Transmural Location Research Articles

Understanding unipolar electrograms and global activation from noninvasive mapping for diagnosing arrhythmias

In the past several decades Electrocardiographic Imaging has been developed, validated, and recently commercialized for clinical use (Medtronic CardioInsight™). The technology noninvasively maps cardiac electrical potentials by combining multi-channel body surface electrocardiograms and patient specific heart-torso geometry. Unlike contact catheter mapping where bipolar electrograms are commonly used, noninvasive mapping is unipolar in nature. While bipolar electrograms reflect local activation time, noninvasive mapping reflects both local and global information. Characterizing arrhythmias using noninvasive mapping requires a different approach than what is used with bipolar mapping during electrophysiological studies. With multiple example cases, this article aims to demonstrate how in-depth analysis of noninvasive electrogram features and global activation patterns can reveal clinically important characteristics of arrhythmias, which may not be evident on the automatically generated maps. These characteristics include transmural location of the ectopic focus, differential locations within structural proximity, engagement of bundle or specific tissue, and substrate properties. The analysis approach involves applying unipolar potential fundamentals and relating electrogram features, global activation with underlying electrophysiology and cardiac anatomy to explain specific mapping phenomenon. If validated, easy-to-use diagnostic criteria or algorithms can be built and automated as product features in the commercial system to facilitate future noninvasive mapping in patients.

Transmural heterogeneity of diffusion anisotropy in the sheep myocardium characterized by MR diffusion tensor imaging

The orientation of MRI-measured diffusion tensor in the myocardium has been directly correlated to the tissue fiber direction and widely characterized. However, the scalar anisotropy indexes have mostly been assumed to be uniform throughout the myocardial wall. The present study examines the fractional anisotropy (FA) as a function of transmural depth and circumferential and longitudinal locations in the normal sheep cardiac left ventricle. Results indicate that FA remains relatively constant from the epicardium to the midwall and then decreases (25.7%) steadily toward the endocardium. The decrease of FA corresponds to 7.9% and 12.9% increases in the secondary and tertiary diffusion tensor diffusivities, respectively. The transmural location of the FA transition coincides with the location where myocardial fibers run exactly circumferentially. There is also a significant difference in the midwall-endocardium FA slope between the septum and the posterior or lateral left ventricular free wall. These findings are consistent with the cellular microstructure from histological studies of the myocardium and suggest a role for MR diffusion tensor imaging in characterization of not only fiber orientation but, also, other tissue parameters, such as the extracellular volume fraction.

ATX‐II Effects on the Apparent Location of M Cells in a Computational Model of a Human Left Ventricular Wedge

The apparent location of the myocytes (M cells) with the longest action potential duration (APD) in a canine left ventricular (LV) wedge have been reported to shift after application of a sea anemone toxin, ATX-II. This toxin slows inactivation of I(Na) and thus prolongs APD. Thus, M cells may exhibit dynamic functional states, rather than being a static, anatomically discrete, myocyte population. In this study, we attempted to further define and understand this phenomenon using a mathematical model of the human ventricular myocyte action potential incorporated into an in silico "wedge" preparation. Our simulations demonstrate that even under conditions of a fixed population and ratio of epicardial, M, and endocardial myocytes, the apparent anatomical position (transmural location) of the myocytes with the longest APD can shift following ATX-II treatment. This arises because the ATX-II effect, modeled as a small increase in the late or persistent Na(+) current, and consequent prolongation of APD significantly changes the electrotonic interactions between ventricular myocytes in this LV wedge preparation. This LV wedge model is based on bidomain equations. It corresponds to a rectangular tissue immersed in a passive and isotropic medium that represents the superfusion bath. In this theoretical work, the three known different and discrete populations of myocytes in the human left ventricle have been included: the epicardial, M, and endocardial cells. The effects of ATX-II on I(Na) were simulated by altering the voltage-dependent steady-state inactivation of the parameters h (fast gate) and j (slow gate). As a result, in these ATX-II simulations a persistent late Na(+) current was generated in all three types of ventricular myocytes. However, the APDs were prolonged in a heterogeneous pattern. Our simulations demonstrate that after the ATX-II effects develop, alterations in transmural electrotonic interactions can produce changes in the transmural location of myocytes with the longest APD. The combination of intercellular electrotonic interactions, which tend to reduce and smooth out the discrete transmural APD variations, and the heterogeneous effects of ATX-II, which preferentially prolong the APD of M cells, can shift the location of the ventricular myocytes. This shift results in significantly altered transmural patterns of action potential durations, which would be expected to change localized refractory period and excitability. These cellular changes give rise to alterations in the corresponding surface electrograms and may change the overall substrates for conduction and rhythm disturbances.

