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A V Sahakian

Publications and source records attributed to A V Sahakian.

At least 19 recordsLinked to original sources
PubMed2001

A simultaneous multichannel monophasic action potential electrode array for in vivo epicardial repolarization mapping.

While the recording of extracellular monophasic action potentials (MAPs) from single epicardial or endocardial sites has been performed for over a century, we are unaware of any previous successful attempt to record MAPs simultaneously from a large number of sites in vivo. We report here the design and validation of an array of MAP electrodes which records both depolarization and repolarization simultaneously at up to 16 epicardial sites in a square array on the heart in vivo. The array consists of 16 sintered Ag-AgCl electrodes mounted in a common housing with individual suspensions allowing each electrode to exert a controlled pressure on the epicardial surface. The electrodes are arranged in a square array, with each quadrant of four having an additional recessed sintered Ag-AgCl reference electrode at its center. A saline-soaked sponge establishes ionic contact between the reference electrodes and the tissue. The array was tested on six anesthetized open-chested pigs. Simultaneous diagnostic-quality MAP recordings were obtained from up to 13 out of 16 ventricular sites. Ventricular MAPs had amplitudes of 10-40 mV with uniform morphologies and stable baselines for up to 30 min. MAP duration at 90% repolarization was measured and shown to vary as expected with cycle length during sustained pacing. The relationship between MAP duration and effective refractory period was also confirmed. The ability of the array to detect local differences in repolarization was tested in two ways. Placement of the array straddling the atrioventricular (AV) junction yielded simultaneous atrial or ventricular recordings at corresponding sites during 1:1 and 2:1 AV conduction. Localized ischemia via constriction of a coronary artery branch resulted in shortening of the repolarization phase at the ischemic, but not the nonischemic, sites. In conclusion, these results indicate that the simultaneous multichannel MAP electrode array is a viable method for in vivo epicardial repolarization mapping. The array has the potential to be expanded to increase the number of sites and spatial resolution.

Action Potentials↗
PubMed2000

Multi-site dual surface monophasic action potential mapping of atrial repolarization in vivo: is atrial repolarization a two- or three-dimensional process?

Although the atrial free wall is a thin structure, atrial depolarization has been shown to have aspects of three-dimensionality. This study asks whether the same is true for atrial repolarization. By using a multi-element dual-surface probe, monophasic action potentials (MAPs) were recorded simultaneously at several opposing sites on the right atrial endocardial and epicardial surfaces in six open-chest pigs. The times of depolarization and repolarization were marked in recordings during sinus, paced rhythms, and during infusions of cold saline to the epicardial surface, which generated a temperature gradient across the atrial wall. Repolarization times were similar on endocardial and epicardial surfaces in some sites, but others showed significant differences during sinus and paced rhythms. Cold saline infusion produced a significant lengthening of MAP duration, and this was more pronounced on the atrial endocardial sites than on the epicardial sites. The observed differences in endocardial and epicardial repolarization times may be due to the presence of atrial pectinate muscles on the endocardial surface. These results suggest that in some regions atrial repolarization is a three-dimensional process. Possible limitations of this study include the fact that the depth of view of MAPs recorded from the atrial wall may extend to the opposing surface.

Animals↗
PubMed1998

Detection of atrial activity from high-voltage leads of implantable ventricular defibrillators using a cancellation technique.

The inability to detect atrial activity limits implantable ventricular cardioverter defibrillators (ICD) in discriminating tachycardias and can result in inappropriate therapy. This study attempted to detect atrial activity on the wide-spaced bipole signals formed by the high-voltage (HV) leads of the ICD during device implantation and to develop an algorithm for the detection of atrial fibrillation (AFib) from these signals. We used a method that canceled ventricular and correlated atrial activity from the HV lead signals and measured frequency and amplitude distribution information to discriminate sinus rhythm (SR) and AFib segments. We analyzed 186 data segments from 21 patients (six AFib, 14 SR, one AFib and SR). For individual segments in this data set, the sensitivity of the algorithm was 78%, specificity 92.65%, positive and negative predictive values 79.59 and 91.97%, respectively. These results demonstrate that atrial activity is present in the HV lead signals, and AFib detection can be achieved in many, but not all cases, using information currently available to ICD's. Prior work from surface electrocardiograms suggests that this algorithm can function during ventricular tachycardias. However, specificity of the algorithm is not high enough for clinical use.

