Non-Contact Intracardiac Potential Mapping Using Mesh-Based and Meshless Inverse Solvers.

Atrial fibrillation (AF) is the most common cardiac dysrhythmia and percutaneous catheter ablation is widely used to treat it. Panoramic mapping with multi-electrode catheters has been used to identify ablation targets in persistent AF but is limited by poor contact and inadequate coverage of the left atrial cavity. In this paper, we investigate the accuracy with which atrial endocardial surface potentials can be reconstructed from electrograms recorded with non-contact catheters. An in-silico approach was employed in which "ground-truth" surface potentials from experimental contact mapping studies and computer models were compared with inverse potential maps constructed by sampling the corresponding intracardiac field using virtual basket catheters. We demonstrate that it is possible to 1) specify the mixed boundary conditions required for mesh-based formulations of the potential inverse problem fully, and 2) reconstruct accurate inverse potential maps from recordings made with appropriately designed catheters. Accuracy improved when catheter dimensions were increased but was relatively stable when the catheter occupied >30% of atrial cavity volume. Independent of this, the capacity of non-contact catheters to resolve the complex atrial potential fields seen in reentrant atrial arrhythmia depended on the spatial distribution of electrodes on the surface bounding the catheter. Finally, we have shown that reliable inverse potential mapping is possible in near real-time with meshless methods that use the Method of Fundamental Solutions.

Intracardiac Inverse Potential Mapping Using the Method of Fundamental Solutions.

Introduction: Atrial fibrillation (AF) is the most prevalent cardiac dysrhythmia and percutaneous catheter ablation is widely used to treat it. Panoramic mapping with multi-electrode catheters can identify ablation targets in persistent AF, but is limited by poor contact and inadequate coverage. Objective: To investigate the accuracy of inverse mapping of endocardial surface potentials from electrograms sampled with noncontact basket catheters. Methods: Our group has developed a computationally efficient inverse 3D mapping technique using a meshless method that employs the Method of Fundamental Solutions (MFS). An in-silico test bed was used to compare ground-truth surface potentials with corresponding inverse maps reconstructed from noncontact potentials sampled with virtual catheters. Ground-truth surface potentials were derived from high-density clinical contact mapping data and computer models. Results: Solutions of the intracardiac potential inverse problem with the MFS are robust, fast and accurate. Endocardial surface potentials can be faithfully reconstructed from noncontact recordings in real-time if the geometry of cardiac surface and the location of electrodes relative to it are known. Larger catheters with appropriate electrode density are needed to resolve complex reentrant atrial rhythms. Conclusion: Real-time panoramic potential mapping is feasible with noncontact intracardiac catheters using the MFS. Significance: Accurate endocardial potential maps can be reconstructed in AF with appropriately designed noncontact multi-electrode catheters.

Cardiac intramural electrical mapping reveals focal delays but no conduction velocity slowing in the peri-infarct region.

Altered electrical behavior alongside healed myocardial infarcts (MIs) is associated with increased risk of sudden cardiac death. However, the multidimensional mechanisms are poorly understood and described. This study characterizes, for the first time, the intramural spread of electrical activation in the peri-infarct region of chronic reperfusion MIs. Four sheep were studied 13 wk after antero-apical reperfusion infarction. Extracellular potentials (ECPs) were recorded in a ~20 × 20-mm2 region adjacent to the infarct boundary (25 plunge needles <0.5-mm diameter with 15 electrodes at 1-mm centers) during multisite stimulation. Infarct geometry and electrode locations were reconstructed from magnetic resonance images. Three-dimensional activation spread was characterized by local activation times and interpolated ECP fields (n = 191 records). Control data were acquired in 4 non-infarcted sheep (n = 96 records). Electrodes were distributed uniformly around 15 ± 5% of the intramural infarct boundary. There were marked changes in pacing success and ECP morphology across a functional border zone (BZ) ±2 mm from the boundary. Stimulation adjacent to the infarct boundary was associated with low-amplitude electrical activity within the BZ and delayed activation of surrounding myocardium. Bulk tissue depolarization occurred 3.5-14.6 mm from the pacing site for 39% of stimuli with delays of 4-37 ms, both significantly greater than control (P < 0.0001). Conduction velocity (CV) adjacent to the infarct was not reduced compared with control, consistent with structure-only computer model results. Insignificant CV slowing, irregular stimulus-site specific activation delays, and obvious indirect activation pathways strongly suggest that the substrate for conduction abnormalities in chronic MI is predominantly structural in nature.NEW & NOTEWORTHY Intramural in vivo measurements of peri-infarct electrical activity were not available before this study. We use pace-mapping in a three-dimensional electrode array to show that a subset of stimuli in the peri-infarct region initiates coordinated myocardial activation some distance from the stimulus site with substantial associated time delays. This is site dependent and heterogeneous and occurs for <50% of ectopic stimuli in the border zone. Furthermore, once coordinated activation is initiated, conduction velocity adjacent to the infarct boundary is not significantly different from control. These results give new insights to peri-infarct electrical activity and do not support the widespread view of uniform electrical remodeling in the border zone of chronic myocardial infarcts, with depressed conduction velocity throughout.

