Ventricular arrhythmias in the subacute myocardial infarction period. High-resolution activation and refractory patterns of reentrant rhythms

Effects of azimilide dihydrochloride on circus movement atrial flutter in the canine sterile pericarditis model

INTRODUCTION

The effects of a Class III agent, azimilide dihydrochloride, on atrial flutter circuits were studies in a functional model of single loop reentrant atrial flutter using dogs, 3 to 5 days after production of sterile pericarditis.

METHODS AND RESULTS

A computerized mapping system was used to construct activation maps from 138 to 222 epicardial sites in the right atrium. Doses of 3, 10, and 30 mg/kg i.v. azimilide dihydrochloride were analyzed in 8 dogs in which sustained atrial flutter lasting more than 30 minutes was induced by burst pacing. Atrial flutter was always due to single loop circus movement reentry in the lower right atrium. At 3 mg/kg, azimilide dihydrochloride terminated atrial flutter in 2 dogs; however, atrial flutter was reinduced. At 10 mg/kg, atrial flutter was terminated in all 8 dogs but was reinduced in 4 dogs with slower rate. At 30 mg/kg, atrial flutter was terminated in the remaining 4 dogs and could not be reinduced. Atrial flutter cycle length always increased prior to termination. Isochronal activation maps showed that the increase in cycle length was due to additional conduction delays in the slow zone of the reentrant circuit. The site of termination was always located within the slow conduction zone situated in the lower right atrium between the line of functional conduction block and the AV ring. Effective refractory periods (ERPs) were measured at selected sites in the slow zone and normal zone at twice diastolic threshold for the 10 mg/kg dose. Azimilide preferentially prolonged ERP in the slow zone (42.4 +/- 20.1 msec, mean +/- SD) compared with the normal zone (23.3 +/- 15.4 msec, P < 0.0001). The increase in cycle length corresponded with the increase in ERP in the slow zone.

CONCLUSIONS

In a functional model of circus movement atrial flutter, azimilide dihydrochloride terminates and prevents reinduction of atrial flutter by a preferential increase in refractoriness leading to further conduction delay and conduction block in the slow zone of the functional reentrant circuit.

Correlation between late potentials duration and QTc dispersion: Is there a causal relationship?

QTc interval dispersion (QTcd) analysis (difference between maximum and minimum QTc calculated from at least five of the standard 12 ECG leads) and signal-averaged electrocardiograms were performed on 23 patients referred to our coronary care unit because of acute myocardial infarction. Late potentials were considered positive if all three of the following criteria were satisfied: (1) total QRS duration (QRSd) > 114 ms; (2) duration of QRS under 40 muV (LAS 40) > 38 ms; (3) root mean square voltage of the last 40 ms of QRS (RMS 40) < 25 muV. Patients were divided into two groups according to the presence (group A, 9 patients) or absence of late potentials (group B, 14 patients). Group A patients showed a significantly higher QTcd (0.0652 +/- 0.0177 s vs. 0.0448 +/- 0.0201 s; P = 0.021) and a significantly longer mean QTcm (0.43117 +/- 0.01817 s vs. 0.40472 +/- 0.03013 s; P = 0.028) than group B patients. Among the three different parameters used to define the presence of late potentials, QTcd was significantly related to LAS 40 (r = 0.418, P = 0.047) and mean QT cm to QRSd (r = 0.497; P = 0.016). We also found a significant correlation between QTcd and mean QTcm (r = 0.426; P = 0.043). In conclusion, our data suggest that (1) the presence of late potentials is associated with a greater dishomogeneity of ventricular recovery time; (2) the longer the duration of late potentials, expressed by LAS 40, the greater the QTcd, suggesting that the dispersion of repolarization could be attributed to slowly conducting areas from which late potentials arise; (3) mean QTcm is not useful to identify these areas because it is more affected by total rather than by terminal QRS duration; (4) regional discrepancies of ventricular recovery time are connected with general repolarization duration.

