A transatrial pericardial access: lead placement as proof of concept.

A safe, easy, and quick access into the pericardial space may provide a window for diagnostics and therapeutics to the heart. The objective of this study was to provide proof of concept for an engagement and access catheter that allows access to the pericardial space percutaneously. A multilumen catheter was developed to allow navigation and suction fixation to the right atrial appendage/wall in a normal swine model. Advancement through the multilumen catheter using a second catheter with a distal needle tip allows access to the pericardial space without pericardial puncture and advancement of a standard guide wire into the space. Navigation into the pericardial space was undertaken by fluoroscopy alone and was accomplished in 10 swine (5 acute and 5 chronic). As a specific application of this pericardial access method, a pacing lead was implanted on the epicardial surface. Five chronic swine experiments were conducted with successful pacing engagement verified by lead impedance and pacing threshold and sensing. Lead impedance exceeded 1,000 Omega preengagement and dropped by an average of 200 Omega upon implant (769 +/- 498 Omega). Pacing thresholds at 0.4 ms ranged from approximately 0.5 to 2.1 V acutely (1.03 +/- 0.92 V). No cardiac effusion or tamponade was observed in any of the acute or chronic studies. The ability to engage, maintain, and retract the right atrial appendage/wall and to engage an epicardial lead was successfully demonstrated. These findings support the feasibility of safe access into the pericardial space in a normal swine model and warrant further investigations for clinical translation.

Differences in organization between acute and chronic atrial fibrillation in dogs.

OBJECTIVES: The purpose of this study was to determine differences in acute and chronic atrial fibrillation (AF) "organization" in canine models. BACKGROUND: Electrophysiologic changes occur during atrial remodeling, but little is known about how remodeling affects AF organization. We hypothesized that atrial remodeling induced by long-term rapid atrial rates heterogeneously decreases AF organization. METHODS: In seven dogs, acute AF was induced by atrial burst pacing, and in eight dogs chronic AF was created by six weeks of continuous rapid atrial pacing. Atrial fibrillation was epicardially mapped from the right atria (RA) and left atria (LA). Atrial cycle length (CL), spatial organization and activation maps were compared. Spatial organization was quantified by an objective signal processing measure between multiple electrograms. RESULTS In acute AF, mean CL was slightly shorter in the LA (124 +/- 16 ms) than it was in the RA (131 +/- 14 ms) (p < 0.0001). In chronic AF, LA CL (96 +/- 14 ms) averaged 24 ms shorter than RA CL (121 +/- 18 ms) (p < 0.0001). Right atria and LA in acute AF had similar levels of organization. In chronic AF, the LA became approximately 25% more disorganized (p < 0.0001) while the RA did not change. In acute AF, a single broad wave front originating from the posterior and medial atrium dominated LA activation. In chronic AF, LA activation was more complex, sustaining multiple reentrant wavelets in the free wall and lateral appendage. CONCLUSIONS: Acute and chronic AF exhibit heterogeneous differences in CL, organization and activation patterns. The LA in chronic AF is faster and more disorganized than it is in acute AF. Differences in the models may be due to heterogeneous electrophysiologic remodeling and anatomic constraints. The design of future AF therapies may benefit by addressing the patient specific degree of atrial remodeling.

An introduction to high-resolution ECG recordings of cardiac late potentials.

High-resolution electrocardiography utilizes computer processing to record low-levels signals not normally observed on standard electrocardiographs. Cardiac late potentials occur at the end of or after the QRS complex and require these methods to be quantified. A brief overview of the methods used to record late potentials is presented. These include lead placement, computer-implemented signal averaging, high-pass filtering, and feature extraction for characterizing the late potential. The major application of late- potential analysis has been in patients after myocardial infarction. Several of these studies are reviewed that demonstrate the usefulness of this new approach in identifying those patients at greatest risk for developing ventricular tachycardia. The most impressive studies have been those that compare late potentials with measures of ventricular performance and ventricular ectopy.

Signal-averaged electrocardiography: a new noninvasive test to identify patients at risk for ventricular arrhythmias.

