Generation of Monophasic Action Potentials and Intermediate Forms.

The monophasic action potential (MAP) is a near replica of the transmembrane potential recorded when an electrode is pushed firmly against cardiac tissue. Despite its many practical uses, the mechanism of MAP signal generation and the reason it is so different from unipolar recordings are not completely known and are a matter of controversy. In this work, we describe a method to simulate realistic MAP and intermediate forms, which are multiphasic electrograms different from an ideal MAP. The key ideas of our method are the formation of compressed zones and junctional spaces-regions of the extracellular and bath or blood pool directly in contact with electrodes that exhibit a pressure-induced reduction in electrical conductivity-and the presence of a complex network of passive components that acts as a high-pass filter to distort and attenuate the signal that reaches the recording amplifier. The network is formed by the interaction between the passive tissue properties and the double-layer capacitance of electrodes. The MAP and intermediate forms reside on a continuum of signals, which can be generated by the change of the model parameters. Our model helps to decipher the mechanisms of signal generation and can lead to a better design for electrodes, recording amplifiers, and experimental setups.

Clinical Performance of Magnetic Resonance Imaging Conditional and Nonconditional Cardiac Implantable Electronic Devices.

OBJECTIVES: This study compared risks associated with magnetic resonance imaging (MRI) in patients with non-MRI conditional and MRI conditional pacing and defibrillator systems with particular attention to clinically actionable outcomes. BACKGROUND: While recipients of new MRI conditional pacemaker and defibrillator systems may undergo MRI scanning with very low risk, safety and regulatory concerns persist regarding such scanning in recipients of non-MRI conditional systems. METHODS: Patients with any cardiac device who were referred for MRI were prospectively enrolled at a single center and underwent scanning at 1.5 Tesla. Pre- and postscan lead characteristic changes, system integrity, and symptoms were analyzed. A comparison was made between non-MRI conditional and MRI conditional devices. RESULTS: 105 patients were evaluated allowing for comparison of 97 scans with non-MRI conditional devices and 16 scans with MRI conditional devices. The cohort included those with pacemaker dependency, defibrillator, and cardiac resynchronization devices. Small, nonsignificant changes were observed in lead characteristics following scanning, and there was no significant difference when comparing non-MRI and MRI conditional devices. Lead parameter changes did not require lead revision or programming changes. No device reset, failures, or premature scan termination was observed. CONCLUSIONS: 1.5 T MRI scanning in patients with MRI conditional and non-MRI conditional cardiac devices was performed with similar, low clinical risk.

Critical phase transitions during ablation of atrial fibrillation.

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia with significant morbidity and mortality. Pharmacological agents are not very effective in the management of AF. Therefore, ablation procedures have become the mainstay of AF management. The irregular and seemingly chaotic atrial activity in AF is caused by one or more meandering spiral waves. Previously, we have shown the presence of sudden rhythm organization during ablation of persistent AF. We hypothesize that the observed transitions from a disorganized to an organized rhythm is a critical phase transition. Here, we explore this hypothesis by simulating ablation in an anatomically-correct 3D AF model. In 722 out of 2160 simulated ablation, at least one sudden transition from AF to an organized rhythm (flutter) was noted (33%). They were marked by a sudden decrease in the cycle length entropy and increase in the mean cycle length. At the same time, the number of reentrant wavelets decreased from 2.99 ± 0.06 in AF to 1.76 ± 0.05 during flutter, and the correlation length scale increased from 13.3 ± 1.0 mm to 196.5 ± 86.6 mm (both P < 0.0001). These findings are consistent with the hypothesis that transitions from AF to an anatomical flutter behave as phase transitions in complex non-equilibrium dynamical systems with flutter acting as an absorbing state. Clinically, the facilitation of phase transition should be considered a novel mechanism of ablation and may help to design effective ablation strategies.

Electrogram Characteristics of Ablated and Non-Ablated Myocardium in Humans: A Comparison of Miniaturized Embedded Electrodes and Conventional Ablation Electrodes.