How Accurate Is Inverse Electrocardiographic Mapping? A Systematic In Vivo Evaluation.

BACKGROUND: Inverse electrocardiographic mapping reconstructs cardiac electrical activity from recorded body surface potentials. This noninvasive technique has been used to identify potential ablation targets. Despite this, there has been little systematic evaluation of its reliability. METHODS: Torso and ventricular epicardial potentials were recorded simultaneously in anesthetized, closed-chest pigs (n=5), during sinus rhythm, epicardial, and endocardial ventricular pacing (70 records in total). Body surface and cardiac electrode positions were determined and registered using magnetic resonance imaging. Epicardial potentials were reconstructed during ventricular activation using experiment-specific magnetic resonance imaging-based thorax models, with homogeneous or inhomogeneous (lungs, skeletal muscle, fat) electrical properties. Coupled finite/boundary element methods and a meshless approach based on the method of fundamental solutions were compared. Inverse mapping underestimated epicardial potentials >2-fold (P<0.0001). RESULTS: Mean correlation coefficients for reconstructed epicardial potential distributions ranged from 0.60±0.08 to 0.64±0.07 across all methods. Epicardial electrograms were recovered with reasonable fidelity at ≈50% of sites (median correlation coefficient, 0.69-0.72), but variation was substantial. General activation spread was reproduced (median correlation coefficient, 0.72-0.78 for activation time maps after spatio-temporal smoothing). Epicardial foci were identified with a median location error ≈16 mm (interquartile range, 9-29 mm). Inverse mapping with meshless method of fundamental solutions was better than with finite/boundary element methods, and the latter were not improved by inclusion of inhomogeneous torso electrical properties. CONCLUSIONS: Inverse potential mapping provides useful information on the origin and spread of epicardial activation. However the spatio-temporal variability of recovered electrograms limit resolution and must constrain the accuracy with which arrhythmia circuits can be identified independently using this approach.

A minimal or maximal ablation strategy to achieve pulmonary vein isolation for paroxysmal atrial fibrillation: a prospective multi-centre randomized controlled trial (the Minimax study).

AIMS: Pulmonary vein isolation (PVI) is the cornerstone of catheter ablation of atrial fibrillation (AF). The intervenous ridge (IVR) may be incorporated into ablation strategies to achieve PVI; however, randomized trials are lacking. We performed a randomized multi-centre international study to compare the outcomes of (i) circumferential antral PVI (CPVI) alone (minimal) vs. (ii) CPVI with IVR ablation to achieve individual PVI (maximal). METHODS AND RESULTS: Two hundred and thirty-four patients with paroxysmal AF underwent CPVI and were randomized to a minimal or maximal ablation strategy. The primary outcome of recurrent atrial arrhythmia was assessed with 7-day Holter monitoring at 6 and 12 months. PVI was achieved in all patients. Radiofrequency ablation time was longer in the maximal group (46.6 ± 14.6 vs. 41.5 ± 13.1 min; P < 0.01), with no significant differences in procedural or fluoroscopy times. At mean follow-up of 17 ± 8 months, there was no difference in freedom from AF after a single procedure between a minimal (70%) and maximal ablation strategy (62%; P = 0.25). In the minimal group, ablation was required on the IVR to achieve electrical isolation in 44%, and was associated with a significant reduction in freedom from AF (57%) compared with the minimal group without IVR ablation (80%; P < 0.01). CONCLUSION: There was no statistically significant difference in freedom from AF between a minimal and maximal ablation strategy. Despite attempts to achieve PVI with antral ablation, IVR ablation is commonly required. Patients in whom antral isolation can be achieved without IVR ablation have higher long-term freedom from AF (the Minimax study; ACTRN12610000863033).