QT dispersion in nonsustained ventricular tachycardia and coronary artery disease

This study examines the relation between QT dispersion and the inducibility of ventricular tachycardia (VT) in 35 consecutive patients with coronary artery disease who underwent electrophysiologic testing for evaluation of nonsustained VT. The mean age of the patients was 66 +/- 9 years (+/- SD) and the mean left ventricular ejection fraction was 0.36 +/- 0.14. In 6 patients in whom sustained, monomorphic VT was inducible by programmed ventricular stimulation, QT dispersion was significantly greater than in the 29 patients in whom VT was not inducible (126 +/- 35 vs 67 +/- 25 ms, p < 0.001). All patients who had a QT dispersion > 120 ms had inducible sustained monomorphic VT, and no patient who had a QT dispersion < 90 ms had inducible VT. The patients who had inducible VT dis not differ significantly from those who did not with regard to age, gender, ejection fraction, RR interval, or mean QT. In conclusion, in patients with coronary artery disease who have nonsustained VT, inducibility of monomorphic VT is associated with an increase in QT dispersion. QT dispersion may be helpful in predicting which patients with nonsustained VT are and are not likely to have inducible VT by programmed stimulation.

Effect of sustained load on dispersion of ventricular repolarization and conduction time in the isolated intact rabbit heart

INTRODUCTION

It is well known that myocardial stretch can elicit ventricular arrhythmias in experimental models. However, previous reports have predominantly documented stretch-induced arrhythmias during short, pulsatile stretch. The arrhythmogenic mechanism of sustained static stretch is incompletely understood.

METHODS AND RESULTS

To examine the influence of sustained load on several electrophysiologic parameters, a latex balloon was placed into the left ventricle of ten isolated Langendorff-perfused rabbit hearts and filled with a neutral volume of fluid. The heart was paced from a catheter inside the right ventricle (apicoseptal endocardial position), and the following parameters were studied during steady-state pacing with a cycle length of 500 msec (S1) and during extrastimulation (S2, base drive of 8 beats): monophasic action potential (MAP) durations at 90% repolarization (APD90) from 5 to 6 epicardial electrodes located on both ventricles and one right ventricular endocardial contact electrode; dispersion of APD90 (range of MAP durations from all electrodes); effective refractory period (ERP) and longest activation time (pacing stimulus to MAP upstroke). After baseline recordings, the balloon inside the left ventricle was filled with a volume of 1.0 mL of fluid by means of a servo-controlled pump. The ERP was significantly shortened from 198 +/- 9 msec at baseline to 183 +/- 8 msec during sustained load (P < 0.03). Similarly, the average APD90 was shortened from 180 +/- 5 msec at baseline to 175 +/- 6 msec during sustained load (P < 0.006) with steady-state pacing and from 178 +/- 6 msec to 170 +/- 8 msec during premature extrastimulation (P < 0.03). At the same time, dispersion of APD90 was increased from 27 +/- 5 msec to 38 +/- 6 msec (P < 0.002) during steady-state pacing and from 28 +/- 4 msec to 38 +/- 6 msec (P = 0.013) during premature extrastimulation. The longest activation time among all MAP recordings was increased from 39 +/- 2 msec to 43 +/- 3 msec (P = 0.003) during steady-state pacing and from 56 +/- 6 msec to 69 +/- 6 msec during premature extrastimulation (P < 0.003).

CONCLUSIONS

Sustained load shortens the ERP and the mean APD90, and at the same time increases dispersion of APD90 and prolongs activation times. These findings provide additional insight into the arrhythmogenic mechanisms of sustained mechanical load.

Activation time determination by high-resolution unipolar and bipolar extracellular electrograms in the canine heart

INTRODUCTION

To identify the optimal criteria for activation time (AT) determination of bipolar electrograms from normal hearts, a high-resolution cross electrode array comprising 128 unipolar electrodes of 500-microns spacing was used to record extracellular potentials from the left ventricular epicardium of 12 dog hearts.

