Impaired contractile function due to decreased cardiac myosin binding protein C content in the sarcomere.

Mutations in cardiac myosin binding protein C (MyBP-C) are a common cause of familial hypertrophic cardiomyopathy (FHC). The majority of MyBP-C mutations are expected to reduce MyBP-C expression; however, the consequences of MyBP-C deficiency on the regulation of myofilament function, Ca²âº homeostasis, and in vivo cardiac function are unknown. To elucidate the effects of decreased MyBP-C expression on cardiac function, we employed MyBP-C heterozygous null (MyBP-C+/-) mice presenting decreases in MyBP-C expression (32%) similar to those of FHC patients carrying MyBP-C mutations. The levels of MyBP-C phosphorylation were reduced 53% in MyBP-C+/- hearts compared with wild-type hearts. Skinned myocardium isolated from MyBP-C+/- hearts displayed decreased cross-bridge stiffness at half-maximal Ca²âº activations, increased steady-state force generation, and accelerated rates of cross-bridge recruitment at low Ca²âº activations (<15% and <25% of maximum, respectively). Protein kinase A treatment abolished basal differences in rates of cross-bridge recruitment between MyBP-C+/- and wild-type myocardium. Intact ventricular myocytes from MyBP-C+/- hearts displayed abnormal sarcomere shortening but unchanged Ca²âº transient kinetics. Despite a lack of left ventricular hypertrophy, MyBP-C+/- hearts exhibited elevated end-diastolic pressure and decreased peak rate of LV pressure rise, which was normalized following dobutamine infusion. Furthermore, electrocardiogram recordings in conscious MyBP-C+/- mice revealed prolonged QRS and QT intervals, which are known risk factors for cardiac arrhythmia. Collectively, our data show that reduced MyBP-C expression and phosphorylation in the sarcomere result in myofilament dysfunction, contributing to contractile dysfunction that precedes compensatory adaptations in Ca²âº handling, and chamber remodeling. Perturbations in mechanical and electrical activity in MyBP-C+/- mice could increase their susceptibility to cardiac dysfunction and arrhythmia.

Minimum Information about a Cardiac Electrophysiology Experiment (MICEE): standardised reporting for model reproducibility, interoperability, and data sharing.

Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work.

Dynamics of spiral pairs induced by unidirectional propagating pulses.

The dynamics of unidirectionally propagating pulses in a two-dimensional uniform excitable reaction-diffusion medium is investigated. It is shown that under weak diffusion coupling between medium points such a pulse can evolve into a pair of counter-rotating spirals (spiral pair). We analyze the drift of such a pair and examine the collisions between several drifting pairs. It is demonstrated that collisions can result in a special type of reflection or, alternatively, in new types of complex stationary spiral structures. A possible application of these findings for the diagnosis of cardiac arrhythmias is suggested.

High resolution optical mapping reveals conduction slowing in connexin43 deficient mice.

UNLABELLED: Analysis of mice with genetically altered expression of cardiac connexins can provide insights into the role of individual gap junction channel proteins in cell-to-cell communication, impulse propagation, and arrhythmias. However, conflicting results have been reported regarding conduction velocity slowing in mice heterozygous for a null mutation in the gene encoding connexin43 (Cx43). METHODS: High-resolution optical mapping was used to record action potentials from 256 sites, simultaneously, on the ventricular surface of Langendorff perfused hearts from 15 heterozygous (Cx43+/-) and 8 wildtype (Cx43+/+) mice (controls). A sensitive method for measuring epicardial conduction velocity was developed to minimize confounding influences of subepicardial breakthrough and virtual electrode effects. RESULTS: Epicardial conduction velocity was significantly slower (23 to 35%, P<0.01) in Cx43+/- mice compared to wildtype. There was no change in conduction patterns or anisotropic ratio (Cx43+/- 1.54+/-0.33; Cx43+/+ 1.57+/-0.17) suggesting that Cx43 expression was reduced uniformly throughout myocardium. The magnitude of reductions in conduction velocity and Cx43 protein expression (45%) were similar in mice in which the null allele occurred in a pure C57BL/6J genetic background versus a mixed (C57BL/6J X 129) background. Action potential duration did not differ between mice of different genotypes. CONCLUSIONS: A approximately 50% reduction of Cx43 expression causes significant conduction velocity slowing in the Cx43+/- mouse heart. The apparent lack of conduction velocity changes reported in previous studies may be related to technical factors rather than variations in genetic background. High-resolution optical mapping is a powerful tool for investigating molecular determinants of propagation and arrhythmias in genetically engineered mice.