Aged↗
PubMed1998

Further observations of "linking" of atrial excitation during clinical atrial fibrillation.

The objective of this article was to look for evidence of nonrandom behavior during atrial fibrillation by examining long (> 15 minutes) recordings. We have previously reported transient "linking" of atrial activation during atrial fibrillation, and showed that activation was not entirely random. Over the few episodes of linking seen during 1 minute, activation directions apparently repeated, indicating a possible anatomical or physiological constraint. In the present study, we examined atrial fibrillation over longer time periods to see if this constancy of direction was stable. Endocardial recordings were made from 12 patients with atrial fibrillation using a catheter with three orthogonal bipoles, allowing measurements of local activation directions in three dimensions. The direction was calculated using Pipberger's half-area method, and episodes of transient linking were identified. An average direction for each episode of linking was calculated and plotted in two dimensions using spherical coordinates (altitude and azimuth). In addition, the nature of initiation and termination of linking was examined. Of the twelve patients, 611 episodes of linking (range 1 to 169 per patient, mean 51) were identified. The episodes for most patients clustered closely in direction. In contrast, directions measured for all activations (i.e., linked and not linked) filled up the entire available range. Linking in most cases subjectively appeared to initiate and terminate suddenly. The results indicate that the local anatomy, pathology, or physiology of the atrium has a strong constraining effect on the electrical activations occurring during atrial fibrillation, and revises our perception of activation during atrial fibrillation as "random." The demonstration that local properties greatly influence conduction during fibrillation has important implications for ablation or pacing therapy.

Aged↗
PubMed1997

Discrimination of atrial fibrillation from regular atrial rhythms by spatial precision of local activation direction.

This study tests the hypothesis that atrial fibrillation (AFib) can be discriminated from regular atrial rhythms by a measure of the variation in local activation direction. Human endocardial atrial recordings of AFib, sinus rhythm, atrial flutter, and supraventricular tachycardia were collected using a catheter with orthogonally placed electrodes, and the direction of each activation was calculated using methods previously described by our laboratory. Each recording was divided into segments containing 100 activations, and the spatial precision for each segment was calculated in three dimensions, as well as in each of the three two-dimensional (2-D) planes. The three-dimensional (3-D) spatial precision for 1161 segments of AFib in 11 recordings ranged from 0.09-0.85 (mean = 0.45), whereas the spatial precision for 138 segments of regular rhythms in 28 recordings was > or = 0.91 in all but four instances. The 2-D spatial precision values overlapped for all rhythms. The results indicate that 3-D spatial precision of local activation direction is a useful discriminator of AFib.

Algorithms↗
PubMed1997

Biomedical information technology: medicine and health care in the digital future.

Advancements in medicine and health care are being significantly influenced by the exploding information technology developments. The IEEE Transactions on Information Technology in Biomedicine will address the applications and the infrastructure innovations that would harness biomedical and health care programs in the 21st century.

Biomedical Engineering↗
PubMed1996

A method for determining high-resolution activation time delays in unipolar cardiac mapping.

This paper presents a method for determining activation time delays in unipolar cardiac mapping data to resolutions considerably smaller than the sample interval. The method involves taking two filtered, differentiated electrograms and computing the Hilbert transform of their cross correlation, which exhibits a negative-to-positive zero crossing at the delay time between the signals. Simultaneous endocardial/epicardial recordings of sinus rhythm were made in the swine right atrium using identical, precisely superpositioned electrode arrays. Data were amplified, lowpass filtered, and digitized at 1000 Hz. A window of data was chosen around each electrogram in an endocardial/epicardial electrogram pair. The windowed electrograms were differentiated and highpass filtered, and the Hilbert transform of the cross correlation between the electrograms was computed. The activation time delay was taken to be the first negative-to-positive zero crossing. Average activation time delays (+/- SD) were computed for 4-s sinus rhythm recordings from each endocardial/epicardial electrode pair. For a representative site, the average transmural activation time delay was 0.71 +/- 0.06 ms (n = 10 electrograms). Time delays estimated using the Hilbert transform method were compared with time delays estimated using the maximum negative slope criterion. The Hilbert transform results exhibited much smaller standard deviations, indicating that the Hilbert transform method may produce more accurate time delay estimates than the maximum negative slope method.