Measuring and Mapping Cardiac Fiber and Laminar Architecture Using Diffusion Tensor MR Imaging

The ventricular myocardium is known to exhibit a complex spatial organization, with fiber orientation varying as a function of transmural location. It is now well established that diffusion tensor magnetic resonance imaging (DTMRI) may be used to measure this fiber orientation at high spatial resolution. Cardiac fibers are also known to be organized in sheets with surface orientation varying throughout the ventricles. This article reviews results on use of DTMRI for measuring ventricular fiber orientation, as well as presents new results providing strong evidence that the tertiary eigenvector of the diffusion tensor is aligned locally with the cardiac sheet surface normal. Considered together, these data indicate that DTMRI may be used to reconstruct both ventricular fiber and sheet organization. This article also presents the large deformation diffeomorphic metric mapping (LDDMM) algorithm and shows that this algorithm may be used to bring ensembles of imaged and reconstructed hearts into correspondence (e.g., registration) so that variability of ventricular geometry, fiber, and sheet orientation may be quantified. Ventricular geometry and fiber structure is known to be remodeled in a range of disease processes; however, descriptions of this remodeling have remained subjective and qualitative. We anticipate that use of DTMRI for reconstruction of ventricular anatomy coupled with application of the LDDMM method for image volume registration will enable the detection and quantification of changes in cardiac anatomy that are characteristic of specific disease processes in the heart. Finally, we show that epicardial electrical mapping and DTMRI imaging may be performed in the same hearts. The anatomic data may then be used to simulate electrical conduction in a computational model of the very same heart that was mapped electrically. This facilitates direct comparison and testing of model versus experimental results and opens the door to quantitative measurement, modeling, and analysis of the ways in which remodeling of ventricular microanatomy influences electrical conduction in the heart.

The transmural location of earliest activation following a defibrillation shock is species dependent

The transmural location of earliest activation following a defibrillation shock is species dependent

Transmural changes in size, contractile and electrical properties of SHR left ventricular myocytes during compensated hypertrophy.

Some hypertrophic stimuli provoke responses from myocytes that vary across the thickness of the left ventricular wall. The Spontaneously Hypertensive Rat (SHR) is a well-established genetic model of hypertension and whole heart hypertrophy. Details of transmural responses to hypertension in the SHR are few, but are needed if the properties of this model are to be fully understood. We therefore tested the hypothesis that left ventricular myocytes of the SHR do not respond uniformly to their hypertensive environment. The volume, contraction and action potentials of enzymically isolated sub-epicardial (EPI) mid-myocardial (MID) and sub-endocardial (ENDO) myocytes from the left ventricle of 20-week-old SHR and normotensive Wistar-Kyoto (WKY) control rats were compared. Compared to WKY, as a single population, SHR myocytes displayed concentric hypertrophy (larger volumes with smaller length:width) increased t-tubule spacing, larger and prolonged cell shortening and intracellular calcium ([Ca2+]i) transients and longer action potentials. However, these responses differed across the left ventricular wall. MID myocytes showed significantly less hypertrophy than EPI and ENDO myocytes. EPI myocytes showed the largest (and significant) increases in cell shortening, [Ca2+]i transients and action potential duration, whilst MID myocytes showed the smallest (and non-significant) changes in these parameters. Real time reverse transcription polymerase chain reaction analysis on cardiac tissue suggest that increased expression of mRNA for fibronectin-1 and protein kinase Cepsilon are involved in the hypertrophic response of the whole heart. Our findings show that in the SHR, the effect of hypertension upon the morphology, mechanical activity and electrical activity of left ventricular myocytes is dependent upon their transmural location. Therefore, in addition to the overall compensating response to hypertension, i.e. increased contractility, there are likely to be regionally specific alterations in mechano-electric interactions that may influence the properties of this important model, e.g. its pre-disposition to arrhythmia whilst still in a compensated state.

