Balanced-force shim system for correcting magnetic-field inhomogeneities in the heart due to implanted cardioverter defibrillators

Background

In the US, 1.4 million people have implanted ICDs for reducing the risk of sudden death due to ventricular arrhythmias. Cardiac MRI (cMR) is of particular interest in the ICD patient population as cMR is the optimal imaging modality for distinguishing cardiac conditions that predispose to sudden death, and it is the best method to plan and guide therapy. However, all ICDs contain a ferromagnetic transformer which imposes a large inhomogeneous magnetic field in sections of the heart, creating large image voids that can mask important pathology. A shim system was devised to resolve these ICD issues. A shim coil system (CSS) that corrects ICD artifacts over a user-selected Region-of-Interest (ROI), was constructed and validated.

Methods

A shim coil was constructed that can project a large magnetic field for distances of ~15 cm. The shim-coil can be positioned safely anywhere within the scanner bore. The CSS includes a cantilevered beam to hold the shim coil. Remotely controlled MR-conditional motors allow 2 mm-accuracy three-dimensional shim-coil position. The shim coil is located above the subjects and the imaging surface-coils. Interaction of the shim coil with the scanner's gradients was eliminated with an amplifier that is in a constant current mode. Coupling with the scanners' radio-frequency (rf) coils, was reduced with shielding, low-pass filters, and cable shield traps. Software, which utilizes magnetic field (B0) mapping of the ICD inhomogeneity, computes the optimal location for the shim coil and its corrective current. ECG gated single- and multiple-cardiac-phase 2D GRE and SSFP sequences, as well as 3D ECG-gated respiratory-navigated IR-GRE (LGE) sequences were tested in phantoms and N = 3 swine with overlaid ICDs.

Results

With all cMR sequences, the system reduced artifacts from >100 ppm to <25 ppm inhomogeneity, which permitted imaging of the entire left ventricle in swine with ICD-related voids. Continuously acquired Gradient recalled echo or Steady State Free Precession images were used to interactively adjust the shim current and coil location.

Conclusion

The shim system reduced large field inhomogeneities due to implanted ICDs and corrected most ICD-related image distortions. Externally-controlled motorized translation of the shim coil simplified its utilization, supporting an efficient cardiac MRI workflow.

Additional coils mitigate elevated defibrillation threshold in right-sided implantable cardioverter defibrillator generator placement: a simulation study

AIMS

The standard implantable cardioverter defibrillator (ICD) generator (can) is placed in the left pectoral area; however, in certain circumstances, right-sided cans may be required which may increase defibrillation threshold (DFT) due to suboptimal shock vectors. We aim to quantitatively assess whether the potential increase in DFT of right-sided can configurations may be mitigated by alternate positioning of the right ventricular (RV) shocking coil or adding coils in the superior vena cava (SVC) and coronary sinus (CS).

METHODS AND RESULTS

A cohort of CT-derived torso models was used to assess DFT of ICD configurations with right-sided cans and alternate positioning of RV shock coils. Efficacy changes with additional coils in the SVC and CS were evaluated. A right-sided can with an apical RV shock coil significantly increased DFT compared to a left-sided can [19.5 (16.4, 27.1) J vs. 13.3 (11.7, 19.9) J, P < 0.001]. Septal positioning of the RV coil led to a further DFT increase when using a right-sided can [26.7 (18.1, 36.1) J vs. 19.5 (16.4, 27.1) J, P < 0.001], but not a left-sided can [12.1 (8.1, 17.6) J vs. 13.3 (11.7, 19.9) J, P = 0.099). Defibrillation threshold of a right-sided can with apical or septal coil was reduced the most by adding both SVC and CS coils [19.5 (16.4, 27.1) J vs. 6.6 (3.9, 9.9) J, P < 0.001, and 26.7 (18.1, 36.1) J vs. 12.1 (5.7, 13.5) J, P < 0.001].

CONCLUSION

Right-sided, compared to left-sided, can positioning results in a 50% increase in DFT. For right-sided cans, apical shock coil positioning produces a lower DFT than septal positions. Elevated right-sided can DFTs may be mitigated by utilizing additional coils in SVC and CS.

Defibrillation Threshold Testing for Right-sided Device Implants: A Review to Inform Shared Decision-making, in Association with the British Heart Rhythm Society

Prevention of sudden death using ICDs requires the reliable delivery of a high-energy shock to successfully terminate VF. Until more recently, the device implant procedure included conducting defibrillation threshold (DFT) testing involving VF induction and shock delivery to ensure efficacy. Large clinical trials, including SIMPLE and NORDIC ICD, have subsequently demonstrated that this is unnecessary, with a practice of omitting DFT testing having no impact on subsequent clinical outcomes. However, these studies specifically excluded patients requiring devices implanted on the right side, in whom the shock vector is significantly different and smaller studies suggest a higher DFT. In this review, the data regarding the use of DFT testing, focusing on right-sided implants, and the results of a survey of current UK practice are presented. In addition, a strategy of shared decision-making when it comes to deciding on the use of DFT testing during right-sided ICD implant procedures is proposed.