Action Potentials↗
PubMed1995

Effects of barriers on propagation of action potentials in two-dimensional cardiac tissue. A computer simulation study.

A two-dimensional anisotropic model of cardiac ventricular muscle was used to study the effects of discontinuities (barriers), such as dead cells or high-resistance areas, on longitudinal plane-wave propagation. Problems in propagation appear when long barriers become thicker and their spacing closer. Short barriers with large widths and small spacing also cause propagation disturbances and significant delays in their vicinity. If the plane wave front propagates through the barriers, the velocity returns to near normal within one-length constant away from the end of the barrier region. For a funnel-like structure, an opening of 13 cells should exist for longitudinal plane wave propagation. For smaller openings, the ratio of openings required for propagation to occur when traveling from a narrow to a wider area of tissue is proportional to the anisotropy ratio, which can cause unidirectional block. Tortuosity, created by spatial distribution of dead cell barriers, can facilitate propagation by changing the effective impedance the wave front sees, and can create multiple local delays, which may result in discrepancies when measuring propagation velocity.

Action Potentials↗
PubMed1995

A frequency domain analysis of spatial organization of epicardial maps.

Mapping of organized rhythms like sinus rhythm uses activation times from individual electrograms, and often assumes that the map for a single activation is similar to maps for subsequent activations. However, during fibrillation, activation times and electrograms are not easy to define, and maps change from activation to activation. Volume and complexity of data make analysis of more than a few seconds of fibrillation difficult. Magnitude Squared Coherence (MSC), a frequency domain measure of the phase consistency between two signals, can be used to help interpret longer data segments without defining activation times or electrograms. Sinus rhythm, flutter, and fibrillation in humans and swine were mapped with an array of unipolar electrodes (2.5 mm apart) at 240 sites on the atrial or ventricular epicardium. Four-second data segments were analyzed. One site near the center of the array was chosen ad hoc as a reference. MSC maps were made by measuring mean MSC from 0-50 Hz between every point in the array relative to the reference. Isocoherence contours were drawn. The effects of bias in the coherence estimate due to misalignment were investigated. Average MSC versus distance from the reference was measured for all rhythms. Results indicate that in a 4-s segment of fibrillation, there can exist some phase consistency between one site and the reference and little or none between a second site and the reference even when both sites are equidistant from the reference. In fibrillation, isocoherence contours are elongated and irregularly shaped, reflecting long-term, but nonuniform, spatial organization. That is, activation during fibrillation cannot be considered as random over a 4-s interval. Bias in the coherence estimate due to misalignment is significant for sinus rhythm and flutter, but can be corrected by manual realignment. Average MSC drops with distance for all rhythms, being most pronounced for fibrillation, MSC maps may provide insights into long-term spatial organization of rhythms that would otherwise be cumbersome and difficult to interpret with standard time domain analysis.

Animals↗
PubMed1994

Observations from intraatrial recordings on the termination of electrically induced atrial fibrillation in humans.