INTRODUCTION: Intracardiac electrogram voltage remains an important metric for radiofrequency lesion application. Embedded micro-electrodes within ablation catheters are now approved for use in humans. OBJECTIVE: This study examined electrogram characteristics of miniaturized electrode bipoles (iEGMmini ) compared to distal ablation tip bipoles (iEGMstandard ) in the setting of typical atrial flutter in humans. METHODS: This was a single-center prospective trial involving 21 patients. A total of 54 non-ablated and 56 ablated sites were analyzed offline by separate blinded observers for iEGM voltage after a clinically successful radiofrequency (RF) ablation for typical atrial flutter. Ablation sites were defined as sites that were localized anatomically within the RF lesion set and did not have atrial capture at 10 mA and 2 milliseconds. Non-ablated sites were defined as sites adjacent to or remote from the ablation lesion set with capture at 10 mA and 2 milliseconds. RESULTS: iEGMmini had greater voltages at non-ablated sites and lower voltages at ablated sites than iEGMstandard (non-ablated sites iEGMstandard 0.64 mV vs. iEGMmini 1.0 mV; ablated sites iEGMstandard 0.15 mV, iEGMmini 0.08 mV, P < 0.001). The difference for each patient between ablated and non-ablated sites was greater for iEGMmini compared to iEGMstandard (0.94 mV ± 0.57 iEGMmini vs. 0.52 mV ± 0.35 iEGMstandard , P < 0.001). Inter-observer agreement was good among all non-ablated sites and iEGMmini ablated sites, but was less robust for ablated sites using iEGMstandard . CONCLUSIONS: Imbedded mini-electrode iEGM bipoles had higher voltages in non-ablated tissue, lower voltages in ablated tissue, and significantly greater intra-patient differences between ablated and non-ablated sites. These data suggest a potential role for miniaturized electrode bipole use in RF lesion monitoring.

Sudden and Fatal Malfunction of a Durata Defibrillator Lead due to External Insulation Failure.

Defibrillator lead malfunction can be a disastrous complication, leading to loss of protection from sudden cardiac death in a high-risk patient population. Recognition of lead-specific risk for failure can assist in development of focused screening or surveillance, as in the case of the Riata lead (St. Jude Medical, St. Paul, MN, USA) or the Sprint Fidelis lead (Medtronic Inc., Minneapolis, MN, USA). A case of defibrillation failure secondary to a Durata lead insulation failure is presented. A brief review of the literature and current St. Jude Medical implantable cardiac defibrillator lead design is presented. Identification of arcing is identified as a potential sign of catastrophic insulation failure.

Outcome of Subcutaneous Implantable Cardioverter Defibrillator Implantation in Patients with End-Stage Renal Disease on Dialysis.

BACKGROUND: Although the subcutaneous ICD (S-ICD®) is an attractive alternative in patients with end-stage renal disease (ESRD), data on S-ICD outcomes in dialysis patients are lacking. METHODS: Patients with cardiomyopathy undergoing S-ICD implantation in our center were stratified by need for chronic dialysis at the time of implant. The primary endpoint was incidence of death, heart failure hospitalization or appropriate S-ICD shocks, and secondary endpoints were incidence of inappropriate shocks or implant related complications requiring surgical re-intervention. Mean follow-up was longer in the nondialysis cohort (514 ± 495 vs. 227 ± 233 days, P = 0.006), so all endpoints were analyzed using time-dependent comparisons and reported as annual event rates. RESULTS: Out of 79 S-ICD implants included in this analysis, 27 patients were on dialysis. Dialysis patients were older and more likely to be diabetic. Mean ejection fraction across the entire cohort was 26.9% without significant difference between dialysis and nondialysis groups. Although not significant, the incidence of the primary endpoint was higher in the dialysis cohort (23.8%/year vs. 10.9%/year, P = 0.317), driven primarily by a higher rate of appropriate shocks. The rate of inappropriate shocks was similar between groups (dialysis 6.0%/year vs. nondialysis 6.8%/year, P = 0.509). No patients in the dialysis cohort had complications requiring surgical re-intervention versus 6 patients in the nondialysis cohort (P = 0.086). CONCLUSIONS: Our data suggest that S-ICD implantation in dialysis patients is not associated with an excess risk of implant related complications or inappropriate shocks.

