Multiparametric Cardiovascular Magnetic Resonance Assessment of Pacemaker-Induced Alterations in Ventricular Activation and Function
A 73-year-old woman with presyncope and frequent episodes of symptomatic 2:1 atrioventricular block underwent implantation of a dual-chamber MRI-conditional pacing system (Advisa MRI SureScan pacing system, Medtronic, Minneapolis, MN) to facilitate cardiovascular magnetic resonance (CMR) assessment of apparent isolated right ventricular (RV) hypertrophy seen on transthoracic echocardiography (Figure 1 and online-only Data Supplement Movies I and II). The ventricular lead was positioned in the outflow tract. This pacing system is approved, outside of the United States, for MRI without isocenter restriction, provided certain conditions are fulfilled, including a static magnetic field strength of 1.5 T, a maximum gradient slew rate ≤200 T/m per second, whole body average specific absorption rate levels <2.0 W/kg, and a minimum period of 6 weeks between implantation and scanning. Within the United States, the isocenter is presently required to be either superior to C1 or inferior to T12 vertebrae.
CMR assessment was performed in accordance with the stated conditions. The impact of pacing on the activation pattern and regional deformation of both ventricles, and aortic and pulmonic forward flow, were studied by imaging in unpaced and paced (VVO; 75 bpm) modes.
On balanced steady-state free precession imaging marked susceptibility artifact led to almost nondiagnostic image quality (Figure 2A and online-only Data Supplement Movie III). Switching to a spoiled gradient echo sequence greatly improved image quality (Figure 2B and online-only Data Supplement Movie IV) and reduced the specific absorption rate.
Assessment of myocardial displacement, using spatial modulation of magnetization (tagging) images with sine wave–based analysis, demonstrated a synchronous pattern of left ventricular (LV) activation when imaging was performed during underlying sinus rhythm, ie, unpaced mode (Figure 3A and 3C and online-only Data Supplement Movie V), but a markedly dyssynchronous pattern during pacing (Figure 3B and 3D and online-only Data Supplement Movie VI). Simultaneous LV and RV myocardial deformation assessment with an endocardial feature-tracking algorithm, applied using spoiled gradient echo cines, revealed a detrimental effect of pacing on biventricular function and synchronicity (Figures 4 and 5). Speckle-tracking echocardiography of the LV revealed analogous findings (Figure 6). As a likely consequence, pacing was associated with a marked acute reduction in per-beat aortic and pulmonic forward-flow volume (Figure 7), assessed using phase-contrast velocity mapping of the aorta and main pulmonary artery, respectively. Blood pressure changed from 136/84 during unpaced mode to 133/83 during pacing.
RV wall thickness was found to be normal; overlying epicardial fat (epicardial lipomatosis) was felt to have accounted for the echocardiographic findings. Pacemaker parameters were unaffected by scanning.
RV apical pacing adversely affects LV systolic function in patients with a bradyarrhythmic indication for pacing and normal baseline ejection fraction.1 Furthermore, in patients with impaired LV systolic function, RV pacing has a detrimental impact on heart failure and mortality.2 The negative effect appears to be related to the cumulative percentage of paced beats. The widely proposed underlying mechanism is that the abnormal pattern of LV electric activation associated with RV pacing leads to dyssynchronous ventricular contraction, as illustrated here, and subsequent cardiac remodeling, including asymmetrical hypertrophy, mitral regurgitation, increased left atrial size, and reduced ejection fraction.3
Echocardiographic parameters of LV dyssynchrony have proved to have only modest utility in cardiac resynchronization therapy patient selection and optimization, largely owing to low reproducibility.4 As is demonstrated by this case, CMR acquisition sequences and image analysis techniques allow simultaneous visualization and quantification of the activation pattern and regional function of both ventricles, thereby enabling a more comprehensive assessment of the impact of pacing. Because CMR-based quantification methods display generally higher reproducibility, it is possible that when MRI-compatible cardiac resynchronization therapy devices become available, these techniques, in conjunction with phase-contrast velocity mapping of aortic and pulmonic flow, may better guide cardiac resynchronization therapy patient selection and optimization.
Sources of Funding
Dr Miller is supported by a Doctoral Research Fellowship from the National Institute for Health Research (United Kingdom).
Dr Schmitt is the United Kingdom Principle Investigator for the Medtronic-sponsored ADVISA study (the patient presented is not partaking in the study) and has received speaker fees and hospitality from St. Jude and Medtronic.
The online-only Data Supplement is available with this article at http://circ.ahajournals.org/lookup/suppl/doi:10.1161/CIRCULATIONAHA.112.092429/-/DC1.
- © 2012 American Heart Association, Inc.
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