Optical measurement of cell-to-cell coupling in intact heart using subthreshold electrical stimulation.

Electrical coupling between myocytes plays a critical role in propagation, repolarization, and arrhythmias. On the basis of predictions from cable theory, we hypothesized that the cardiac space constant (lambda) measured from the decay of subthreshold transmembrane potential (ST-Vm) in space would provide an index of regional cell-to-cell coupling in the intact heart. With the use of voltage-sensitive dyes, the distribution of ST-Vm was measured from hundreds of sites in close proximity to the site of subthreshold stimulation. lambda was calculated from the exponential decay of ST-Vm in space. Consistent with known directional differences in axial resistance, the spatial distribution of ST-Vm was strongly dependent on fiber orientation, because lambda was significantly (P < 0.001) longer along (1.5 +/- 0.1 mm) compared with across (0.8 +/- 0.1 mm) fibers. There was a close linear relationship (P < 0.001) between conduction velocity (CV) and lambda along all fiber angles tested. Reducing gap junctional conductance by heptanol reversibly decreased CV and lambda in parallel by approximately 50%. In contrast, sodium channel blockade by flecainide slowed CV by 40% but had no effect on lambda, reaffirming that lambda was an index of passive but not active membrane properties. These data establish the feasibility of measuring lambda as an index of cell-to-cell coupling in the intact heart, and indicate strong dependency of lambda on fiber orientation and pharmacological alterations of gap junction conductance.

Role of wavelength adaptation in the initiation, maintenance, and pharmacologic suppression of reentry.

INTRODUCTION: The stability of reentry is thought to depend on a critical balance between the spatial extent of refractory tissue in a reentrant wave (i.e., wavelength lambda) and the reentrant path length. Because considerable evidence suggests that lambda changes continuously in space and time during abrupt rate changes associated with the onset of tachycardia, we hypothesized that beat-by-beat adaptation of A to the dimensions of the reentrant path plays a central role in the mechanism of initiation of reentry. METHODS AND RESULTS: To investigate the dynamic relationship between lambda and path length during initiation of reentry, optical mapping with voltage-sensitive dyes was used in a guinea pig model of reentrant ventricular tachycardia (VT). In this model, a computer-guided laser obstacle precisely controlled the position and dimensions of the reentrant path. Under control perfusion and after addition of 15 microM d-sotalol, lambda was monitored during steady-state pacing, premature stimulation, and the initiating beats leading to nonsustained and sustained VT. During control perfusion, reentrant VT was reproducibly induced in 8 of 8 hearts, whereas in the presence of d-sotalol, reentry could only be initiated in 1 of 8 hearts due primarily to the failure of lambda to adapt to the reentrant path length. During successful initiation of VT, a consistent sequence was observed. The sequence was characterized by antidromic and orthodromic propagation around both sides of the anatomic obstacle, followed by unidirectional block of the antidromic impulse and persistence of reentry only if the A of the orthodromic impulse adapted to the reentrant path (lambda < path length). d-Sotalol prevented initiation of VT by altering lambda adaptation of the orthodromic wave; however, it failed to terminate ongoing VT because reverse use-dependence developed after several beats of tachycardia. CONCLUSION: In an experimental model where lambda, path length, and cellular action potentials were monitored during initiation of reentry, we found that, in contrast to termination, the initiation of reentry and the transition from nonsustained to sustained VT is strongly dependent on beat-to-beat adaptation of lambda to the dimensions of the reentrant path.

Electrocardiographic prediction of abnormal genotype in congenital long QT syndrome: experience in 101 related family members.

INTRODUCTION: Previous studies showed that diagnosing congenital long QT syndrome (LQTS) is difficult due to variable penetrance and genetic heterogeneity, especially when subjects from multiple families with diverse mutations are combined. We hypothesized that a combination of clinical and ECG techniques could identify gene carriers within a single family with congenital LQTS. METHODS AND RESULTS: One hundred one genotyped members of a family with LQTS, including 26 carriers of a HERG mutation, underwent history and ECG analysis. Forty-eight family members also underwent exercise testing with QT and T wave alternans (TWA) analysis and 24-hour Holter monitoring with QT and heart rate variability analysis. A logistic regression model, which included age, gender, QTc, and QTc by age, provided the best prediction of gene carrier status, although there was substantial overlap (78%) of QTc among subjects with and without the mutation. QTc was not helpful as a discriminator in children < or = 13 years. TWA (observed infrequently) did not add significantly to the model's ability to predict abnormal genotype. CONCLUSION: Even in this homogeneous LQTS population, the phenotype was so variable that clinical and detailed ECG analyses did not permit an accurate diagnosis of gene carrier status, especially in children. Sustained microvolt TWA was a specific (100%) but insensitive (18%) marker for LQTS. Its ability to predict risk of arrhythmia in this population remains to be determined. Genetic testing serves an essential role in screening for carriers of LQTS.