METHODS AND RESULTS

Recordings were made during broad wavefront propagation (B wave) and local elliptical wavefront propagation (E wave). Characteristics of 863 bipolar electrograms (1-mm spacing) were constructed from unipolar data standardized for differences in polarity, then classified morphologically. Features for bipolar AT determination were compared to the time of the negative peak of the first temporal derivative of a unipolar electrogram situated mid-way between the bipoles. During B wave, three distinct morphologies were observed: uniphasic (61%), biphasic (23%), and triphasic (16%). Peak voltage of uniphasic and triphasic signals was the best predictor of AT (error: 0.6 +/- 0.6 msec and 0.6 +/- 0.8 msec, respectively). During E wave, parallel orientation of the bipoles with respect to the direction of impulse propagation wavefront resulted in uniphasic signals (> 99%), while for perpendicular orientation of the bipoles, electrogram morphology was variable. For parallel orientation of the bipoles, peak negative voltage was the best predictor of AT for both longitudinal and transverse propagation, while for perpendicular bipole orientation, peak negative voltage was a less reliable predictor for propagation along both fiber axes. Increasing interpolar distance resulted in a degradation in AT accuracy for B wave (from 0.6 +/- 0.6 msec at 1 mm to 1.1 +/- 1.2 msec at 7 mm) and for E wave (from 0.4 +/- 0.3 msec at 1 mm to 3.1 +/- 2.9 msec at 7 mm).

CONCLUSIONS

(1) The accuracy of bipolar electrograms is sensitive to wavefront direction, bipole orientation, and interpolar distance; (2) peak negative voltage of uniphasic and triphasic signals is a reliable predictor of AT, but only for B wave; (3) a maximum interpolar distance of 2 mm and bipole orientation parallel to the direction of the impulse wavefront are minimally required for accurate determination of AT during impulse propagation initiated near the recording electrodes; and (4) for impulses initiated near the recording site in normal tissue, a biphasic or triphasic morphology almost certainly indicates that the bipolar electrode is oriented perpendicular to the wavefront direction, irrespective of fiber orientation.

Anisotropic conduction in monolayers of neonatal rat heart cells cultured on collagen substrate

Anisotropic impulse conduction was studied in neonatal rat heart cell monolayers produced by culturing cells on a growth-directing substrate of collagen. Monolayers consisting of parallel-oriented cells without visible intercellular clefts were selected for experiments; cell lengths and widths were 65.8 +/- 12.5 and 12.2 +/- 3.2 microns (n = 49), respectively. Action potential upstrokes were measured by using 12 photodiodes selected within a 10x10 diode array and a voltage-sensitive dye (RH-237). The size of the area sensed by a single diode was 14 x 14 microns. High-density multiple recordings (resolution, up to 15 microns) demonstrated the variability of local activation delays and of the maximal rate of rise of the action potential upstroke (Vmax), which are presumably related to the microscopic cellular architecture. Mean macroscopic conduction velocities measured over distances of 135 microns were 34.6 +/- 4.5 and 19.0 +/- 4.3 cm/s (mean +/- SD, n = 13, P < .0001) in longitudinal and transverse directions, respectively. The anisotropic velocity ratio was 1.89 +/- 0.38 (n = 13). Mean Vmax was not significantly different in two directions (122.0 +/- 17.4 V/s longitudinally versus 125.2 +/- 15.6 V/s transversely, n = 13, P = NS). In conclusion, we developed an anisotropic cell culture model suitable for studying impulse conduction with cellular resolution. The anisotropic velocity ratio was close to values measured in vivo. By contrast, Vmax was not dependent on the direction of propagation.

A logical state model of circus movement atrial flutter role of anatomic obstacles, anisotropic conduction and slow conduction zones on induction, sustenance, and overdrive paced modulation of reentrant circuits

Mapping studies of atrial flutter in both the canine sterile pericarditis model and the right atrial enlargement model commonly reveal single loop reentrant circuits in the lower posterior part of the right atrium. Functional bidirectional conduction block and natural anatomical obstacles comprise the central obstacle for reentrant impulse during circus movement atrial flutter. Because the relative roles of anatomical obstacles, in combination with functional barriers, anisotropic conduction, and slow conduction can not be readily assessed with current electrophysiological techniques, an atrial activation model was developed to study the mechanisms of circus movement atrial flutter. A discrete state model consisting of 4096 logically connected cardiac elements was used to simulate atrial activation; an inexcitable region simulating the inferior vena cava (IVC) was also incorporated in the model. Atrial flutter was induced by programmed premature stimulation. Anisotropic conduction velocity properties, regional variations in slow conduction, regional refractory gradients and stimulation parameters were specified for each simulation. The reentrant circuit generally consisted of a single reentrant impulse which circulated around a continuous line of functional bidirectional conduction block joined to the IVC. Rapid pacing, 5-30 ms shorter than the spontaneous reentrant cycle length, was applied to entrain and/or terminate the rhythm. The results of this study demonstrate that patterns of initiation, entrainment, termination and reinitiation of circus movement atrial flutter mimic results from in vivo activation mapping studies. We find that sustained circus movement atrial flutter circuits depend on: 1) natural anatomical obstacles to stabilize reentrant circuits, and 2) anisotropic conduction properties to reduce the degree of functional conduction block needed to maintain circus movement. Rapid pacing of simulated circus movement atrial flutter demonstrated that the entrainment criteria can be satisfied in a two-dimensional syncytium.