Predictors of Long-Term Survival Following Transvenous Extraction of Defibrillator Leads.

BACKGROUND: Little data exist on long-term outcomes following extraction of implantable cardioverter defibrillator (ICD) leads, particularly for noninfectious indications. We sought to identify predictors of long-term survival after ICD lead extraction. METHODS: We retrospectively reviewed ICD lead extractions at our institution (n = 508). Procedural outcomes and long-term survival were ascertained by medical records review. RESULTS: Indication for lead extraction was infection in 32.5% and lead failure in 61.8%. Mean dwell time of the oldest extracted lead was 5.1 ± 5.9 years. Complete procedural success was achieved in 96.5% of cases. Major procedure-related complications occurred in 1.6% with six periprocedural deaths. During a mean follow-up of 866 ± 798 days, survival was significantly worse among patients with infection as the indication for extraction. At 1 year after extraction, survival among those with infection was 88.2%, compared to 95.0% in the lead failure cohort (P < 0.001). Procedural failure was a significant predictor of long-term mortality, even after excluding periprocedural deaths. In multivariate models, the presence of chronic kidney disease, increased number of leads requiring extraction, lower ejection fraction, and procedural failure were predictors of mortality. CONCLUSION: Despite high rates of procedural success, infectious indication for ICD lead extraction is associated with increased long-term mortality. In contrast, among patients undergoing extraction for lead failure, long-term survival was excellent. The presence of procedural failure was a significant predictor of long-term mortality. Further studies will be necessary to better understand the mechanisms by which procedural failure may adversely impact long-term outcomes.

Pulse Generator Exchange Does Not Accelerate the Rate of Electrical Failure in a Recalled Small Caliber ICD Lead.

BACKGROUND: St. Jude Riata/Riata ST defibrillator leads (St. Jude Medical, Sylmar, CA, USA) were recalled by the Food and Drug Administration in 2011 for an increased rate of failure. More than 227,000 leads were implanted and at least 79,000 patients still have active Riata leads. Studies have examined clinical predictors of lead failure in Riata leads, but none have addressed the effect of implantable cardioverter defibrillator (ICD) generator exchange on lead failure. The purpose of this study is to assess the effect of ICD generator exchange on the rate of electrical failure in the Riata lead at 1 year. METHODS: A retrospective chart review was conducted in patients who underwent implantation of a Riata/Riata ST lead at one center. Patients with a functioning Riata lead (with/without externalized conductor) at the time of ICD exchange were compared to controls with Riata leads implanted for a comparable amount of time who did not undergo generator replacement. RESULTS: Riata leads were implanted in 1,042 patients prior to the recall and 153 of these patients underwent generator exchange without lead replacement. Conductor externalization was noted in 21.5% of Riata leads in the ICD exchange cohort, which was not different from the control group (19.2%; P = 0.32). Two leads failed in the first year after generator replacement (1.5%) which did not significantly differ from the control group (2.0%; P = 0.57). At change-out, 54% received a commanded shock (18.6 ± 0.9 J) that did not result in any change in the high-voltage lead impedance (46.1 ± 1.1 ohms). CONCLUSIONS: Conductor externalization was seen frequently in our cohort of patients. ICD generator exchange did not accelerate the rate of Riata lead failure at 1 year. Although both the control and the change-out cohorts failed at a rate much greater than nonrecalled leads, generator exchange did not appear to add to the problem.

Cardiac resynchronization therapy in adult patients with repaired tetralogy of fallot and left ventricular systolic dysfunction.