Reducing cardiac implantable electronic device-induced artefacts in cardiac magnetic resonance imaging

OBJECTIVES

Cardiac implantable electronic device (CIED)-induced metal artefacts possibly significantly diminish the diagnostic value of magnetic resonance imaging (MRI), particularly cardiac MR (CMR). Right-sided generator implantation, wideband late-gadolinium enhancement (LGE) technique and raising the ipsilateral arm to the generator during CMR scanning may reduce the CIED-induced image artefacts. We assessed the impact of generator location and the arm-raised imaging position on the CIED-induced artefacts in CMR.

METHODS

We included all clinically indicated CMRs performed on patients with normal cardiac anatomy and a permanent CIED with endocardial pacing leads between November 2011 and October 2019 in our institution (n = 171). We analysed cine and LGE sequences using the American Heart Association 17-segment model for the presence of artefacts.

RESULTS

Right-sided generator implantation and arm-raised imaging associated with a significantly increased number of artefact-free segments. In patients with a right-sided pacemaker, the median percentage of artefact-free segments in short-axis balanced steady-state free precession LGE was 93.8% (IQR 9.4%, n = 53) compared with 78.1% (IQR 20.3%, n = 58) for left-sided pacemaker (p < 0.001). In patients with a left-sided implantable cardioverter-defibrillator, the median percentage of artefact-free segments reached 87.5% (IQR 6.3%, n = 9) using arm-raised imaging, which fell to 62.5% (IQR 34.4%, n = 9) using arm-down imaging in spoiled gradient echo short-axis cine (p = 0.02).

CONCLUSIONS

Arm-raised imaging represents a straightforward method to reduce CMR artefacts in patients with left-sided generators and can be used alongside other image quality improvement methods. Right-sided generator implantation could be considered in CIED patients requiring subsequent CMR imaging to ensure sufficient image quality.

KEY POINTS

• Cardiac implantable electronic device (CIED)-induced metal artefacts may significantly diminish the diagnostic value of an MRI, particularly in cardiac MRIs. • Raising the ipsilateral arm relative to the CIED generator is a cost-free, straightforward method to significantly reduce CIED-induced artefacts on cardiac MRIs in patients with a left-sided generator. • Right-sided generator implantation reduces artefacts compared with left-sided implantation and could be considered in CIED patients requiring subsequent cardiac MRIs to ensure adequate image quality in the future.

Influence of "high" defibrillation thresholds on patient survival and impact of system modification

OBJECTIVE

To test whether a high defibrillation threshold (DFT) marks patients with poor outcomes which are improved when DFT is decreased by system modification (subcutaneous coil implant; SM).

BACKGROUND

The electrical substrate generating fast ventricular arrhythmias may generate poor outcomes among patients treated with implantable cardioverter-defibrillators (ICDs), even when arrhythmias are treated successfully. Since patients with high DFTs have increased mortality, we contrasted survival among patients with high DFT treated with and without SM.

METHODS

We studied consecutive patients undergoing ICD implantation and DFT testing at Cleveland Clinic over a 14-year period. High DFT was defined as successful defibrillation by shock strength >25 J or ≤10 J of maximal device output. Mortality was recorded using the Social Security Death Index. Survival was compared among those high DFT patients receiving SM versus the remainder.

RESULTS

Out of 6353 patients tested, 191 (3%) had high DFT (32.1 ± 3.7 J) versus 13.9 ± 4.9 J in the remainder ("acceptable DFT," p < .001). One hundred twenty-one high DFT patients (63%; 33.3 ± 3.4 J) underwent SM, which significantly decreased DFT (24.8 ± 5.9 J; p < .001). Seventy patients (37%; 30.3 ± 3.3 J) did not undergo SM. During follow-up, 38% (2363/6162; 7.8 yrs) patients with acceptable DFT died versus 48% high DFT patients (91/191; 5.6 yrs.; p < .001). Concomitantly, 48% patients with SM (58/121) died, as compared to 47% patients (33/70) without SM (p = .91); median follow-up 4.9 yrs).

CONCLUSION

Patients with high DFT have a higher mortality than those with acceptable DFT. The additional subcutaneous coil implant decreases DFT to an acceptable range but does not appear to improve survival. The electrical substrate underlying high DFT appears to determine survival.

EHRA expert consensus statement and practical guide on optimal implantation technique for conventional pacemakers and implantable cardioverter-defibrillators: endorsed by the Heart Rhythm Society (HRS), the Asia Pacific Heart Rhythm Society (APHRS), and the Latin-American Heart Rhythm Society (LAHRS)

With the global increase in device implantations, there is a growing need to train physicians to implant pacemakers and implantable cardioverter-defibrillators. Although there are international recommendations for device indications and programming, there is no consensus to date regarding implantation technique. This document is founded on a systematic literature search and review, and on consensus from an international task force. It aims to fill the gap by setting standards for device implantation.

Should Single-Coil Implantable Cardioverter Defibrillator Leads Be Used in all Patients?

The historical preference for dual-coil implantable cardioverter defibrillator leads stems from high defibrillation thresholds associated with old device platforms. The high safety margins generated by contemporary devices have rendered the modest difference in defibrillation efficacy between single- and dual-coil leads clinically insignificant. Cohort data demonstrating worse lead extraction outcomes and higher all-cause mortality have brought the incremental utility of an superior vena cava coil into question. This article summarizes the current literature and re-evaluates the utility of dual-coil leads in the context of modern device technology.