BACKGROUND: The circulating wavelet hypothesis suggests that atrial fibrillation could terminate by either progressive fusion or simultaneous block of all wavelets. METHODS: Intraatrial recordings from the right atrial free wall were made during procainamide induced (n = 8) or spontaneous (n = 7) termination of electrically induced atrial fibrillation in 14 patients. Atrial rate, mean magnitude squared coherence, and direction of activation during sequential electrograms were measured. Rate and coherence were calculated from the earliest point within 5 minutes prior to termination as well as from the 4-second interval just prior to termination. RESULTS: Termination was directly to sinus rhythm (13 episodes) or to atrial flutter (2 episodes). For the eight procainamide induced terminations, rate decreased between the first measurement and the measurement just prior to termination, from 443 +/- 127 beats/min to 322 +/- 119 beats/min. For the seven spontaneous terminations, rate also decreased from 373 +/- 119 beats/min to 323 +/- 88 beats/min; however, a slight increase in atrial rate prior to termination was observed in three episodes. No specific patterns of atrial cycle lengths were seen during the final few seconds of fibrillation. No increase in coherence was observed. In seven episodes, recordings were made using orthogonal bipoles in the x, y, and z directions, allowing direction of activation of wavefronts to be measured. Three episodes showed multiple instances where direction of activation remained similar over several electrograms as we have previously reported for chronic fibrillation. However, no such instances precipitated termination in any of the seven episodes. CONCLUSIONS: Atrial fibrillation usually terminates directly to sinus rhythm and does so abruptly and without forewarning. While we and others have previously reported that the rate of atrial fibrillation decreases with procainamide infusion, a decrease in the rate of atrial fibrillation is not required for the rhythm to terminate and consequently may not be a part of the termination process at all. Coherence does not demonstrate a progressive increase in the organization of atrial fibrillation prior to termination. Lack of stabilization in the direction of activation of wavefronts in the final few seconds also fails to support fusion of wavefronts as the mechanism of termination of atrial fibrillation. Simultaneous block of all wavelets is consistent with, but not proven by, our observations.

Adolescent↗
PubMed1992

Unidirectional block in cardiac fibers: effects of discontinuities in coupling resistance and spatial changes in resting membrane potential in a computer simulation study.

The mechanisms for conduction and unidirectional block (UDB) in cardiac tissue under spatial changes in cell-to-cell coupling resistivity (Ri) and resting potential (Vrest) were studied. Cable theory was used to simulate the cardiac fiber, and the Beeler and Reuter model, or a modified model based on the Ebihara-Johnson formulation was used to describe the ionic currents. The effects of discontinuities in Ri as would result from collagenous or fibrotic tissue on propagation characteristics were studied. We were especially interested in the effects on propagation characteristics of discontinuities in Ri in the border zone between normal and ischemic tissue. We found that conduction block is more likely to occur when an abrupt decrease in Ri is encountered as compared to an abrupt increase in Ri. Discontinuities in Ri were found to cause changes in propagation characteristics, changing regions of bidirectional block to UDB or bidirectional propagation. Spatial changes in Vrest were also studied. We found that when Vrest alone was altered, block was not likely to occur, while discontinuities in Ri superimposed with Vrest gradients increased the likelihood of block. We also found that Ri discontinuities located in the border zone between normal and ischemic tissue can create exit block or propagation of a parasystolic focus.

Action Potentials↗
PubMed1992

Evidence for transient linking of atrial excitation during atrial fibrillation in humans.

BACKGROUND: Atrial fibrillation is usually thought of as a "random" pattern of circulating wavelets. However, local atrial activation should be influenced by the constant anatomy and receding tail of refractoriness from the previous activation. The general tendency for wave fronts to follow paths of previous excitation has been termed "linking." We examined intra-atrial electrograms recorded during atrial fibrillation for evidence of linking. METHODS AND RESULTS: Two minutes of atrial fibrillation were recorded in 15 patients with an orthogonal catheter. We have previously demonstrated that this catheter can be used to detect changes in the direction of local atrial activation. A mean vector was calculated for each electrogram. The similarity of the direction of the vectors from two consecutive electrograms can be quantified on a scale of 1 to -1 by calculating the cosine (cos) of the smallest angle (theta) between them. Two vectors pointing in the same or opposite directions then have cos(theta) = 1 or -1, respectively. For the entire group of patients, mean cos(theta) was significantly greater than 0 (mean, 0.36; p less than 0.001). In nine of 15 patients, there were groups of six or more consecutive beats (total, 44 groups; range, six to 14 beats per group) in which the direction of activation of each beat was within 30 degrees of the previous beat. The likelihood of one group of six or 14 consecutive similar beats occurring by chance in any one patient in 1 minute is less than 0.05 and less than 0.0000001, respectively. There was a significant correlation (r = 0.90) between the amount of linking during the first and second minutes of atrial fibrillation in each patient. CONCLUSIONS: Transient similarities in the direction of wavelet propagation in the majority of patients with atrial fibrillation is consistent with the presence of transient linking. To our knowledge, this is the first direct evidence that atrial activation during atrial fibrillation in humans is not entirely random.