BACKGROUND: Although left ventricular (LV) systolic dysfunction is known to occur in adults with repaired Tetralogy of Fallot (TOF), the effects of cardiac resynchronization therapy (CRT) are not well characterized. METHODS: We retrospectively divided all patients with repaired TOF and impaired LV ejection fraction (LVEF ≤ 40%) undergoing CRT at our institution (n = 10) into two groups: de novo CRT (group A, n = 6) or upgrade from existing device (group B, n = 4). Echocardiograms were reviewed at baseline, medium-term (>6 months post-CRT), and long-term follow-up. CRT response was defined as reduction in LV end-systolic volume (LVESV) ≥15% at medium term. RESULTS: Age at surgical repair was 13.1 ± 16.0 years, age at CRT was 44.4 ± 12.5 years, and baseline LVEF was 24.0 ± 10.5%. Group A demonstrated a preponderance of right ventricular (RV) conduction delay, whereas all patients in group B demonstrated RV pacing at baseline. At medium-term follow-up, patients in group A showed significant improvements in LVEF, LV end-diastolic volume (LVEDV), and LVESV. Group B also demonstrated a significant improvement in LVEF with favorable trends in LV volumes. Of nine patients with complete data at medium term, eight showed evidence of CRT response. Average long-term follow-up was 53.4 ± 29.3 months. At long-term follow-up, LVEF, LVEDV, and LVESV remained numerically better than baseline, although the results were no longer significant. CONCLUSIONS: Adult patients with repaired TOF and LV systolic dysfunction demonstrate significant medium-term response to CRT, even among those with RV conduction delay. The long-term impact of CRT in this population requires further characterization.

Transient pacing in pigs with complete heart block via myocardial injection of mRNA coding for the T-box transcription factor 18.

The adenovirus-mediated somatic transfer of the embryonic T-box transcription factor 18 (TBX18) gene can convert chamber cardiomyocytes into induced pacemaker cells. However, the translation of therapeutic TBX18-induced cardiac pacing faces safety challenges. Here we show that the myocardial expression of synthetic TBX18 mRNA in animals generates de novo pacing and limits innate and inflammatory immune responses. In rats, intramyocardially injected mRNA remained localized, whereas direct myocardial injection of an adenovirus carrying a reporter gene resulted in diffuse expression and in substantial spillover to the liver, spleen and lungs. Transient expression of TBX18 mRNA in rats led to de novo automaticity and pacemaker properties and, compared with the injection of adenovirus, to substantial reductions in the expression of inflammatory genes and in activated macrophage populations. In rodent and clinically relevant porcine models of complete heart block, intramyocardially injected TBX18 mRNA provided rate-adaptive cardiac pacing for one month that strongly correlated with the animal's sinus rhythm and physical activity. TBX18 mRNA may aid the development of biological pacemakers.

Left atrial flutter accelerates during ablation of atrial fibrillation: a paradoxical effect of electrical remodelling.

AIMS: Atrial fibrillation (AF)-induced electrical remodelling causes shortening of refractory period and slowing of conduction velocity. During the course of catheter ablation of AF, there are often transitions from AF to left atrial flutter (AFL) and from faster to slower AFL. The purpose of this study was to characterize the time course of change in AFL rate during AF ablation. METHODS AND RESULTS: Fourier transformation was performed on 16 s segments of coronary sinus and ablation catheter bipolar electrograms. Ablation-induced AF-to-AFL and AFL-to-AFL transitions were defined as a sudden drop in the dominant frequency (DF) of at least 10 bpm, followed by a regular rhythm. Forty-five transitions were detected in 24 ablation procedures. The mean DF in AF was 5.31 ± 0.79 Hz, which was significantly faster than AFL, 4.52 ± 0.62 Hz (P< 0.05). The mean ΔDF at transitions was -51 ± 16 bpm in AF and -40 ± 14 bpm in AFL. Dominant frequency slope was positive (rate increased) after all the transitions during AF (P< 0.0001) and in 11 of 14 transitions in AFL (P= 0.033). The time constant of the DF recovery curve was 161 ± 105 s. CONCLUSIONS: After ablation-induced transition from AF to AFL, or faster to slower AFL, there is a progressive increase in AFL rate over time. The mechanism of this acceleration is uncertain, but the time constant of this rate increase is consistent with the recovery of the slow/ultraslow sodium current in the setting of established electrical remodelling.