Myocardial contrast echocardiography can be used to assess the microvascular response to vascular endothelial growth factor-121.

Therapeutic angiogenesis is a new approach to treating ischemic heart disease, and the optimal method for assessing its efficacy is unclear. We used myocardial contrast echocardiography (MCE) to evaluate the therapeutic response to the angiogenic agent, vascular endothelial growth factor-121 (VEGF121). After placement of an ameroid constrictor (day 0) around the left anterior descending artery (LAD), dogs were given intracoronary VEGF121 protein (108 microg, n=6) or placebo (n=6) on days 7 and 21, and subcutaneous VEGF121 (1 mg) or placebo on days 8 to 20 and 22 to 27. On day 48, MCE was performed during rest and dobutamine stress. Videointensity (y) and pulsing interval (t) were fit to an exponential model (y=A[1-e(-beta(t))]) used to derive indices of red cell velocity (beta) and capillary area (A), and parameters were compared with radiolabeled microsphere flow data. VEGF(121) treatment resulted in higher resting left anterior descending artery/left circumflex flow ratio compared with placebo (P<0.03) and improved collateral flow reserve. Beta was 0.94+/-0.37 in VEGF121 dogs versus 0.38+/-0.31 in controls (P<0.02), with the greatest difference in the endocardium. The parameter A was comparable in both groups, suggesting that microvascular changes did not alter capillary cross-sectional area, and histology indicated a trend toward higher arteriolar density in VEGF121-treated animals. VEGF121 protein improves collateral flow and reserve. MCE can evaluate the transmural location and structural and functional responses of the microvasculature to angiogenic interventions.

Relating myocardial laminar architecture to shear strain and muscle fiber orientation.

Cardiac myofibers are organized into laminar sheets about four cells thick. Recently, it has been suggested that these layers coincide with the plane of maximum shear during systole. In general, there are two such planes, which are oriented at +/-45 degrees to the main principal strain axes. These planes do not necessarily contain the fiber axis. In the present study, we explicitly added the constraint that the sheet planes should also contain the muscle fiber axis. In a mathematical analysis of previously measured three-dimensional transmural systolic strain distributions in six dogs, we computed the planes of maximum shear, adding the latter constraint by using the also-measured muscle fiber axis. Generally, for such planes two solutions were found, suggesting that two populations of sheet orientation may exist. The angles at which the predicted sheets intersected transmural tissue slices, cut along left ventricular short- or long-axis planes, were strikingly similar to experimentally measured values. In conclusion, sheets coincide with planes of maximum systolic shear subject to the constraint that the muscle fiber axis is contained in this plane. Sheet orientation is not a unique function of the transmural location but occurs in two distinct populations.

Ultrastructural assessment of myocardial necrosis occurring during ischemia and 3-h reperfusion in the dog.

To determine whether myocardial necrosis may occur during postischemic reperfusion, electron microscopy was used to identify morphological features of irreversible injury in myocardial samples taken from anesthetized dogs with 90-min ischemia and 0-, 5-, 90-, or 180-min reperfusion. In samples without detectable collateral blood flow, necrosis was almost complete, whether or not the myocardium was reperfused. In samples with collateral flow, necrosis was more frequent after 180-min reperfusion than in the absence of reperfusion, despite similar collateral flows in the two groups. Excess of necrosis after 180-min reperfusion was evident in endocardium (ischemia only: 4 of 13, 180-min reflow: 14 of 20; P = 0. 03) and midwall (ischemia only: 9 of 25, 180-min reflow: 29 of 45; P = 0.02). Multiple logistic regression with variables of collateral flow and transmural position was used to determine risk of irreversible injury in 111 samples from ischemic myocardium without reperfusion (model predictive accuracy = 75%, P < 0.00001) and to predict risk of necrosis in myocardium reperfused for 180 min. Of 65 samples from endocardium and midwall with detectable collateral flow, the model predicted necrosis in 23 samples but necrosis was observed in 43 samples (P < 0.01). Reperfusion duration was a determinant of frequency of irreversible injury. Multiple logistic regression for 186 samples from myocardium reperfused for 5, 90, or 180 min showed that reperfusion duration was an independent predictor of irreversible injury (P = 0.0003) when collateral flow and transmural location were accounted for. These findings are consistent with the occurrence of necrosis during reperfusion in myocardium exposed to substantial, prolonged ischemia but with sufficient residual perfusion to avoid necrosis during the period of flow impairment.

