CRT performance,” Prof. Sade points out, adding: “Identification of the site is feasible by echo, localisation of the scar is best performed by cardiac MR imaging, and computed tomography (CT) may accurately show cardiac vein anatomy prior to implantation.”

Also, PROMISE-CRT, a large multicentre retrospective study, last year released what Prof. Sade calls “promising results for a readily applicable parameter” on the use of septal flash and apical rocking, although she warns, “caution is needed until this is validated in prospective studies”.

Other recent findings reinforce the theory that mechanical dyssynchrony after CRT is an important response parameter. Ongoing work is of value, according to Prof. Sade, “because the 2016 European Society of Cardiology heart failure guidelines introduced more stringent criteria for QRS duration in CRT candidates, resulting in lower use of CRT without increasing response rate”.

The merits of imaging have not been settled enough so far; the only echo criterion has been left ventricular ejection fraction in addition to the pattern and duration of QRS on ECG, but these can be incomplete for assigning patients to CRT.

Prof. Sade believes this is because myocardial disease alters local electrical properties of excitability so that the activation pattern in heart failure and left bundle branch block (LBBB) is heterogeneous and unpredictable, challenging resynchronisation. If fibrosis or other damage in the failing heart abolishes the typical LBBB contraction pattern, there will be no suitable mechanical substrate for CRT. Furthermore, severely dilated and very poorly contracting fibrotic ventricles are unlikely to be resynchronised and reverse remodelled by CRT.

MR imaging

Worldwide, approximately 5 million people currently have a pacemaker or implantable cardioverter-defibrillator (ICD), and an indication to undergo MR imaging potentially occurs in >50% of these patients over the device’s lifetime. The safety of MR imaging-conditional devices has been demonstrated at 1.5T. But off-label MR imaging scanning with non-MR imaging conditional devices is “unsafe” from a practical perspective, cautions Prof. Sade, particularly in pacemaker-dependent patients and those with ICDs. Possible adverse effects include tissue heating of the lead tips, pacing inhibition or dysfunction, induction of atrial or ventricular tachyarrhythmias, change or loss of programmed data, and changes in capture thresholds. However, MR imaging may be clinically indispensable for patient management if there is a need for follow-up with serial MR imaging—for example, in the case of tumours, musculoskeletal disease, or central nervous system disease.

Prof. Sade says: “The benefits of MR imaging might outweigh the risks, but alternative imaging techniques have to be considered. ESC guidelines indicate the use of MR imaging at 1.5T with a low risk of complications, provided appropriate precautions are taken to minimise risks, including supervision and continuous monitoring by an electrophysiologist.”

Patients with leads that have not matured (<6 weeks since implantation, prone to spontaneous dislodgement) and those with epicardial and abandoned leads should not undergo off-label MR imaging under any circumstances. As yet, no information exists for MR imaging at 3T, even for MR imaging-compatible devices.

Imaging also plays a role in monitoring patients by assessing the response to CRT and analysing risk prediction afterwards. The lack of reverse remodelling is associated with relatively poor survival compared to those with significant reverse remodelling. Persistent, in particular worsened, dyssynchrony after CRT is associated with an increased ventricular arrhythmic event or death rate, and scar extent and location is another predictor of prognosis that can help fine tune lead placement and patient management.

Image-guided adjustment of atrioventricular and ventriculo-ventricular delays may help to increase the rate of responders to CRT by facilitating the haemodynamic augmentation in selected patients with suboptimal response. Changes in atrial and right ventricular function and mitral regurgitation are also important response modifiers to track by imaging.

Several sessions at EuroEcho-Imaging focus on important issues in CRT. One of these, “Do you understand CRT?” (held yesterday) ran through the main stages. “Patient selection, guidance of the procedure for lead implantation, post-implant care, and assessment of CRT success are integral to understanding CRT,” outlines Prof. Sade. “Imaging plays a pivotal role in all of these steps.”

And, the added value of imaging was demonstrated in her talk yesterday, which was called “Standards and options - How to predict and measure CRT success”. It provided a review of CRT.

“Response to CRT is a complex interaction between myocardial properties, the pattern of regional electromechanical activation, clinical characteristics of the patient, and implant-related factors such as LV lead position,” Prof. Sade explains. “ECG criteria are oversimplifying the application of CRT and are incomplete for predicting success, while imaging has important implications for defining the myocardial substrate properties and mechanical discoordination, refining patient selection, and assessing response to CRT.”

Data from MADIT-CRT demonstrate CRT effect is valid regardless of comorbidities, underlining the importance of conceiving reverse remodelling as a relatively “continuous parameter that seems to have no cut-off value to dichotomise patients into responder or non-responder”, she says, and which depends on the severity and natural progression of the underlying disease.

But defining the success of CRT is “a moving target”, according to Prof. Sade. She notes: “Reverse remodeling can be a meaningful target for treatment in patients with mild symptoms, whereas a reduced hospitalisation rate or symptom improvement can be a meaningful target for class IV ambulatory patients with severely dilated ventricles.”