Sleep quality and sleepiness in persons with implantable cardioverter defibrillators: outcome from a clinical randomized longitudinal trial.

BACKGROUND: Patients receiving an implantable cardioverter defibrillator (ICD) report various types and degree of sleep disruptions, but little is known regarding their characteristics, duration, and associated factors. The purposes of this study were: (1) to describe the effect of a psychoeducational intervention on sleep quality and daytime sleepiness, (2) to describe patterns of sleep over time, and (3) to identify predictors of poor sleep in an ICD population. METHODS: A randomized longitudinal intervention trial was designed to test the effects of a psychoeducational intervention, which included a sleep education and counseling session in patients receiving their initial ICD. Patients (n=236; 75% men; mean age 58.4 [±11.2] from the PsychoEducationAl Intervention for ICD PatiEnts (PEACE) trial comprised the study population. Variables related to sleep were measured by the Pittsburgh Sleep Quality Inventory (PSQI) and Epworth Sleepiness Scale (ESS). RESULTS: No psychoeducational intervention effects on sleep outcomes were observed. However, 67.2% of the patients reported poor sleep quality at baseline, and 56.8% had low sleep quality at 6 months based on PSQI scores>5; one-third (32.6%) were excessively sleepy based on ESS scores≥10 at 6 months. Anxiety, depression, physical function, pain intensity, and pain severity were all highly correlated to each other across time. Female gender was a significant covariate for the PSQI. New York Heart Association (NYHA) class was a significant covariate for sleepiness (Epworth). CONCLUSIONS: Low sleep quality and daytime sleepiness are found at time of insertion and over time in patients with ICD. Female gender, higher NYHA class, as well as two latent factors encompassing increased anxiety, depressive symptoms, and decreased physical function and increased pain, were significant predictors of poor sleep quality and sleepiness over time. These data help identify those at higher risk for sleep problems after ICD.

Complication rates associated with pacemaker or implantable cardioverter-defibrillator generator replacements and upgrade procedures: results from the REPLACE registry.

BACKGROUND: Prospective studies defining the risk associated with pacemaker or implantable cardioverter-defibrillator replacement surgeries do not exist. These procedures are generally considered low risk despite results from recent retrospective series reporting higher rates. METHODS AND RESULTS: We prospectively assessed predefined procedure-related complication rates associated with elective pacemaker or implantable cardioverter-defibrillator generator replacements over 6 months of follow-up. Two groups were studied: those without (cohort 1) and those with (cohort 2) a planned transvenous lead addition for replacement or upgrade to a device capable of additional therapies. Complications were adjudicated by an independent events committee. Seventy-two US academic and private practice centers participated. Major complications occurred in 4.0% (95% confidence interval, 2.9 to 5.4) of 1031 cohort 1 patients and 15.3% (95% confidence interval, 12.7 to 18.1) of 713 cohort 2 patients. In both cohorts, major complications were higher with implantable cardioverter-defibrillator compared with pacemaker generator replacements. Complications were highest in patients who had an upgrade to or a revised cardiac resynchronization therapy device (18.7%; 95% confidence interval, 15.1 to 22.6). No periprocedural deaths occurred in either cohort, although 8 later procedure-related deaths occurred in cohort 2. The 6-month infection rates were 1.4% (95% confidence interval, 0.7 to 2.3) and 1.1% (95% confidence interval, 0.5 to 2.2) for cohorts 1 and 2, respectively. CONCLUSIONS: Pacemaker and implantable cardioverter-defibrillator generator replacements are associated with a notable complication risk, particularly those with lead additions. These data support careful decision making before device replacement, when managing device advisories, and when considering upgrades to more complex systems.