Signal-averaged electrocardiography (ECG) is a new noninvasive test for identifying patients at risk for ventricular arrhythmias. This computerized method of analyzing standard ECGs identifies particular microvolt-level signals called late potentials. Late potentials have been correlated with clinical ventricular tachycardia, are predictive of ventricular tachycardia inducibility at the time of electrophysiologic testing, and are predictive of arrhythmic events after myocardial infarction. In this review, we describe late potentials, the method of obtaining and processing the signal-averaged ECG, and clinical studies in various patient groups that have assessed the predictive value of the signal-averaged ECG for identification of patients at risk for subsequent ventricular arrhythmias.

High-resolution electrocardiography.

The electrocardiogram (ECG) is arguably the most common noninvasive diagnostic test performed by physicians. The instrumentation for recording the ECG has followed technological trends but the addition of new information has not generally been forthcoming from these advances. Instead, the elucidation of the standard waves P, QRS, T, and U is still the primary focus of electrocardiographic recording and interpretation. High-resolution electrocardiography can be defined as the use of methods to record physiological information not measurable with the standard ECG while still maintaining a noninvasive approach. Often these newer methods require digital signal processing for enhancing very low-level signals, deriving parameters from stored waveforms, or adaptively changing filter characteristics to record the normal wave when contaminated with high levels of noise. This review examines most of these approaches but concentrates on the methods which allow the recording of new information which may have significant diagnostic and, perhaps, prognostic value.

Time-frequency plane Wiener filtering of the high-resolution ECG: background and time-frequency representations.

This paper introduces the concept of a posteriori Wiener filtering (APWF), performed in the time-frequency plane. The objective is to improve the signal-to-noise ratio (SNR) of the ensemble-averaged high-resolution electrocardiogram (HRECG). APWF was developed to address the problem of a limited ensemble size for estimating ensemble-averaged evoked potentials. For the HRECG, we identify the major challenge as adapting the time-frequency structure of the filter to that of low-level cardiac signals. Technical limitations and the characteristics of HRECG signals make time-frequency analysis of the ensemble average problematic. Normal and abnormal signal components are difficult to distinguish due to low time-frequency energy concentration and limited spectrotemporal resolution. However, considering the entire ensemble of repetitive ECG records, signal and noise components are separable in the time-frequency plane. This forms the basis of the new time-frequency plane Wiener (TFPW) filter, applicable to any ensemble averaging problem involving repetitive deterministic signals mixed with uncorrelated noise.

Time-frequency plane Wiener filtering of the high-resolution ECG: development and application.

The time-frequency plane Wiener (TFPW) filter is a new method, based on a posteriori Wiener filtering principles, to enhance the performance of ensemble averaging. This paper develops the mathematical aspects of the TFPW filter, and assesses its performance with elementary signals, such as sine waves and chirps, and authentic high-resolution electrocardiogram (HRECG) ensembles. The principal feature of the TFPW filter is its use of the time-frequency plane to accommodate signal nonstationarity. Using a posteriori computed statistics of the ensemble, the filter matches itself to the time-frequency structure of the signals to be estimated. The method is sufficiently general to be applicable to any class of repetitive signal with a deterministic time-frequency structure and additive noise in the ensemble. It is concluded that significant improvements in both estimated signal fidelity and noise reduction are possible with the TFPW filter, compared to conventional ensemble averaging.

Model studies of extracellular electrograms arising from an excitation wave propagating in a thin layer.

Extracellular potentials were calculated for a wave of activation propagating through a thin layer of tissue. Sources were represented on the basis of the bidomain model, with the observation point at the tissue surface. Calculated potentials were found to be insensitive to the length of the wavefront if it passed under the recording electrode provided its length exceeded 1 mm, but to decrease rapidly when the wave passed to a side of the electrode. Contributions of source regions with a lateral displacement from the electrode greater than about 0.3 mm were substantially diminished. The effects of the thickness of the layer and velocity of propagation on amplitude was investigated.

Analysis of abnormal signals within the QRS complex of the high-resolution electrocardiogram.