QT interval dispersion: a non-invasive marker of susceptibility to arrhythmia in patients with sustained ventricular arrhythmias?

OBJECTIVE

To assess QT interval dispersion on the surface electrocardiogram in patients with sustained ventricular arrhythmias.

DESIGN

A retrospective and prospective blinded controlled study of patients referred for investigation of ventricular arrhythmias at a tertiary cardiac centre.

PATIENTS AND METHODS

89 consecutive patients with sustained ventricular arrhythmias due to chronic ischaemic heart disease, cardiomyopathy, or ventricular tachycardia (VT) in a normal heart. 32 patients did not meet the inclusion criteria; therefore 57 patients were compared with a control group of 40 patients with myocardial disease but no history of arrhythmias and 12 normal controls with no myocardial disease. Standard 12 lead electrocardiograms were enlarged, the QT intervals for each lead measured, and QT dispersion calculated.

RESULTS

There was a significantly greater mean QT dispersion (77 ms) in patients with sustained ventricular arrhythmias compared with the control group (38 ms, p < 0.01). This held for all groups; after myocardial infarction VT (82 (22) ms v control 38 (10) ms; p < 0.01), dilated cardiomyopathy VT (76 (18) ms v control 40 (11) ms, p < 0.01), and normal heart VT (65 (7) ms v control 32 (8), p < 0.05). There was also a greater QT dispersion in patients with impaired left ventricular function and VT, with a correlation between left ventricular function and QT dispersion in patients with VT (r = 0.56, p < 0.01).

CONCLUSION

QT interval dispersion may be a further non-invasive marker of susceptibility to ventricular arrhythmias.

Effects of activation sequence on the spatial distribution of repolarization properties

The electrotonic effects of activation spread on the spatial distribution of repolarization properties were studied in animal experiments and with computer simulations. Refractory periods (RPs) were measured at 36 sites within a 1.0 cm2 region of the epicardial surface of the canine pulmonary conus during 37 drives in 11 experiments. In each experiment three or four sites along the perimeter of the region bounding the RP test sites were driven. Activation propagated uniformly during some and nonuniformly during other drives in the same animals. In general, RPs were distributed uniformly when activation spread uniformly and nonuniformly when activation spread nonuniformly. The authors observed RP differences as large as 16 ms between sites with 2 mm separation during drive from some epicardial sites in these normal canine hearts. Indices of nonuniformity of activation and of relative RP values were used to quantify the relation between nonuniformity of activation spread and the spatial distribution of the RP. There was a significant negative correlation between nonuniformity of activation and RP indices during the 19 drives in which activation spread nonuniformly. This indicated that RPs were relatively long at sites where activation spread decelerated and relatively short at sites where activation spread accelerated. When nonuniform activation spread was simulated by introducing high-resistance barriers in a model with fixed anisotropic conductivities, there were marked spatial variations in action potential duration. The spatial variations in action potential duration were negatively correlated to acceleration and deceleration of activation spread. The major new finding of this study is that the spatial distributions of RPs are markedly affected by activation spread. Since both characteristics of activation sequence and nonuniformity of RP distributions have roles in reentrant arrhythmias, the findings suggest that some sites of origin of premature activity may be more arrhythmogenic than others. The findings may also explain why ventricular tachycardia can sometimes be initiated from one but not from other sites in patients undergoing electrophysiologic testing.