A comparison of T-wave alternans, signal averaged electrocardiography and programmed ventricular stimulation for arrhythmia risk stratification.

OBJECTIVES: The goal of this study was to compare T-wave alternans (TWA), signal-averaged electrocardiography (SAECG) and programmed ventricular stimulation (EPS) for arrhythmia risk stratification in patients undergoing electrophysiology study. BACKGROUND: Accurate identification of patients at increased risk for sustained ventricular arrhythmias is critical to prevent sudden cardiac death. T-wave alternans is a heart rate dependent measure of repolarization that correlates with arrhythmia vulnerability in animal and human studies. Signal-averaged electrocardiography and EPS are more established tests used for risk stratification. METHODS: This was a prospective, multicenter trial of 313 patients in sinus rhythm who were undergoing electrophysiologic study. T-wave alternans, assessed with bicycle ergometry, and SAECG were measured before EPS. The primary end point was sudden cardiac death, sustained ventricular tachycardia, ventricular fibrillation or appropriate implantable defibrillator (ICD) therapy, and the secondary end point was any of these arrhythmias or all-cause mortality. RESULTS: Kaplan-Meier survival analysis of the primary end point showed that TWA predicted events with a relative risk of 10.9, EPS had a relative risk of 7.1 and SAECG had a relative risk of 4.5. The relative risks for the secondary end point were 13.9, 4.7 and 3.3, respectively (p < 0.05). Multivariate analysis of 11 clinical parameters identified only TWA and EPS as independent predictors of events. In the prespecified subgroup with known or suspected ventricular arrhythmias, TWA predicted primary end points with a relative risk of 6.1 and secondary end points with a relative risk of 8.0. CONCLUSIONS: T-wave alternans is a strong independent predictor of spontaneous ventricular arrhythmias or death. It performed as well as programmed stimulation and better than SAECG in risk stratifying patients for life-threatening arrhythmias.

Interdependence of modulated dispersion and tissue structure in the mechanism of unidirectional block.

We previously showed that a premature stimulus can significantly alter vulnerability to arrhythmias by modulating spatial gradients of ventricular repolarization (ie, modulated dispersion). However, it is not clear if such changes in arrhythmia vulnerability can be attributed to the formation of an electrophysiological substrate for unidirectional block and what the potential role is of tissue structure in this process. Therefore, the main objective of the present study was to examine the concomitant effect repolarization gradients and tissue structure have on unidirectional block. Optical action potentials were recorded from 128 ventricular sites (1 cm(2)) in 8 Langendorff-perfused guinea pig hearts. Propagation was confined to the epicardial surface using an endocardial cryoablation procedure, and a 12-mm barrier with a 1.5-mm isthmus was etched with a laser onto the epicardium. A premature stimulus (S2) was delivered over a range of S1S2 coupling intervals to modulate repolarization gradients in a predictable fashion. When a second premature stimulus (S3) was delivered from the center of the isthmus, the occurrence and orientation of unidirectional block were highly dependent on repolarization gradients created by the S2 beat. In this model, a local repolarization gradient of 3.2 ms/mm was required for unidirectional block at this isthmus. In addition, the formation of unidirectional block was critically dependent on the presence of the source-sink mismatch imposed by the isthmus. These results may explain how the interplay between spatial heterogeneities of repolarization and tissue structure form a substrate for unidirectional block and reentry.

Influence of heart rate and sympathetic stimulation on arrhythmogenic T wave alternans.