This paper presents a new, quantitative approach to measuring abnormal intra-QRS signals, using the high-resolution electrocardiogram (HRECG). These signals are conventionally known as QRS "notches and slurs." They are measured qualitatively and form the basis for the ECG identification of myocardial infarction. The HRECG is used for detection of ventricular late potentials (LP), which are linked with the presence of a reentry substrate for ventricular tachycardia (VT) after a myocardial infarction. LP's are defined as signals from areas of delayed conduction which outlast the normal QRS period. Our objective is to quantify very low-level abnormal signals that may not outlast the normal QRS period. In this work, abnormal intra-QRS potentials (AIQP) were characterized by removing the predictable, smooth part of the QRS from the original waveform. This was represented as the impulse response of an ARX parametric model, with model order selected empirically from a training data set. AIQP were estimated using the residual of the modeling procedure. Critical AIQP parameters to separate VT and non-VT subjects were obtained using discriminant functions. Results suggest that AIQP indexes are a new predictive index of the HRECG for VT. The concept of abnormal intra-QRS potentials permits the characterization of pathophysiological signals contained wholly within the normal QRS period, but related to arrhythmogenesis. The new method may have other applications, such as detection of myocardial ischemia and improved ECG identification of the site of myocardial infarction, particularly in the absence of Q waves.

A high-temporal resolution algorithm for quantifying organization during atrial fibrillation.

Atrial fibrillation (AF) has been described as a "random" or "chaotic" rhythm. Evidence suggests that AF may have transient episodes of temporal and spatial organization. We introduce a new algorithm that quantifies AF organization by the mean-squared error (MSE) in the linear prediction between two cardiac electrograms. This algorithm calculates organization at a finer temporal resolution. (approximately 300 ms) than previously published algorithms. Using canine atrial epicardial mapping data, we verified that the MSE algorithm showed nonfibrillatory rhythms to be significantly more organized than fibrillatory rhythms (p < .00001). Further, we compared the sensitivity of MSE to that of two previously published algorithms by analyzing AF with simulated noise and AF manipulated with vagal stimulation or by adenosine administration to alter the character of the AF. MSE performed favorably in the presence of noise. While all three algorithms distinguished between low and high vagal AF, MSE was the most sensitive in its discrimination. Only MSE could distinguish baseline AF from AF with adenosine. We conclude that our algorithm can distinguish different levels of organization during AF with a greater temporal resolution and sensitivity than previously described algorithms. This algorithm could lead to new ways of analyzing and understanding AF as well as improved techniques in AF therapy.

Critical overview of late potential recordings.

High resolution ECG (HRECG) techniques have been used for several years to quantify low level cardiac potentials which occur at the end of and after the normal registration of the QRS complex. These potentials have been recorded with endocardial catheter electrodes in man and epicardial electrodes in canine models of myocardial infarction. In order to record these small signals on the body surface, signal averaging is used to improve the signal-to-noise ratio. These so-called "late potentials" have been the focus of a few animal studies and an ever increasing number of clinical studies. The primary focus of the clinical studies has been in the patients with inducible ventricular tachycardias. Long-term follow-up of patients with myocardial infarction has also been reported. There is continual support for the conclusion that the presence of late potentials, measured with a variety of approaches, is an independent measure of vulnerability to life-threatening ventricular arrhythmias. Methods used for recording and analyzing the late potentials have varied widely among investigators. Four aspects of these various studies will be examined: lead selection, signal processing, parameter selection, and quality control. Specific data with regard to lead selection and high pass filtering will be presented.

The signal-averaged electrocardiogram: update on clinical applications.

The signal-averaged electrocardiogram (SAECG) facilitates noninvasive recording of low-amplitude cardiac signals such as ventricular late potentials. The SAECG has been used to accurately predict life-threatening ventricular tachyarrhythmias in patients after acute myocardial infarction and with nonischemic dilated cardiomyopathy, and to screen for inducible ventricular tachycardia in patients with unexplained syncope and with nonsustained ventricular tachycardia. This review focuses on currently accepted methodology and clinical and research applications of the SAECG.

Use of high-pass filtering to detect late potentials in the signal-averaged ECG.

The authors investigate the use of digital filters for analysis of the signal-averaged ECG. They consider the basic types of digital filters and examine respective advantages and disadvantages for ECG analysis. An approach to analyzing the signal-averaged ECG using separate filters for measuring QRS offset and amplitudes is proposed. A study of 19 subjects explores the use of filters to detect low-level activity anywhere in the QRS complex.