Circus movement atrial flutter in the canine sterile pericarditis model. Cryothermal termination from the epicardial site of the slow zone of the reentrant circuit

BACKGROUND

We have shown that atrial flutter (AF) in dogs with sterile pericarditis is commonly due to a single-loop reentrant circuit in the lower right atrium comprised of a functional or functional/anatomic obstacle and a slow zone of conduction (SZ) between the central obstacle and the atrioventricular (AV) ring. The goals of the present study were 1) to establish that the epicardial SZ is the critical component of circus movement AF and 2) to identify the optimal site within the epicardial SZ at which interruption of circus movement can be accomplished by ablative techniques.

METHODS AND RESULTS

We analyzed the atrial activation patterns during epicardial cooling of the SZ with as N2O-cooled probe in eight dogs (five with clockwise [CW] reentrant circuit, one with counterclockwise [CCW] reentrant circuit, and two with both CW and CCW reentrant circuits around the same pathway). In all eight dogs, cooling (-5 to +5 degrees C for 5-20 seconds) the narrow isthmus at the inferoposterior part of the SZ between the central obstacle and the AV ring reversibly terminated the reentrant circuit, whereas cooling outside this area failed to terminate the reentrant circuit. The circus movement was not observed to continue along alternate pathways when conduction in this critical zone was interrupted. Both CW and CCW reentrant circuits could be terminated from the same site within the SZ. Cooling resulted in slowing of conduction in the SZ (55 +/- 15 msec) in both CW and CCW reentrant circuits. Cooling-induced termination of CW reentrant circuits was characteristically associated with oscillations of conduction in the cooled zone of the last three cycles before termination and conduction block occurred within the cooled zone. The last "manifest" reentrant cycle was associated with the longest conduction delay in the cooled zone. However, this delay was not necessarily reflected in the length of the last reentrant cycle because of compensatory acceleration of conduction in the rest of the pathway. On the other hand, in CCW reentrant circuits, conduction block occurred abruptly at the distal border of the SZ and without significant oscillations of conduction.

CONCLUSIONS

The present study provides convincing evidence that single-loop circus movement in this model is critically dependent on an obligatory conduction in a SZ in the inferoposterior portion of the free right atrial wall between a functional obstacle and the AV ring. Because the atrial myocardium behaves electrophysiologically as a two-dimensional surface, the results of this study may help to guide the endocardial electrode catheter ablative technique for treatment of clinical AF.

Computer simulations of three-dimensional propagation in ventricular myocardium. Effects of intramural fiber rotation and inhomogeneous conductivity on epicardial activation

Three-dimensional membrane-based simulations of action potential propagation in ventricular myocardium were performed. Specifically, the effects of the intramural rotation of the fiber axes and inhomogeneous conductivity on the timing and pattern of epicardial activation were examined. Models were built, with approximately 400,000 microscopic elements arranged in rectangular parallelepipeds in each model. Simulations used the nonlinear Ebihara and Johnson membrane equations for the fast sodium current. Constructed models had histological features of ventricular myocardium. All models were anisotropic. In a subset of the models, an abrupt intramural rotation of the fiber axes was included. This feature was also combined with randomly distributed inhomogeneous conductivity and regions of high transverse resistance to represent nonuniform anisotropy in a further subset of the models. Epicardial stimuli were applied for each simulation. Three-dimensional activation patterns and epicardial isochron maps were constructed from the simulations. We noted that the rotation of fiber axes accelerated epicardial activation distant from the stimulus site. The inhomogeneous conductivity caused regional acceleration and deceleration of activation spread. We also noted features of epicardial activation that resulted from the fiber rotation, and the inhomogeneous conductivity corresponded to that observed in maps from experimental animals.

Criteria for local myocardial electrical activation: effects of electrogram characteristics

Detection of local electrical myocardial activation by means of extracellular recordings is often difficult in the presence of polyphasic electrograms. The purpose of this investigation was to compare the ability of several variables to distinguish unipolar deflections due to local activation from those due to nonlocal activity. A model of polyphasic deflections based on atrial recordings during reentrant tachycardia was used to facilitate distinction of local and distant activity by methods independent of the test variables. The performance of variables were assessed by comparing areas under receiver operating characteristic curves. Optimal thresholds of test variables were identified by maximizing statistics which corrected for the pretest probability of local activation. We found that the greatest negative first derivative of the unipolar potential discriminated between local and distant ventricular signals, but performed less well than the ratio of the first derivative to the potential for distinguishing between local atrial signals and distant ventricular signals. A linear combination of the potential and the ratio of the first derivative and the potential performed well for all groups of signals studied. We conclude that optimal criteria for detecting local activation depends on the characteristics of the population of signals and that a statistical approach can be used to identify optimal criteria for a given population.

