C. Cabo

Activation in unipolar cardiac electrograms: a frequency analysis

Several detectors of local activations in unipolar cardiac electrograms are discussed. The detectors are based on the frequency content of the waveforms. For this study, myocardial regions with no local electrical activity were created with cryoablation in canine ventricles, so that the characteristics of electrograms reflecting local activation could be compared with those with only distant electrical activity. For each electrogram, representations, of the original signal were created using the output of bandpass filters; for each representation, the value of the maximum amplitude was taken as a measurement of the frequency content of the electrogram in that frequency band. The content of each frequency band and the first derivative of the signal were tested as discriminators between local and distant electrical activity. Combinations of frequency bands were also tested using a logistic regression technique. It was found that a detector based on multivariate analysis of different frequency components of a signal may be more effective than single-band filtering in discriminating between local and distant electrical activity in the heart, especially when those components have very different magnitudes.<<ETX>>

Propagation model using the DiFrancesco-Noble equations

Propagation, re-entry and the effects of stimuli within the conduction system can be studied effectively with computer models when the pertinent membrane properties can be represented accurately in mathematical form. To date, no membrane models have been shown to be accurate representations during repolarisation and recovery of excitability, although for the Purkinje membrane the DiFrancesco-Noble (DN) model has become a possibility. The paper examines the DN model, restates its equations and compares simulated waveforms in a number of propagation contexts to experimental measurements reported in the literature. The objective is to determine whether or not the DN model reproduced phenomena such as supernormality, shortening in action potential duration during pacing rate increases, alternation of duration with changes in rhythm, graded responses and ‘all-or-none’ repolarisation in a quantitatively realistic way, as each of these come from time and space dependencies not directly a part of the ionic current measurements on which the DN model is based. The results show that the DN equations correctly simulate these situations and support the goal of having a model that is broadly applicable to Purkinje tissue, including refractory period properties and response to electrical stimulation.

Reflection after delayed excitation in a computer model of a single fiber.

Reflection (reflected reentry) is a case of reentry in a one-dimensional structure, divided into proximal and distal segments, in which tissue excited by a wave front propagating in a forward direction is reexcited by electrical activity coming backward from the original direction of propagation. Cases of reflection have been demonstrated in Purkinje fibers and in ventricular muscle preparations containing multiple fibers. Several mechanisms possibly responsible for reflected reentry have been proposed. However, the difficulty in the interpretation of the experimental results, as well as the limited number of different conditions in which reflection was obtained, has kept open the question about conditions and mechanisms for reflection. We have developed a computer model in which reflection occurs. The model involves a single fiber and uses the DiFrancesco-Noble equations for the Purkinje fiber to model the ionic currents. The results show that reflection is possible in a single fiber and that diastolic depolarization (automaticity) is not a requirement for reflection. Active membrane responses to a just-above-threshold stimulus were important for achieving the necessary time delay. Systematic simulations showed further that reflection occurred only when the right coupling conditions linked a short or long proximal fiber to a short distal segment.

Evaluation of an automatic cardiac activation detector for bipolar electrograms

The identification of local activation events in bipolar cardiac electrograms, the first step of isochronal map construction, is a time-consuming and difficult process. Owing to the variability among bipolar activation complexes and the lack of practical knowledge concerning the relationship of the bipolar waveform to action potential characteristics, a set of empirical rules to guide the assignment of local activation times have been adopted. A computer program, called AP, has been designed, which implements these rules in the form of a syntactic analyser. Canine epicardial recordings were used to evaluate AP by comparing local activation times, assigned by AP, with times assigned independently by three investigators. The Hermes-Cox model for detector evaluation and a bootstrap statistical method were used in conjunction with ROC analysis to evaluate the ability of AP to detect events. Analysis of discrepancies among investigator-assigned times showed that the reliabilities of AP event detection and AP-assigned times were comparable to those of the investigators. The methods used in system design and evaluation are applicable to a broad range of problems in the detection and localisation of waveform components.