We determined the temporal stability of T wave alternans (TWA) during constant rate stimulation and the dependence of alternans on heart rate (HR) and beta-adrenergic stimulation. Although it is established that exercise can provoke microvolt-level TWA in patients at risk for reentrant ventricular arrhythmias, the mechanisms underlying TWA in humans are not well understood. Specifically, the temporal stability of alternans at any given HR and the influence of HR vs. sympathetic activation on alternans remain unclear. TWA was measured during prolonged fixed-rate atrial pacing at multiple cycle lengths (CLs) in 10 subjects referred for electrophysiological testing and in 14 additional subjects in whom atrial pacing was performed at identical pacing CLs with and without isoproterenol. During constant CL stimulation, TWA amplitude oscillated significantly over time (typically by 10 microV) in a quasiperiodic fashion with periodicity of approximately 2-3 min. Alternans amplitude was strongly dependent on HR but not on adrenergic stimulation. There was a patient-specific threshold HR over which alternans appeared. At higher HR, alternans amplitude increased and oscillations were less prominent. Adrenergic stimulation was required to produce TWA that was not already elicited by moderate elevation of HR in only 2 of 14 (14%) patients. In conclusion, TWA 1) fluctuates spontaneously over 2-3 min and 2) increases monotonically with increased HR (without a major adrenergic contribution in most patients). These data suggest that increased HR rather than sympathetic activation is responsible for arrhythmogenic microvolt-level TWA measured during exercise.

Role of structural barriers in the mechanism of alternans-induced reentry.

Previously, using an animal model of T-wave alternans in structurally normal myocardium, we demonstrated that repolarization can alternate with opposite phase between neighboring myocytes (ie, discordant alternans), causing spatial dispersions of repolarization that form the substrate for functional block and reentrant ventricular fibrillation (VF). However, the mechanisms responsible for cellular discordant alternans and its electrocardiographic manifestation (ie, T-wave alternans) in patients with structural heart disease are unknown. We hypothesize that electrotonic uncoupling between neighboring regions of cells by a structural barrier (SB) is a mechanism for discordant alternans. Using voltage-sensitive dyes, ventricular action potentials were recorded from 26 Langendorff-perfused guinea pig hearts in the absence (ie, control) and presence of an insulating SB produced by an epicardial laser lesion. Quantitative analysis of magnitude and phase of cellular alternans revealed that in controls, action potential duration alternated in phase at all ventricular sites above a critical heart rate (269+/-17 bpm), ie, concordant alternans. Also, above a faster critical heart rate threshold (335+/-24 bpm), action potential duration alternated with opposite phase between sites, ie, discordant alternans. In contrast, only discordant but not concordant alternans was observed in 80% of hearts with the SB, and discordant alternans always occurred at a significantly slower heart rate (by 68+/-28 bpm) compared with controls. Therefore, the SB had a major effect on the alternans-heart rate relation, which served to facilitate the development of discordant alternans. Whether a SB was present or not, discordant alternans produced considerable increases (by approximately 170%) in the maximum spatial gradient of repolarization, which in turn formed the substrate for unidirectional block and reentry. However, by providing a structural anchor for stable reentry, discordant alternans in the presence of a SB led most often to sustained monomorphic ventricular tachycardia rather than to VF, whereas in the absence of a SB discordant alternans caused VF. SBs facilitate development of discordant alternans between cells with different ionic properties by electrotonically uncoupling neighboring regions of myocardium. This may explain why arrhythmia-prone patients with structural heart disease exhibit T-wave alternans at lower heart rates. These data also suggest a singular mechanism by which T-wave alternans forms a substrate for initiation of both VF and sustained monomorphic ventricular tachycardia.

Cellular basis for dispersion of repolarization underlying reentrant arrhythmias.

Substantial heterogeneity in ion channel density and expression exists in cells isolated from various regions of the heart. Cell-to-cell coupling in the intact heart, however, is expected to attenuate the functional expression of the ion channel heterogeneities. Due to limitations of conventional electrophysiological recording techniques, the extent to which cellular electrical heterogeneities are functionally present in intact myocardium remains unknown. High-resolution optical mapping with voltage-sensitive dyes was used to measure transepicardial and transmural repolarization gradients in the Langendorff perfused guinea pig ventricle and the canine wedge preperation, respectively. Diversity of repolarization kinetics in the transepicardial direction modulated dispersion of repolarization in a biphasic fashion as premature coupling interval was shortened. Moreover, modulation of repolarization paralleled arrhythmia vulnerability in a predictable fashion. Transmural optical mapping revealed significant gradients of repolarization across the ventricular wall that were markedly increased in a surrogate model of LQTS. Transmural gradients of repolarization in LQTS were associated with an enhanced susceptibility to TdP. Therefore, despite strong cell-to-cell coupling in the normal heart, heterogeneities in the ionic make-up of cells across the epicardial and transmural surfaces result in functional heterogeneities of repolarization leading to arrhythmias.

Mechanism linking T-wave alternans to the genesis of cardiac fibrillation.