A unified functional/anatomic substrate for circus movement atrial flutter: activation and refractory patterns in the canine right atrial enlargement model

OBJECTIVES

This study was designed to test the concept of a functional/anatomic interaction in a canine model of reentry based on right atrial enlargement and to elucidate the electrophysiologic basis for functional conduction block.

BACKGROUND

The monotonic feature of atrial flutter suggests a uniform substrate for the arrhythmia. Atrial flutter in the sterile pericarditis model is due to single-loop circus movement around a functional or a functional/anatomic obstacle near the atrioventricular (AV) ring. Sustained circus movement requires a critical interaction of a functional arc of block, a natural obstacle, the AV ring and a zone of slow conduction. The location of the inferior vena cava predisposes the lower right atrium to single-loop reentry.

METHODS

In 11 dogs with right atrial enlargement, 127 bipolar epicardial electrograms were obtained during atrial flutter. For correlation of activation and refractory maps, the effective refractory period under each electrode was determined using the extrastimulus technique.

RESULTS

Atrial flutter was due to single-loop reentry around functional arcs of block near the AV ring (n = 2) or around functional/anatomic obstacles (n = 8) involving the inferior vena cava. A slow zone was located between the arc and the AV ring and between the inferior vena cava and AV ring, respectively. During initiation, the arc joined the AV ring, forcing activation to proceed around the free end of the arc before breaking through the arc near the AV ring. Arrhythmia termination required the arc of block to rejoin the AV ring. Inducibility of sustained atrial flutter was associated with a marked spatial dispersion of refractoriness. The configuration of the functional arc of block was critically dependent on the spatial pattern of refractoriness.

CONCLUSIONS

Atrial flutter requires a similar functional or functional/anatomic substrate independent of the underlying etiology. The spatial distribution of refractoriness in enlarged canine atria provides an adequate substrate for the development of functional conduction block.

Reentrant and focal mechanisms underlying ventricular tachycardia in the human heart

BACKGROUND

To determine the mechanisms of ventricular tachycardia (VT) in humans, three-dimensional intraoperative mapping of up to 156 intramural sites was performed in 13 patients with healed myocardial infarction and refractory VT.

METHODS AND RESULTS

Mapping was of sufficient density to define the mechanism of 10 VTs in eight patients. In five of 10 cases, sustained VT was initiated in the subendocardium or epicardium by intramural reentry with marked conduction delay as well as functional and anatomic block most prominent in the subendocardium and midmyocardium. The initiating beats of reentrant VT induced by programmed electrical stimulation arose in the endocardium or midmyocardium by progressive slowing of conduction leading to unidirectional block. Multiple simultaneous reentrant circuits can be present. In contrast, five of the 10 sustained VTs were initiated by a focal mechanism as defined by the absence of electrical activity between the termination of one beat and the initiation of the next despite the presence of multiple intervening intramural electrode recording sites. Comparisons of the mapping data with results of histopathological analysis of tissue demonstrated that the location of infarction as well as that of adjacent fibrotic muscle determined sites of both fixed and functional conduction block during macroreentrant VT and that slowing of conduction occurred in a direction transverse rather than longitudinal to fiber orientation.

CONCLUSIONS

Both intramural reentry and a focal mechanism underlie sustained VT in patients with healed myocardial infarction.

Determination of local myocardial electrical activation for activation sequence mapping. A statistical approach