Flow-function relations during graded coronary occlusions in the dog: effects of transmural location and segment orientation.

The sensitive relationship between regional myocardial perfusion and local systolic deformation during acute myocardial ischemia is not independent of the transmural location or segment orientation. The aim of this study was to determine the effects of fiber orientation and transmural location on the relationships between regional myocardial flow and three-dimensional systolic wall strain during graded coronary artery occlusions. Transmural distributions of three-dimensional strain (by biplane radiography of implanted radiopaque markers) and myocardial blood flows (using fluorescent microspheres) were measured in the ischemic region during graded left anterior descending (LAD) coronary artery occlusions in 12 anesthetized dogs. Occlusion of the coronary artery did not significantly alter mean heart rate or end-systolic pressure. As flow decreased during graded occlusions, ischemia significantly changed systolic circumferential, longitudinal, radial, fiber and cross-fiber strains (p < 0.004). There was a significant effect of transmural position on circumferential, cross-fiber and radial strains, but not on fiber or longitudinal strains. Ischemia significantly altered all normal strains: circumferential, longitudinal, fiber, cross-fiber and radial. There was a strong interaction effect between transmural location and blood flow for circumferential, cross-fiber and radial strains, but not fiber or longitudinal strains. During non-transmural ischemia, there is evidence of strong transmural tethering in the cross-fiber direction, whereas the fiber-strain flow relation is independent of transmural position. Thus, whether the relationship between local myocardial bloodflow and systolic strain during acute ischemia is dependent on transmural location, depends on segment orientation.

Distribution of coronary collateral blood flow at different levels of collateral growth in conscious ponies.

Coronary collateral growth was stimulated in chronically instrumented conscious ponies by a previously validated intermittent coronary occlusion method. Changes in regional myocardial function (sonomicrometry) and reactive hyperemia (Doppler method) were used to monitor collateral growth and to program measurements of regional myocardial blood flow (microsphere method). A serial analysis of the transmural and lateral distributions of collateral blood flow was performed at the native and three superimposed levels of collateral growth. Results in nine animals undergoing an average of 553 +/- 188 brief coronary occlusions over 68 +/- 18 days demonstrated that as collateral conductance increased, the perfusion field within the ischemic region increased from the epicardium to the endocardium but not from the lateral edges to the center of the ischemic region. The findings are consistent with an analog model consisting of interarterial collaterals whose collective resistance is in series with arteriolar resistance of the recipient artery. No special protection of deeper myocardial layers by a subendocardial plexus or intramural collaterals was noted. Instead, the findings suggest that coronary extravascular compressive forces play a more important role than the transmural location of collaterals in determining the volume and spatial distribution of collateral blood flow during collateral growth in the pony.

Three-dimensional characterization of human ventricular myofiber architecture by ultrasonic backscatter.