Forward problem of electrocardiography: is it solved?

BACKGROUND: The relationship between epicardial and body surface potentials defines the forward problem of electrocardiography. A robust formulation of the forward problem is instrumental to solving the inverse problem, in which epicardial potentials are computed from known body surface potentials. Here, the accuracy of different forward models has been evaluated experimentally. METHODS AND RESULTS: Body surface and epicardial potentials were recorded simultaneously in anesthetized closed-chest pigs (n=5) during sinus rhythm, and epicardial and endocardial ventricular pacing (65 records in total). Body surface potentials were simulated from epicardial recordings using experiment-specific volume conductor models constructed from magnetic resonance imaging. Results for homogeneous (isotropic electric properties) and inhomogeneous (incorporating lungs, anisotropic skeletal muscle, and subcutaneous fat) forward models were compared with measured body surface potentials. Correlation coefficients were 0.85±0.08 across all animals and activation sequences with no significant difference between homogeneous and inhomogeneous solutions (P=0.85). Despite this, there was considerable variance between simulated and measured body surface potential distributions. Differences between the body surface potential extrema predicted with homogeneous forward models were 55% to 78% greater than observed (P<0.05) and attenuation of potentials adjacent to extrema were 10% to 171% greater (P<0.03). The length and orientation of the vector between potential extrema were also significantly different. Inclusion of inhomogeneous electric properties in the forward model reduced, but did not eliminate these differences. CONCLUSIONS: These results demonstrate that homogeneous volume conductor models introduce substantial spatial inaccuracies in forward problem solutions. This probably affects the precision of inverse reconstructions of cardiac potentials, in which this assumption is made.

Novel methods for characterization of paroxysmal atrial fibrillation in human left atria.

INTRODUCTION: More effective methods for characterizing 3D electrical activity in human left atrium (LA) are needed to identify substrates/triggers and microreentrant circuit for paroxysmal atrial fibrillation (PAF). We describe a novel wavelet-based approach and wave-front centroid tracking that have been used to reconstruct regional activation frequency and electrical activation pathways from non-contact multi-electrode array. METHODS: Data from 13 patients acquired prior to ablation for PAF with a 64 electrode noncontact catheter positioned in the LA were analysed. Unipolar electrograms were reconstructed at 2048 locations across each LA endocardial surface. Weighted fine- and coarse-scale electrograms were constructed by wavelet decomposition and combined with peak detection to identify atrial fibrillation (AF) activation frequency and fractionated activity at each site. LA regions with upper quartile AF frequencies were identified for each patient. On the other hand, a wave-front centroid tracking approach was introduced for this first time to detect macro-reentrant circuit during PAF. RESULTS: The results employing wavelet-based analysis on atrial unipolar electrograms are validated by the signals recorded simultaneously via the contacted ablation catheter and visually tracking the 3D spread of activation through the interest region. Multiple connected regions of high frequency electrical activity were seen; most often in left superior pulmonary vein (10/12), septum (9/12) and atrial roof (9/12), as well as the ridge (8/12). The wave-front centroid tracking approach detects a major macro circuit involving LPVs, PLA, atrial floor, MV, septum, atrial roof and ridge. The regions with high frequency by wave-front tracking are consistent with the results using wavelet approach and our clinical observations. CONCLUSIONS: The wavelet-based technique and wave-front centroid tracking approach provide a robust means of extracting spatio-temporal characteristics of PAF. The approach could facilitate accurate identification of pro-arrhythmic substrate and triggers, and therefore, to improve success rate of catheter ablation for AF.

Geographic, ethnic and socioeconomic factors influencing access to implantable cardioverter defibrillators (ICDs) in New Zealand.

BACKGROUND: We examined equity of access to implanted cardioverter defibrillators (ICDs) in New Zealand in 2010 by district health board (DHB), ethnicity and socioeconomic status. METHODS: All new ICD recipients in 2010 were examined according to home district health board, ethnicity according to the national health database, and socioeconomic status using the NZDep index. RESULTS: During 2010, 352 new ICDs were implanted nationwide, giving an overall implantation rate of 80.6/million. However, implant rates varied significantly across the 20 DHBs with the highest implant rate observed in Tairawhiti at 192.3/million, and the lowest at 22/million in the Nelson region. There was also significant variation in implant rate by ethnicity, with Maori ethnicity at an implant rate of 114/million, European patients at 83/million, Pacific Island patients at 47/million and Asian patients an implant rate of 32/million. There was no significant difference in number of implants by socioeconomic decile. CONCLUSIONS: The variance in implantation rate by district health board and by ethnicity suggests that access to ICD therapy is not equitable in New Zealand. Investigation into causes of inequity of access is required.