Electrical activation sequence mapping requires accurate identification of local activation, but because extracellular recordings do not exclusively reflect local events, complex electrograms may be difficult to interpret. In such cases, the assignment of local activation is subject to error that could affect interpretation of the resulting activation maps. The purpose of this investigation was to develop an approach that would provide quantitative indexes of error in the determination of local activation. An electrode array with 64 closely spaced unipolar electrodes was used to record from the left ventricular surface during open heart surgery. Electrograms with multiple deflections were recorded from four patients with scarred myocardium; two other patients with normal myocardial function served as controls. Each of 784 deflections was scored on the basis of three features: evidence for propagation, the configuration of the bipolar signal, and the effect of changing from the chest to an average reference. Local activation was considered probable if evidence for all three features was present and improbable if none of the three features was present. Deflections that were ambiguous with respect to this standard were excluded. Of over 30 test variables analyzed, the three with the greatest power to discriminate signals due to local activation from those due to distant activity were 1) a linear combination of the extracellular potential plus the ratio of the second derivative and the extracellular potential, 2) the second derivative, and 3) the minimum (greatest negative) first derivative. For each of these variables, the threshold value providing the greatest performance was identified by the maximum quality of efficiency, an index of agreement. This statistical approach provides an objective basis for determining local activation and provides a quantitative assessment of error that could enhance interpretation of electrical activation sequence maps.

Electrophysiologic mechanisms of ventricular arrhythmias

In this work the electrophysiologic mechanisms of ventricular arrhythmias have been briefly summarized. Ventricular arrhythmias can be caused either by pacemaker activity or by reentrant excitation. Enhancement of normal automaticity can generate a parasystolic rhythm in normal fibers. Abnormal automaticity may arise from fibers in which maximum diastolic potential has been reduced by a variety of interventions. Triggered activity is caused by either an early (EAD) or delayed (DAD) afterdepolarization and requires a prior normal action potential for initiation. While there is growing evidence that EAD-induced triggered activity plays a significant role in the Long QTU syndrome and Torsade de Pointes, no clinical arrhythmias has definitely been ascribed to DADs, although DADs have been recorded in man after acute digoxin intoxication. Ventricular arrhythmias can be also caused by reentrant excitation, which can be subdivided into reflection or circus movement reentry (CMR). In the reflection model impulses in both directions are transmitted over the same pathway. In the CMR three models can be differentiated: the ring model, which requires a fixed anatomical obstacle; the figure-eight model and the leading circle model, where functional rather than fixed anatomical obstacles are involved.

Circus movement atrial flutter in canine sterile pericarditis model. Activation patterns during entrainment and termination of single-loop reentry in vivo

BACKGROUND

Recently, we used a custom designed "jacket" electrode with 127 bipolar electrodes in a flexible nylon matrix to map the total atrial epicardial surface in the in situ canine heart. Atrial flutter in dogs with sterile pericarditis was shown to be due to a single wave front circulating around a combined functional/anatomic obstacle, with the arc of functional conduction block contiguous with one or more of the atrial vessels.

METHODS AND RESULTS

In the present study, this model was used to analyze the activation pattern during pacing-induced entrainment and termination of single reentrant loops in a syncytium without anatomically predetermined pathways. Sustained atrial flutter was induced in five dogs with 3-5-day-old sterile pericarditis. Atrial pacing at a cycle length 5-30 msec shorter than the spontaneous cycle length entrained the arrhythmia and could result in a "classical" activation pattern, characterized by an antidromic stimulated wave that collided with the reentrant orthodromic wave front of the previous beat at a constant site. However, two variations of this classical activation pattern were also observed: 1) Pacing at short cycle lengths could lead to localized conduction block in antidromic direction, forcing a change in the pathway of the antidromic wave front. This could prevent the expected shift of the site of collision in antidromic direction. 2) The stimulated orthodromic wave front could also use a pathway different from that of the original reentrant impulse, so that a different circuit was active during the pacing period. Termination of atrial flutter by rapid atrial stimulation was associated with progressive slowing and finally blocking of the paced orthodromic wave front and a progressive shift of the site of collision in antidromic direction. The occurrence of conduction block was determined by the cycle length of stimulation and the number of stimulated beats. A longer train at the critical cycle length or the critical number of beats at a shorter cycle length could reinduce the same reentrant circuit or a different reentrant circuit, respectively, during stimulated cycles following the beat that terminated reentry.

CONCLUSIONS

The epicardial activation sequence during entrainment of reentrant arrhythmias does not necessarily follow a standard activation pattern. Instead, the stimulated orthodromic as well as the antidromic wave front might use a pathway different from that of the original reentrant wave front. The mechanisms of termination, failure of termination, and reinitiation of single-loop reentry are similar to those in the "figure-eight" reentrant circuit.