Aged↗
PubMed1991

Detection of changes in atrial endocardial activation with use of an orthogonal catheter.

The ability of a catheter with an orthogonal electrode configuration to sense differences in the direction of local atrial endocardial activation was tested in 18 consecutive patients with intact retrograde conduction. In all 18, discrimination of anterograde from retrograde conduction at a single atrial site was examined; in 5 of the 18, multiple sites were examined to determine if the discriminatory ability of the catheter was site dependent. The catheter was specially designed with bipoles in the x, y and z directions. A vector was computed for each electrogram during anterograde and retrograde conduction. Electrogram amplitude along the standard bipole was also compared for anterograde and retrograde conduction. Mean electrogram amplitude for the standard bipole was significantly different for anterograde than for retrograde conduction in 17 of 18 patients (mean +/- SD 4 +/- 1.9 vs. 2.7 +/- 1.3 mV; p less than 0.005), with complete separation of amplitude distributions in 4 patients. The electrogram vector during anterograde conduction was significantly different from that during retrograde conduction in all 18 patients (p less than 0.0001), with complete separation of vector distributions in 14. In some patients with multiple site recordings, the choice of site greatly affected separation based on electrogram amplitude or vector, or both. The orthogonal catheter can be used to sense directional differences in local endocardial activation. The catheter shows promise for discriminating anterograde from retrograde conduction and examining the direction of endocardial activation in the heart during an electrophysiologic examination.

Atrial Function↗
PubMed1990

Effect of bipole configuration on atrial electrograms during atrial fibrillation.

Despite an increasing body of work on the nature of fibrillatory rhythms, and the application of different bipole configurations in antifibrillatory devices, little published work has assessed the effect of bipole configuration on the endocardial recordings of fibrillatory rhythms. To address this issue, a specially designed 6 Fr decapolar catheter was used to record intra-atrial electrograms during sustained atrial fibrillation in 15 patients. Simultaneous filtered (30-500 Hz) and unfiltered (0.05-5,000 Hz) recordings of atrial fibrillation were performed of four different bipole configurations: (a) 1-mm interelectrode spacing adjacent to the atrial wall; (b) 10-mm interelectrode spacing adjacent to the atrial wall; (c) 10-mm inter-electrode spacing 24 mm from the distal catheter tip; (d) 1-mm interelectrode spacing 24 mm from the distal catheter tip. One minute of such data was recorded, and each 4.27-second segment (x 14 segments) was analyzed for atrial rate, electrogram amplitude, amplitude probability density function (apdf), median frequency in the 2-9 Hz band, and electrogram morphology. Changes in bipole configuration resulted in profound changes in calculated atrial rate, amplitude, and apdf (P less than 0.001 by two-way ANOVA in each instance). Specifically, closer interbipole spacing and closer proximity to the atrial wall resulted in lower calculated atrial rates, higher electrogram amplitudes, and higher apdf values. In contrast, median frequency proved to be a more robust measure despite multiple configurations (P greater than 0.10 by two-way ANOVA). These changes significantly affected the predictive value of previously published detection criteria for rate (P less than 0.01) and apdf (P less than 0.00001). Bipole location also affected morphology, with locations adjacent to the atrial wall and with closer interbipole spacing having more discrete electrograms and greater apparent organization (P less than 0.0001). Further, when data segments from all patients and bipole configurations were grouped, rate and apdf were found to be strongly inversely correlated (r = -0.808). (r = -0.808).(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1990

Differentiation of ventricular tachyarrhythmias.