An unmet need for implantable cardioverter-defibrillators in New Zealand.

AIMS: This study examined the prior history of all patients presenting to the regional ambulance service with community cardiac arrest to determine what proportion of these patients had prior indications for implanted cardioverter-defibrillator (ICD) therapy. METHODS AND RESULTS: We reviewed the medical history of all adult patients presenting to our regional ambulance service with cardiac arrest between 1 June 2007 and 31 May 2008 (n= 144). Patients were classified as either not having an ICD indication, having a possible ICD indication, or having an ICD indication by two electrophysiologists. Eighty-seven patients (60%) had no pre-existing indication for an ICD. Twenty-two patients (15%) had a possible indication for an ICD but required further investigation to confirm this. This group consisted of 6 patients (4%) with previously documented left ventricular ejection fraction <35%, but without a measurement in the last 12 months, 14 patients (10%) with heart failure (n= 10) or syncope (n= 4) without appropriate investigations, and 2 patients with an ICD indication but with co-morbidities that required further investigation. Thirty-five patients (24%) had a documented indication for an ICD. In 11 (8%) there was no evidence of a contraindication, in 3 (2%) alternative therapy was judged more appropriate, and in 21 (15%) contraindications to ICD implantation were also present. Addition of the 11 patients with an ICD indication and the 6 patients with a documented indication requiring updated measurement, 17 patients (12%) had a prior documented ICD indication but had not been referred for this therapy. CONCLUSIONS: Our observation that 12% of sudden cardiac arrest patients had prior indications for an ICD demonstrates that there is an unmet need for ICDs in New Zealand.

Use of implantable cardioverter defibrillators in the New Zealand context from 2000 to 2007.

BACKGROUND: Implantable Cardioverter Defibrillator (ICD) therapy is now standard of care for prevention of sudden cardiac death in high-risk patient groups. In order to determine if the potential benefit of ICD therapy is being realised, ongoing monitoring of ICD therapy is required. This study was conducted to examine ICD therapy in two New Zealand tertiary hospitals. METHODS: We retrospectively audited patient notes for all patients receiving a first ICD between 2000 and 2007 at two tertiary referral hospitals in New Zealand. RESULTS: 702 patients received their first ICD within the study period, 73% male, mean age 53 years (range 1 to 83), with 73% of devices for secondary prevention. The implant rate increased from 15/million in 2000 to a peak of 44/million in 2004. Antitachycardia pacing was delivered to 21% of patients, appropriate defibrillation to 26% and inappropriate defibrillation to 16% of patients, with frequency of all types of therapy increasing with time since implantation. All cause mortality was 8.6%, and only 7 (1%) died as a consequence of sudden cardiac arrest. CONCLUSIONS: While increasing across the study period, the ICD implant rate remains low, with a high therapy rate and low mortality rate. This suggests that those receiving ICD therapy are benefiting, but may also imply that the group of patients receiving ICDs is too restricted.

A national survey of clinician's knowledge of and attitudes towards implantable cardioverter defibrillators.

AIMS: This study surveyed referring clinicians to identify barriers that may contribute to New Zealand's low national implantable cardioverter defibrillator (ICD) implant rate. METHODS: We conducted a telephone survey of 100 cardiologists and general physicians working at 30 different New Zealand hospitals who routinely manage patients with ischaemic heart disease and heart failure. RESULTS: The majority of those surveyed (76%) rated their knowledge as satisfactory or better, although only 62% reported familiarity with international guidelines for ICD therapy. When asked to identify ICD indications 80% identified symptomatic or sustained ventricular arrhythmias and 73% left ventricular dysfunction. While 82% believed that the use of ICD therapy for secondary prevention was cost effective, only 53% believed they were cost effective for primary prevention. Lack of financial resource (88%), lack of local expertise (61%), lack of New Zealand guidelines (51%), and the referral process (43%) were seen as significant barriers to ICD referral by many participants. The majority of rural clinicians (71%) identified restricted access to investigations as a barrier to implantation, significantly higher than urban clinicians (18%, P = 0.001). CONCLUSION: We have identified a number of potential barriers that will need to be addressed to raise the New Zealand ICD implantation rate.