Hands-on defibrillation: an analysis of electrical current flow through rescuers in direct contact with patients during biphasic external defibrillation.

BACKGROUND: Brief interruptions in chest compressions reduce the efficacy of resuscitation from cardiac arrest. Interruptions of this type are inevitable during hands-off periods for shock delivery to treat ventricular tachyarrhythmias. The safety of a rescuer remaining in contact with a patient being shocked with modern defibrillation equipment has not been investigated. METHODS AND RESULTS: This study measured the leakage voltage and current through mock rescuers while they were compressing the chests of 43 patients receiving external biphasic shocks. During the shock, the rescuer's gloved hand was pressed onto the skin of the patient's anterior chest. To simulate the worst case of an inadvertent return current pathway, a skin electrode on the rescuers thigh was connected to an electrode on the patient's shoulder. In no cases were shocks perceptible to the rescuer. Peak potential differences between the rescuer's wrist and thigh ranged from 0.28 to 14 V (mean 5.8+/-5.8 V). The average leakage current flowing through the rescuer's body for each phase of the shock waveform was 283+/-140 microA (range 18.9 to 907 microA). This was below several recommended safety standards for leakage current. CONCLUSIONS: Rescuers performing chest compressions during biphasic external defibrillation are exposed to low levels of leakage current. The present findings support the feasibility of uninterrupted chest compressions during shock delivery, which may enhance the efficacy of defibrillation and cardiocerebral resuscitation.

Pacing features that mimic malfunction: a review of current programmable and automated device functions that cause confusion in the clinical setting.

Modern implantable devices capable of pacing are armed with a multitude of programmable and automated features. While some features represent important advances in device safety and performance, many can also mimic device malfunction. This article discusses these features in terms of the confusion they may cause and highlights important clinical clues that aid in their recognition.

The relationship of bipolar left ventricular pacing stimulus intensity to cardiac depolarization and repolarization in humans with cardiac resynchronization devices.

BACKGROUND: Myocardial depolarization can be altered by varying pacing stimulus output. This may have implications on response rates for cardiac resynchronization therapy (CRT). The purpose of our study was to determine the relationship of left ventricular (LV) pacing stimulus intensity to measures of depolarization and repolarization in humans with CRT devices. METHODS: In 37 patients with a CRT device and bipolar LV leads, bipolar LV-only pacing from maximum output to threshold was performed. The presence of changes in depolarization was defined by predetermined changes in ECG morphology that accompanied a change in bipolar stimulus amplitude. ECG parameters and the EGMs were analyzed at various LV pacing stimulus intensities. RESULTS: Changes in ECG morphology were apparent in 70% of patients. These occurred at a mean LV stimulus amplitude of greater than 2.7 +/- 0.8 V at 1 ms. Of the patients with changes in surface ECG, the transventricular conduction time decreased from 155 +/- 41 ms at low output to 141 +/- 39 ms at high output (P < 0.01). Despite a significant reduction in QRS duration with high output, mean QTc and JTc interval increased with increasing LV stimulus strength (539 +/- 45 vs 559 +/- 46 ms (P < 0.01) and 353 +/- 31 ms vs 377 +/- 32 (P < 0.01)). CONCLUSION: Increased LV stimulus intensity, independent of RV anodal capture, is associated with faster transventricular conduction time, changes in myocardial depolarization, and longer QT intervals. These findings have important implications on the relationship of programmed LV pacing output to pacing-induced proarrhythmia and clinical CRT response rates.

Effect of a psychoeducational intervention on depression, anxiety, and health resource use in implantable cardioverter defibrillator patients.