Implantable devices capable of several modes of therapy will require differentiation of various ventricular tachyarrhythmias. Three methods of arrhythmia analysis, magnitude-squared coherence, ventricular rate, and irregularity of cycle length were performed for 45 episodes of induced ventricular tachyarrhythmia in 15 patients. Differentiation of monomorphic ventricular tachycardia from polymorphic ventricular tachycardia and ventricular fibrillation was possible by mean magnitude-squared coherence, less possible by rate, and not possible by beat-to-beat irregularity. Faster monomorphic ventricular tachycardia overlapped with rates of polymorphic ventricular tachycardia and ventricular fibrillation. Differentiation of polymorphic ventricular tachycardia and ventricular fibrillation was not possible by rate or irregularity. A progressive decrease in mean magnitude-squared coherence from monomorphic ventricular tachycardia to polymorphic ventricular tachycardia to ventricular fibrillation strengthens previous observations that coherence is a measure of rhythm "organization."

Adult↗
PubMed1989

The coherence spectrum. A quantitative discriminator of fibrillatory and nonfibrillatory cardiac rhythms.

Previous work has suggested that a comparison of electrograms from two or more sites may best differentiate fibrillatory from nonfibrillatory rhythms. The coherence spectrum is a measure by which two signals may be compared quantitatively in the frequency domain. In the present study, the coherence spectrum was used to quantify the relation between spectral components of electrograms from two sites in either the atrium or ventricle during both fibrillatory and nonfibrillatory rhythms. Bipolar recordings of 35 rhythms from 20 patients were analyzed for coherence in the 1-59 Hz band. The 17 nonfibrillatory rhythms were sinus rhythm (six), paroxysmal supraventricular tachycardia (two), atrial flutter (four), and monomorphic ventricular tachycardia (five). The 18 fibrillatory rhythms were atrial fibrillation (12) and ventricular fibrillation (six). Nonfibrillatory rhythms exhibited moderate-to-high levels of coherence throughout the 1-59 Hz band, with peaks concentrated at the rhythm's fundamental frequency and its harmonics. Fibrillatory rhythms exhibited little coherence throughout the 1-59 Hz band, and harmonics were not evident. The mean magnitude-squared coherence (scale of 0 to 1) for the 1-59 Hz band ranged from 0.22 to 0.86 (mean +/- SD, 0.52 +/- 0.19) for nonfibrillatory rhythms and from 0.042 to 0.12 (0.067 +/- 0.021) for fibrillatory rhythms. Separation of fibrillatory and nonfibrillatory rhythms was possible whether signals were recorded by floating or fixed-electrode configurations. These findings indicate that comparison of two electrograms with magnitude-squared coherence measurements differentiates fibrillatory from nonfibrillatory rhythms. A recognition algorithm based on coherence spectra may provide a major variations in lead configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Ventricular response to atrial fibrillation: role of atrioventricular conduction pathways.

Irregularity of the ventricular rhythm is a hallmark of patients with atrial fibrillation, yet the genesis of the irregularity is not yet fully understood. The role of the atrioventricular (AV) node in determining the irregularity of the ventricular response to atrial fibrillation was investigated by comparing the frequency distributions of the atrial (AA) and the ventricular (RR) intervals. Atrial electrograms and surface electrocardiographic leads were recorded during sustained atrial fibrillation in 12 patients with conduction over the AV node. The scaling factor (mean RR interval/mean AA interval) quantified the ability of the conduction pathway to scale the atrial input to a slower ventricular response and ranged from 2.55 to 5.92 (mean +/- SD 3.77 +/- 0.92). The coefficient of variation (SD/mean) measured the relative variability of the AA and RR interval distributions. The atrial and ventricular coefficients of variation were not significantly different (0.20 +/- 0.04 versus 0.21 +/- 0.03, p greater than 0.27). Similar recordings were analyzed in six patients with conduction over a accessory AV pathway. The scaling factor ranged from 1.54 to 2.46 (2.02 +/- 0.39) and, as was the case for patients with conduction over the AV node, the atrial and ventricular coefficients of variation did not significantly differ (0.24 +/- 0.08 versus 0.27 +/- 0.10, p greater than 0.6). For both groups of patients, ventricular variability and the maximal RR intervals were predicted by the product of the scaling factor and either atrial variability or maximal AA intervals, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