Circus movement atrial flutter in the canine sterile pericarditis model. Differential effects of procainamide on the components of the reentrant pathway

To evaluate the mechanisms of action of procainamide on the components of the reentrant pathway, drug-induced changes in activation patterns, effective refractory periods (ERPs), and stimulation thresholds were analyzed in nine dogs with sterile pericarditis and sustained atrial flutter. Activation maps were based on 127 close bipolar recordings from a special "jacket" electrode. From the control map, 22 +/- 2 sites covering the slow zone and the normal zone of the reentrant circuit were selected to measure ERPs and thresholds. The excitable gap was estimated from the longest ERP during pacing at the tachycardia cycle length. During atrial flutter, epicardial activation proceeded as a single wave around an arc of functional conduction block in the proximity of the atrioventricular (AV) ring or around a combined functional/anatomic obstacle, with the arc being contiguous with one of the venae cavae. An area of slow conduction, which accounted for 53 +/- 15% of the revolution time within 35 +/- 15% of the total length of the reentrant pathway, was bordered by the arc of block and the AV ring or a caval vein and the AV ring, respectively. Procainamide (5-10 mg/kg i.v.) prolonged the cycle length of atrial flutter from 144 +/- 17 to 190 +/- 24 msec (p less than 0.05) and then terminated the arrhythmia in all studies. The increase in cycle length was due to an increase in conduction time in the slow zone by 37 +/- 11 msec (86 +/- 17% of the total cycle length increase). During the last reentrant beat, conduction failed in the slow zone, with the arc of block joining the AV ring. At termination, procainamide had prolonged conduction time, stimulation threshold, and ERP in the normal zone by 11 +/- 18%, 40 +/- 80%, and 5 +/- 15%, respectively, compared with 51 +/- 16%, 86 +/- 93%, and 14 +/- 21%, respectively, in the slow zone (p less than 0.05 for all three parameters). The duration of the excitable gap did not change significantly. We conclude that procainamide preferentially affected the slow zone of single loop reentrant circuits. The drug terminated circus movement atrial flutter without abolishing the excitable gap, and its effect on conduction seemed the major determinant of the antiarrhythmic action.

Reentrant ventricular arrhythmias in the late myocardial infarction period: mechanism by which a short-long-short cardiac sequence facilitates the induction of reentry

The electrophysiological mechanism by which a short-long-short stimulated cardiac sequence facilitates the induction of ventricular tachyarrhythmia was investigated in dogs 4 days after ligation of the left anterior descending coronary artery. In these dogs, reentry develops in the surviving electrophysiologically abnormal epicardial layer that overlies the infarct zone when premature stimulation results in a critically long arc of functional conduction block. The activation wavefront circulates around both ends of the arc, coalesces, and conducts slowly distal to the arc before reactivating sites proximal to the arc to initiate a figure-eight reentrant circuit. Epicardial isochronal activation maps and effective refractory periods (ERPs) were determined during three different stimulation protocols: A, a basic train of eight beats at a cycle length of 300 msec followed by a single premature stimulus (S2); B, a basic train of eight beats at a cycle length of 300 msec with abrupt lengthening of the last cycle of the train before S2 to 600 msec; C, a basic train of eight beats at a cycle length of 600 msec followed by S2. Protocol B was found to result in a differential lengthening of ERP at adjacent sites within the border of the epicardial ischemic zone, whereas protocols A and C induced, respectively, comparable shortening and lengthening of ERPs at the same sites. The differential lengthening of ERPs at adjacent sites resulted in an increased dispersion of refractoriness so that a premature stimulus induced functional conduction block between those sites. The development of a longer arc of conduction block and, hence, a longer reentrant pathway as well as slower conduction of the circulating wavefront during protocol B allowed more time for refractoriness to expire proximal to the arc and for the circulating wavefront to reexcite those sites to initiate reentry. The lengthening of ERP, associated with a single long cycle (protocol B), ranged from 44% to 79% of the total increase in ERP after a series of eight long cycles (protocol C). Epicardial sites with longer ERPs located close to the center of the ischemic zone showed more lengthening of refractoriness during protocol B compared with more normal sites near the border of the ischemic zone. This strongly suggests that the increased dispersion of refractoriness during protocol B is caused by the shorter memory of ischemic myocardium to the cumulative effects of preceding cycle lengths.