Normal human left ventricular architecture comprises a highly aligned array of cardiac myofibers whose orientation depends on transmural location. This study was designed to determine whether measurement of integrated backscatter could be used detect the progressive transmural shift of myofiber alignment that occurs from epicardium to endocardium in human ventricular wall segments. Integrated backscatter was measured at 32 transmural levels in seven cylindrical biopsy specimens (1.4 cm diam) sampled from normal regions of six explanted fixed human hearts by insonification of samples at 180 independent angles in 2 degrees steps around their entire circumference with a 5-MHz broadband piezoelectric transducer. Histologic analysis was performed to determine fiber orientation. Integrated backscatter varied approximately as a sinusoidal function of the angle of insonification at each transmural level. Greater integrated backscatter was observed for insonification perpendicular as compared with parallel to fibers (difference = 14.5 +/- 0.6 dB). Ultrasonic analysis revealed a progressive transmural shift in fiber orientation of approximately 9.2 +/- 0.7 degrees/mm of tissue. Histologic analysis revealed a concordant shift in fiber orientation of 7.9 +/- 0.8 degrees/mm of tissue. Thus, human myocardium manifests anisotropy of ultrasonic scattering that may be useful for characterization of the intramural fiber alignment and overall three-dimensional organization of cardiac myofibers.

Determination of transmural location of onset of activation from cardiac surface electrograms.

Methods of estimating depth of origin of ventricular activation from cardiac surface electrograms were evaluated in experiments on eight dogs. The ventricles were paced via multielectrode needle arrays placed transmurally in from four to six locations in the wall of the left ventricle. A multiplexed data-recording system was used to simultaneously record from 64 unipolar cardiac surface electrodes during pacing at each multielectrode needle site. The four indexes evaluated were the maximum and average gradients of activation isochrones around the site of earliest epicardial activation, the QRS area at the site of earliest epicardial activation, the interval between the QRS onset computed from all 64 epicardial surface electrograms, and the time of the minimum dV/dt in the electrogram displaying the earliest epicardial activation time (t(ee)-t(rmso) interval). Correlation coefficients between depth of stimulation and average and maximum gradients of isochrones, QRS area at the site of earliest epicardial activation, and t(ee)-t(rmso) interval were 0.985 or higher. These methods, particularly those involving gradients of isochrones, should be useful for evaluating electromaps of patients undergoing surgery for ablation of tachyarrhythmias.

Morphometry of the coronary microvasculature of the canine left ventricle.

The objective of this study was to test for the presence of transmural gradients of various components of the coronary microvasculature of the canine left ventricle. In order to achieve study objectives, the heart and coronary circulation were fixed in a reproducible state of myocardial and vascular tone (diastolic cardiac arrest and maximal coronary vasodilation). Morphometric methods which treat the coronary microvasculature as anisotropically arranged structures were applied for quantitative structural analysis. Eight dog hearts were fixed with a glutaraldehyde-cacodylate-buffered fixative by retrograde perfusion of the aorta with the heart in diastolic arrest and with maximal coronary vasodilation. Tissue samples were taken from areas near to the anterior and posterior papillary muscles from the subendocardium, subepicardium, and intermediate transmural locations. Morphometric results showed a homogeneously arranged array of microvascular and myocardial components with no significant differences in any of the primary morphometric measurements, down to the ultrastructural level, in myocytes relative to transmural location. The results suggest that transmural differences in coronary blood flow are not due to transmural structural differences but rather are due to physiological regulatory mechanisms of coronary blood flow. Further, the results indicate that failure to correct for anisotropy of myocardial structures can lead to erroneous conclusions concerning the structural basis of function in the heart.

Transmural distribution of isomyosin in rabbit ventricle during maturation examined by immunofluorescence and staining for calcium-activated adenosine triphosphatase.

Mammalian ventricle contains two major isomyosins, V1 and V3, which differ in the primary structure of their heavy chains (HC alpha alpha and HC beta beta, respectively) and in their adenosine triphosphatase activity. The distribution of the HC alpha isomyosin in the left ventricle of the rabbit was followed as a function of age and transmural location. HC alpha was detected with a monoclonal antibody found to be specific for the hinge region of V1 myosin molecules when viewed in the electron microscope after low-angle rotary shadowing. Frozen sections were observed with indirect immunofluorescence developed to this anti-HC alpha hinge antibody. Serial sections were observed with the histochemical assay for calcium-activated myosin adenosine triphosphatase, using preincubation at various pH levels. Results show that all the ventricular myocytes in baby rabbits (2 weeks) are stained by the HC alpha-antibody from the epi- to endocardium. The isomyosin content of myocytes varies through the epi- to endocardium of the right ventricular wall of the adult (1-year-old) rabbit, with the HC alpha form predominating in the outer epicardial third of the wall and the lowest amount of HC alpha in the middle third of the wall. A mixture of stained and unstained myocytes is seen in the endo- and subendocardial regions. The spatial distribution of HC alpha in 4-month-old rabbits varies between that of the baby and adult. There is good agreement between myocyte classifications made by histochemical and antibody staining methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Preocclusive perfusion area as a determinant of infarct size in a canine model.