BACKGROUND: Although T-wave alternans has been closely associated with vulnerability to ventricular arrhythmias, the cellular processes underlying T-wave alternans and their role, if any, in the mechanism of reentry remain unclear. METHODS AND RESULTS: -T-wave alternans on the surface ECG was elicited in 8 Langendorff-perfused guinea pig hearts during fixed-rate pacing while action potentials were recorded simultaneously from 128 epicardial sites with voltage-sensitive dyes. Alternans of the repolarization phase of the action potential was observed above a critical threshold heart rate (HR) (209+/-46 bpm) that was significantly lower (by 57+/-36 bpm) than the HR threshold for alternation of action potential depolarization. The magnitude (range, 2.7 to 47.0 mV) and HR threshold (range, 171 to 272 bpm) of repolarization alternans varied substantially between cells across the epicardial surface. T-wave alternans on the surface ECG was explained primarily by beat-to-beat alternation in the time course of cellular repolarization. Above a critical HR, membrane repolarization alternated with the opposite phase between neighboring cells (ie, discordant alternans), creating large spatial gradients of repolarization. In the presence of discordant alternans, a small acceleration of pacing cycle length produced a characteristic sequence of events: (1) unidirectional block of an impulse propagating against steep gradients of repolarization, (2) reentrant propagation, and (3) the initiation of ventricular fibrillation. CONCLUSIONS: Repolarization alternans at the level of the single cell accounts for T-wave alternans on the surface ECG. Discordant alternans produces spatial gradients of repolarization of sufficient magnitude to cause unidirectional block and reentrant ventricular fibrillation. These data establish a mechanism linking T-wave alternans of the ECG to the pathogenesis of sudden cardiac death.

Modulated dispersion explains changes in arrhythmia vulnerability during premature stimulation of the heart.

BACKGROUND: Previously, we have shown that a premature stimulus can significantly modulate spatial gradients of ventricular repolarization (ie, modulated dispersion), which result from heterogeneous electrophysiological properties between cells. The role modulated dispersion may play in determining electrical instability in the heart is unknown. METHODS AND RESULTS: To determine if premature stimulus-induced changes in repolarization are a mechanism that governs susceptibility to cardiac arrhythmias, optical action potentials were recorded simultaneously from 128 ventricular sites (1 cm2) in 8 Langendorff-perfused guinea pig hearts. After baseline pacing (S1), a single premature stimulus (S2) was introduced over a range of S1S2 coupling intervals. Arrhythmia vulnerability after each premature stimulus was determined by measurement of a modified ventricular fibrillation threshold (VFT) during the T wave of each S2 beat (ie, S2-VFT). As the S1S2 interval was shortened to an intermediate value, spatial gradients of repolarization and vulnerability to fibrillation decreased by 51+/-9% (mean+/-SEM) and 73+/-45%, respectively, compared with baseline levels. As the S1S2 interval was further shortened, repolarization gradients increased above baseline levels by 54+/-30%, which was paralleled by a corresponding increase (37+/-8%) in vulnerability. CONCLUSIONS: These data demonstrate that modulation of repolarization gradients by a single premature stimulus significantly influences vulnerability to ventricular fibrillation. This may represent a novel mechanism for the formation of arrhythmogenic substrates during premature stimulation of the heart.

T-wave alternans and dispersion of the QT interval as risk stratification markers in patients susceptible to sustained ventricular arrhythmias.

T-wave alternans and QT dispersion were compared as predictors of the outcome of electrophysiologic study and arrhythmia-free survival in patients undergoing electrophysiologic evaluation. T-wave alternans was a highly significant predictor of these 2 outcome variables, whereas QT dispersion was not.

Angiotensin-converting enzyme inhibition produces electrophysiologic but not antiarrhythmic effects in the intact heart.

Although angiotensin-converting enzyme (ACE) inhibitors are known to influence favorably the structural remodeling of the heart after myocardial infarction, the mechanisms by which ACE inhibitors improve survival are not well understood. The hypothesis that ACE inhibitors may possess antiarrhythmic activity has been studied in various isolated tissue preparations. However, the electrophysiologic effects of ACE inhibitors in the intact heart are not well understood. The effect of the ACE inhibitor enalaprilat on intact heart electrophysiology was studied by using multisite optical action-potential recordings with voltage-sensitive dyes. Action potentials were recorded simultaneously from 128 left ventricular epicardial sites in 15 Langendorff perfused hearts subjected to an endocardial cryoablation procedure, which was used to restrict propagation to a thin viable rim of epicardium. Action-potential duration (APD) was significantly prolonged in 67% of preparations perfused with 5 mg/L enalaprilat. Higher concentration of enalaprilat (50 mg/L) prolonged APD in all preparations tested. This APD-prolonging effect persisted over a broad range of stimulus rates, indicating the absence of reverse use-dependent properties. Enalaprilat did not modify conduction velocity, nor did it affect spatial dispersion of repolarization times. In addition, enalaprilat had no effect on ventricular fibrillation threshold and failed to suppress the initiation of ventricular tachycardia using an anatomically defined reentrant circuit. These findings indicate that in the intact heart, enalaprilat does indeed have electrophysiologic effects that cause APD prolongation, particularly at high drug concentrations. However, this effect was not of sufficient magnitude in the guinea pig to suppress the initiation of ventricular fibrillation or reentrant ventricular tachycardia.