Ventricular fibrillation frequency from implanted cardioverter defibrillator devices.

AIMS: The dominant frequency (DF) of ventricular fibrillation (VF) provides a measure of cycle length that may relate to the underlying complexity of the arrhythmia. Dominant frequency analysis may therefore provide insights into VF mechanisms, and potentially guide future therapies. Dominant frequency analysis can be undertaken on stored electrograms (EGMs) from implanted cardioverter defibrillator devices (ICDs). Demonstration of the reproducibility of the DF during separate VF events is necessary before using this tool. METHODS AND RESULTS: We identified 82 patients receiving a Medtronic ICD who had two episodes of VF induced during ICD testing. We extracted EGMs recorded during both episodes and determined DF using the fast Fourier transform. The mean DF for the population was 4.7 +/- 0.6 Hz, corresponding to a cycle length of 213 ms. First and second episodes of VF were very highly correlated (interclass correlation = 0.87, P < 0.01) demonstrating that DF was highly reproducible. The 18 patients on Class III agents had a significantly lower DF than the remaining 63 (4.4 +/- 0.4 vs. 4.8 +/- 0.6 Hz, P < 0.01, n = 18). However, the DF of patients with ischaemic heart disease (n = 34) did not differ when compared with dilated cardiomyopathy patients (n = 25) (4.7 +/- 0.6 vs. 4.6 +/- 0.4 Hz, P = 0.3). CONCLUSION: The DF of short intervals of induced VF is highly reproducible and is sensitive to pharmacological interventions that extend effective refractory period. Such estimates of DF may therefore have clinical utility and in patients with ICDs provide a means of investigating mechanisms underlying the initiation and early phases of VF.

Are our medical graduates in New Zealand safe and accurate in ECG interpretation?

AIM: We aimed to assess the skills of final year medical students and resident medical officers in recognising and interpreting important common or life-threatening abnormalities in the electrocardiogram (ECG). METHODS: 102 participants at two study sites (52 of whom were final year medical students) attempted to determine the heart rate and rhythm and identify and interpret any abnormalities present in 15 ECGs in a 30-minute time period. RESULTS: Accurate determination of heart rate was poor, ranging from 0% to 89% correct across the 15 ECGs. Normal sinus rhythm in 8 ECGs was identified 81% to 95% of the time, and ventricular tachycardia was identified by 98% of participants. Atrial fibrillation (55%), second degree heart block (19%) and ventricular pacing (9%) were not well identified. Four ECGs showed acute ischaemic ST segment changes, and these were correctly identified in 87% to 93% of cases, although interpretation of these abnormalities was less accurate. Long QT interval (7%) and pre-excitation (WPW pattern, 11%) were not well recognised. Nearly half of the participants rated their ability to interpret ECGs as less than satisfactory while just over half rated the ECG teaching they had received as less than satisfactory. CONCLUSIONS: Overall study participants did not achieve what we would consider an adequate standard in recognising and interpreting important common or life-threatening abnormalities in the ECG. To address this we need to define minimum standards in ECG interpretation, to improve our teaching to meet these standards, and to assess our graduates against these.

Differences in the characteristics of induced and spontaneous episodes of ventricular fibrillation.

AIMS: The degree of organization of ventricular fibrillation (VF) can be examined in terms of the regularity of the electrical activity within the ventricle. Using electrograms (EGMs) stored within implanted cardioverter defibrillators (ICDs), we examined the hypothesis that the degree of organization, or regularity, was different if the VF was induced by electrical stimulation as opposed to occurring clinically due to ischemia or scar. METHODS AND RESULTS: We compared the statistical characteristics of EGMs recorded by ICDs during spontaneous episodes with those induced during device testing in the laboratory in nine subjects. Regularity of the VF EGM signals was quantified using autocorrelation, Shannon entropy (derived from cycle to cycle activation complexes), and Kolmogorov entropy (derived from eight second long episodes of VF). All three measurements showed a statistically greater degree of regularity for induced VF than in spontaneous episodes. CONCLUSION: Analysis of VF EGMs using these techniques is novel and robust, providing a new way for assessing electrical organization during VF. The clinical significance and utility of differences in VF waveform regularity is unclear at this stage.