The extent of myocardial infarction related to the pre-occlusive perfusion are was quantitatively evaluated in a canine model. The perfusion area was determined preocclusively by selective injection of tracer microspheres into the coronary artery in question, then the main trunk and a small lateral branch of the left circumflex coronary artery was occluded in group 1 (10 dogs) and group 2 (10 dogs), respectively. Rapid freezing was used to prepare autoradiographic samples for determination of the perfusion are along with samples for dehydrogenase staining in order to delineate the extent of myocardial infarction. The relationship between perfusion area and infarct size was examined in 50 micrometers thick samples from base, middle and apex of the ventricle and a direct linear correlation between the perfused (P) and infarcted area (I) in the left ventricle (LV) was noted regardless of the perfusion size or the location in the ventricle. The linear relationship I = 0.93P - 4.9 (r = 0.95, P less than 0.001) was obtained in all 20 dogs. Transmural extent of necrosis was larger in the endocardium than in the epicardium, thereby representing a more salvageable myocardium in the epicardium. It is concluded that the preocclusive determination of perfusion are by high resolution autoradiography is a useful method of obtaining physiological perfusion not affected by ischaemia. Also the infarct size when standardized by the perfusion area is mainly influenced by transmural location but not by the spacial geometry or the size of the occluded area.

Morphometric study of endomyocardium and epimyocardium of the left ventricle in adult dogs.

Quantitative light- and electron-microscopic data are reported for the endomyocardium and epimyocardium of the left ventricle in adult dogs. The endomyocardium has a markedly lower capillary density, and larger myocyte cross-sectional area, than the epimyocardium. The sarcolemmal-surface/cell-volume ratio is significantly less in endocardial myocytes. No regional differences were found with regard to capillary diameter, interstitial space volume, or volume percent occupied by blood vessels and myocytes. Likewise, there are no differences in proportions of cardiac cell volume occupied by myofibrils (57%), mitochondria (25%), nuclei (1.5%), T tubes (1.2%), and sarcoplasmic reticulum (2.0%). The ratios of surface areas of T tubes and SR to their respective volumes and to cell volumes do not vary with transmural location. The regional differences and similarities are discussed in relation to general problems of blood flow in the heart and cardiac hypertrophy.

Precordial and epicardial surface potentials during Myocardial ischemia in the pig. A theoretical and experimental analysis of the TQ and ST segments.

The solid angle theorem was used to analyze the relationships between TQ and ST segment deflections recorded from precordial and epicardial locations and the time course, size, shape, and transmural location of the ischemic process in the ventricular myocardium. Mathematical predictions were compared with experimental data from the intact heart. Precordial electrograms obtained in anesthetized close-chest pigs were compared with epicardial electrograms recorded directly from the heart's surface. Various areas of ischemia were produced by occluding large and small coronary artery branches, and the resultant changes in ischemic shape were delineated with Thioflavin S injections and postmortem ultraviolet photography. Formally derived equations and cumulative experimental data were in close agreement, suggesting that in the ischemic ventricle (1) TQ depression always accompanies ST elevation, (2) TQ and ST segment changes in magnitude and polarity are complex functions of ischemic size, shape, and transmural location; (3) precordial electrocardiogram (ECG) ST segment elevation is directly related to ischemic size; and (4) epicardial ECG ST segment elevation is inversely related to ischemic size. It is thus concluded that precordial and epicardial ECG TQ and ST segment deflections are complex functions of ischemic geometry and that their accurate interpretation with respect to ischemic size and shape and in the presence of pharmacological interventions is often difficult and may be misleading.