Hypoxia and hypothermia enhance spatial heterogeneities of repolarization in guinea pig hearts: analysis of spatial autocorrelation of optically recorded action potential durations.

INTRODUCTION: Regional dispersions of repolarization (DOR) are arrhythmogenic perturbations that are closely associated with reentry. However, the characteristics of DOR have not been well defined or adequately analyzed because previous algorithms did not take into account spatial heterogeneities of action potential durations (APDs). Earlier simulations proposed that pathologic conditions enhance DOR by decreasing electrical coupling between cells, thereby unmasking differences in cellular repolarization between neighboring cells. Optical mapping indicated that gradients of APD and DOR are associated with fiber structure and are largely independent of activation. We developed an approach to quantitatively characterize APD gradients and DOR to determine how they are influenced by tissue anisotropy and cell coupling during diverse arrhythmogenic insults such as hypoxia and hypothermia. METHODS AND RESULTS: Voltage-sensitive dyes were used to map APs from 124 sites on the epicardium of Langendorff-perfused guinea pig hearts during (1) cycles of hypoxia and reoxygenation and (2) after 30 minutes of hypothermia (32 degrees to 25 degrees C). We introduce an approach to quantitate DOR by analyzing two-dimensional spatial autocorrelation of APDs along directions perpendicular and parallel to the longitudinal axis of epicardial fibers. A spatial correlation length L was derived as a statistical measure of DOR. It corresponds to the distance over which APDs had comparable values, where L is inversely related to DOR. Hypoxia (30 min) caused a negligible decrease in longitudinal thetaL (from 0.530 +/- 0.138 to 0.478 +/- 0.052 m/sec) and transverse thetaT (from 0.225 +/- 0.034 to 0.204 +/- 0.021 m/sec) conduction velocities and did not alter thetaL/thetaT or activation patterns. In paced hearts (cycle length [CL] = 300 msec), hypoxia decreased APDs (123 +/- 18.2 to 46 +/- 0.6 msec; P < 0.001) within 10 to 15 minutes and enhanced DOR, as indicated by reductions of L from 1.8 +/- 0.9 to 1.1 +/- 0.5 mm (P < 0.005). Hypothermia caused marked reductions of thetaL (0.53 +/- 0.138 to 0.298 +/- 0.104 m/sec) and thetaT (0.225 +/- 0.034 to 0.138 +/- 0.027 m/sec), increased APDs (128 +/- 4.4 to 148 +/- 14.5 msec), and reduced L from 2.0 +/- 0.3 to 1.3 +/- 0.6 mm (P < 0.05). L decreased with increased time of hypoxia and recovered upon reoxygenation. Hypoxia and hypothermia reduced L measured along the longitudinal (L(L)) and transverse (L(T)) axes of cardiac fibers while the ratio of L(L)/L(T) remained constant. CONCLUSION: Conventional indexes of DOR (i.e., APD "range" or "standard deviation," evaluated with extracellular electrodes) did not convey the spatial inhomogeneities of repolarization revealed by L. Spatial autocorrelation analysis provides a statistically significant measurement of DOR, which can take into account intrinsic heterogeneities of APDs and fiber orientation. The data show that hypoxia and hypothermia produce reductions of L, even though they have different effects on mean APD and conduction velocity. The preservation of a constant L(L)/L(T) ratio during hypoxia and hypothermia, despite large reductions in L, is consistent with a mechanism in which reduced cell-to-cell coupling unmasks intrinsic dispersions of APD and reduces L(L) and L(T) by the same factor. Thus, the spatial autocorrelation of APDs provides a sensitive index of DOR under normal and arrhythmogenic conditions. It incorporates the anisotropic nature of the myocardium and therefore is preferable to conventional indexes of DOR.