BACKGROUND: Psychological responses have been reported for some patients after the insertion of an implantable cardioverter defibrillator (ICD). This study tested the effects of a psychoeducational intervention on anxiety, depressive symptoms, functional status, and health resource use during the first year after ICD implantation. METHODS: ICD patients (n = 246) were randomized to usual care (UC), group (GRP), or telephone counseling (TC) intervention that included education, symptom management, and coping skill training. Participants were 58 +/- 11 years, 73% men, and 23% minorities. Anxiety (State-Trait Anxiety Inventory [STAI]), depressive symptoms (Beck Depression Inventory II [BDI-II]), and functional status (Duke Activity Status Inventory [DASI]) were measured at baseline and after 1, 3, 6, and 12 months. Health resource use and disability days were tracked. Analyses were repeated-measures analysis of covariance to assess Group x Time effects,chi(2)for percentage with clinically significant anxiety and depression at each time point, and logistic regression. RESULTS: All groups experienced decreased anxiety and depressive symptoms over the 12 months; GRP intervention had lower STAI (P = 0.03) than UC at 3 months. Logistic regression revealed group differences for predicted probability of having depressive symptoms at 12 months (UC = 0.31, GRP = 0.17, TC = 0.13, P = 0.03). UC had greater calls to providers at 1 and 6 months (P < 0.05) and more sick/disability days at 12 months (P = 0.01) than intervention groups. CONCLUSIONS: A psychoeducational intervention reduced anxiety and depressive symptoms early after ICD implant, lowered probability of depressive symptoms at 1 year, and decreased disability days/calls to providers. These findings support further study and clinical use of both group and telephone interventions to yield better psychological outcomes after ICD implant.

A review of bioelectrodes for clinical electrophysiologists.

The theory of bioelectrodes describes the rules governing the passage of electrical charge between electrodes and electrolytes. In this review, we explain the basis of bioelectrodes with focus on clinical electrophysiology. The central concept is the double-layer capacitance that forms in the interface between the electrode and tissue. This phenomenon controls charge transfer between electrodes and tissues and contributes to detrimental effects such as electrode polarization and motion artifacts. Many methods critical to the practice of electrophysiology, including fractally coated pacemaker leads, biphasic stimuli, signal filtering, and the use of nonpolarizable electrodes, are devised to mitigate these problems. Our goal is to provide a robust and intuitive background on these topics for practicing electrophysiologists to help them better understand how catheters and leads work and to assist them with optimizing and troubleshooting electrophysiology systems.

Theoretical Modeling and Experimental Detection of the Extracellular Phasic Impedance Modulation in Rabbit Hearts.

Theoretical cardiac electrophysiology focuses on the dynamics of the membrane and sarcoplasmic reticulum ion currents; however, passive (e.g., membrane capacitance) and quasi-active (response to small signals) properties of the cardiac sarcolemma, which are quantified by impedance, are also important in determining the behavior of cardiac tissue. Theoretically, impedance varies in the different phases of a cardiac cycle. Our goal in this study was to numerically predict and experimentally validate these phasic changes. We calculated the expected impedance signal using analytic methods (for generic ionic models) and numerical computation (for a rabbit ventricular ionic model). Cardiac impedance is dependent on the phase of the action potential, with multiple deflections caused by a sequential activation and inactivation of various membrane channels. The two main channels shaping the impedance signal are the sodium channel causing a sharp and transient drop at the onset of action potential and the inward rectifying potassium channel causing an increase in impedance during the plateau phase. This dip and dome pattern was confirmed in an ex-vivo rabbit heart model using high-frequency sampling through a monophasic action potential electrode. The hearts were immobilized using a myosin-inhibitor to minimize motion artifacts. We observed phasic impedance changes in three out of four hearts with a dome amplitude of 2 - 4Ω. Measurement of phasic impedance modulation using an extracellular electrode is feasible and provides a non-invasive way to gain insight into the state of cardiac cells and membrane ionic channels. The observed impedance recordings are consistent with the dip and dome pattern predicted analytically.