Propagation Versus Delayed Activation During Field Stimulation of Cardiac Muscle

This modeling study seeks to explain the experimentally detected delay between the application of an electric field and the recorded response of the transmembrane potential. In this experiment, conditions were deliberately set so that the field should excite all cells at once and so that no delay should be caused by a propagating wave front. The explanation of the observed delay may lie in the intrinsic properties of the membrane. To test this hypothesis, the strength latency curves were determined for three cases: (1) for a membrane patch model, in which the membrane is uniformly polarized and its intrinsic properties can be studied; (2) for the cardiac strand directly excited by the electric field; and (3) for the cardiac strand excited by a propagating wave front. The models of the membrane patch and the directly excited strand yield excitation delays that are comparable to those observed experimentally in magnitude and in the overall shape of the strength latency curves. The delays resulting from propagation are, in general, dependent on the position along the strand, although for some positions the strength latency curves for propagation are similar to those obtained from the directly activated strand and from the patch model. Therefore, the delay in excitation does not necessarily imply the presence of propagating wave fronts and can be attributed to intrinsic membrane kinetics.

Unidirectional block in a computer model of partially coupled segments of cardiac Purkinje tissue

The initiation of a reentrant circuit requires a zone of slow conduction and a zone of unidirectional block. This study used computer model conditions under which partial coupling between segments of cardiac Purkinje tissue resulted in unidirectional block. The structure used was one-dimensional and divided into three segments: a middle segment of variable length coupled to two long (semi-infinite in concept) segments. The DiFrancesco-Noble equations represented the ionic currents of the membrane. The results show that the possibility of unidirectional block depends on the size of the middle segment and the coupling resistances between the segments. No combination of coupling resistances allowed unidirectional block for middle segments with a length of two space constants (4 mm) or longer. Unidirectional block occurred for many combinations of coupling resistances as the length of the middle segment decreased to around half a space constant (1 mm). The number of length combinations that caused unidirectional block decreased again as segment length further decreased. These results provide a possible mechanism of unidirectional block for situations where islands of viable tissue are connected through nonviable tissue, such as in a healed myocardial infarction.

Digital filters for activation detection in unipolar cardiac electrograms

A limitation on the use of cardiac activation mapping is the inability to distinguish reliably between electrical activity occurring next to a sensing electrode and the effects of distant wavefronts. The authors modeled electrograms without local electrical activity by recordings in lesions created in canine epicardium by freezing, where, by definition, local electrical activity is not possible. Electrograms with local electrical activity were recorded in healthy myocardium. The authors present discriminators based on the frequency characteristics of unipolar electrograms. Different frequency bands were tested as discriminators and compared to the typically used first-derivative discriminator.<<ETX>>

Neural networks for improved automation of ventricular activation mapping

Whether or not a neural network could discriminate between electrograms from viable tissue (local myocardial activation) and nonviable tissue (distant electrical events) was investigated as a step toward improving automation of cardiac activation mapping. Cryolesions were made in canine myocardium. Five days later, plunge needles with eight electrodes were inserted through normal tissue and the lesions, and during atrial pacing unipolar electrograms were recorded, sampling at 8000 Hz with 12-bit resolution. Data were analyzed from 19 electrodes. The type of tissue from which electrograms were recorded was correctly identified for 18 of the 19 electrodes, indicating that the network was able to identify features of the electrograms that distinguished viable from nonviable tissue sources, and that this generalized to new electrograms upon which the network had not been trained. It is concluded that neural networks may have application in automated cardiac activation mapping.<<ETX>>

Use of coherence in activation detection during ventricular fibrillation

The use of cardiac activation mapping to study ventricular fibrillation (VF) is discussed. The locations of isochrones in maps are based on decisions on whether and when segments of the electrogram have local electrical activation. The authors compare two quantitative criteria for making decisions about local activity in electrograms. The criteria are based on the comparison of electrograms recorded in those excitable zones with electrograms recorded in normal myocardium. One uses the first derivative of the electrogram (only one electrogram affects the decision); the second uses the first derivative of the primarily local component of the waveform.<<ETX>>

Implementation, performance and characterization of the DiFrancesco-Noble model

The authors describe their experience in the implementation and the use of the DiFrancesco-Noble model. Clarifying references for the equations are given as well as a way to implement the equations in a propagation model. An estimate of the number of floating point operations required is given as well as the running times of functional programs in some widely used computers. A number of parameters useful in the design of numerical experiments like resting membrane resistance are also given. A short discussion on the accuracy of the equations for the fast sodium current is also included as well as some modifications made to the DiFrancesco-Noble equations, so that the depolarization rate was closer to real tissue experiments.<<ETX>>