Mary N. Morrow

Left Heart Volume Estimation in Infancy and Childhood: Reevaluation of Methodology and Normal Values

Left ventricular (LV) volume determinations by the area-length method were reevaluated in postmortem studies of left ventricles ranging from 0.5 to 90 cm3 absolute volume. The regression equation relating known and calculated volumes for calculated volumes <15 cm3 (V′ = 0.733V) was found to be significantly different from that for calculated volumes >15 cm3 (V′ = 0.974V - 3.1). From these equations, normal values for cinecardiographic LV end-diastolic volume (LVEDV), LV ejection fraction (LVEF), LV systolic output (LVSO), LV mass (LVM), and left atrial maximal volume (LAMax) were derived from 56 children (19 < 2 years) with normal left ventricles who underwent cardiac catheterization. Values for LVEDV/BSA were significantly less for infants (< 2 years) than for older children (42 ± 10 versus 73 ± 11 cm3/m2, P <0.001). Values for LAMax/BSA were also less for infants than for older children (26 ± 5 versus 38 ± 8 cm3/ m2, P <0.001), and LVEF was significantly increasel for infants (0.68 ± 0.05 versus 0.63 ± 0.05, P <0.01). The values for LVM/BSA (88 ± 12 g/m2) and LVSO/BSA (4.42 ± 0.95 liters/min/m2) were not significantly different for infants and older children. Multiple regression equations were derived for the prediction of normal volume and mass variables from a patients height, weight, and age. The predicted values can be obtained from nomograms, and estimations of normalcy can be made by comparisons of observed and predicted values with the 95% limits as defined.

Electrical Potential Distribution Surrounding the Atria during Depolarization and Repolarization in the Dog

The potential distribution at the atrial surface during depolarization and repolarization was studied in intact dogs. A preparation was developed by implanting 30 to 40 miniature electrodes permanently on each atrium to record unipolar electrograms in the intact animal. Heart block was created to dissociate atrial and ventricular activity. The electrograms were recorded on magnetic tape and atrial isopotential heart maps produced with the use of a digital computer. The changing potential distribution during excitation indicated the early presence of multiple wave fronts which were related primarily to the crista terminalis, Bachmanns bundle, and a special bundle to the base of the right appendage. The interatrial septum provided a conducting bridge which had an important influence of global atrial excitation, depending on the site of impulse formation. Colliding excitation wave fronts were quite prominent. During terminal atrial excitation, repolarization maxima were present simultaneously with depolarization maxima. Repolarization was characterized by a changing potential distribution which followed the same general pattern as excitation spread; and, furthermore, the earliest areas of excitation were associated with a repolarization maximum and terminal areas of excitation were associated with repolarization minima.

The Effects of Thoracic Inhomogeneities on the Relationship Between Epicardial and Torso Potentials

This study examines the effects of the lungs, spine, sternum, and the anisotropic skeletal muscle layer on the relationship between torso and epicardial potentials. Boundary integral equations representing potentials on the epicardial surface, the torso surface, and the internal conductivity interfaces were solved yielding a set of transfer coefficients valid for any source inside the epicardium and for any conductivity configuration outside the epicardial surface. These transfer coefficients relate potentials on the torso to potentials on the epicardial surface. Calculated torso potentials are generated via the transfer coefficients and measured epicardial potentials for comparison to measured torso potentials. This comparison indicates whether including the thoracic inhomogeneities improves attainable accuracy in calculations relating torso potentials to epicardial potentials.

A volume conductor model of the thorax for the study of defibrillation fields

The authors develop a physiologically realistic volume conductor model for calculating epicardial potentials during transthoracic stimulation. The objective of the study is to measure cardiac potentials during a transthoracic stimulus and compare the measurements to calculated epicardial potentials obtained from the model. The results for all four stimulus configurations (anterior-posterior, neck-waist, precordial, and right-left) on the torso consistently yield correlation coefficients of about 0.90 and RMS errors of 47% between calculated and measured epicardial potentials for a homogeneous torso. Incorporating the effects of the skeletal muscle layer improves the agreement, i.e., correlation coefficients increase to about 0.914 and RMS errors decrease to about 42%. At the same time, the lungs and heart have little influence on the agreement between measured and calculated epicardial potentials. The results of the study demonstrate the importance of the skeletal muscle layer in physiologically realistic volume conductor models.<<ETX>>

An assessment of variable thickness and fiber orientation of the skeletal muscle layer on electrocardiographic calculations

A realistic model of a canine torso which includes extensive detail about skeletal muscle layer thickness and fiber orientation is compared with two other uniformly anisotropic models. A transfer coefficient is developed which relates torso potentials to epicardial potentials for a given anisotropic skeletal muscle layer thickness and muscle fiber orientation. The transfer coefficient is valid for any set of measured epicardial potentials and is independent of the conductivity of the heart. Transfer coefficients calculated for different thicknesses and muscle fiber orientations of the skeletal muscle layer are used to compute torso potentials directly from measured epicardial potentials. A comparison of the measured torso potentials with the potentials calculated from the different transfer coefficients indicates the effectiveness of including variations in fiber orientation and in thickness of the skeletal muscle layer in the model.<<ETX>>

Comparison of measured and calculated epicardial potential during transthoracic stimulation

The authors develop a physiologically realistic volume conductor model for calculating epicardial potentials during transthoracic stimulation to measure cardiac potentials during a transthoracic stimulus, and compare the measurements to calculated epicardial potentials obtained from the model. Potential measurements during a stimulus are recorded for three closed chest dogs. Four different torso electrode combinations (anterior-posterior, neck-waist, precordial and right-left lateral) are used to deliver the stimulus. A boundary integral model is developed which utilizes electrode, heart, and torso geometry, the surface geometry from lung and skeletal muscle interfaces, and internal inhomogeneity conductivities to compute epicardial potentials from a knowledge of stimulus strength and location.<<ETX>>

Electrocardiographic Modeling Of The Fiber Orientation Of The Skeletal Muscle Layer

The effectiveness of including variable thickness and fiber orientation characteristics of the skeletal muscle layer in calculations relating epicardial and torso potentials was examined in this study. The comparison of calculated and measured torso potentials indicates that a simple model consisting of a uniformly anisotropic skeletal muscle layer of 1.0 to 1.5 cm constant thickness significantly improves the model. However, when more detailed data about the variation in skeletal muscle thickness or fiber orientation is introduced into the model, the agreement between calculated and measured torso potentials decreased, although a finite element mesh of over 5000 nodes was used to describe the skeletal muscle in the more detailed model.

Modeling E\t The Bioelectric Stimulation Unit NSF/ERC Unit 1.1C

The modeling projects in Unit 1.1, Bioelectric Stimulation, are focused on the study of electric fields in the heart, both natural and artificial. The simulations range over forward and inverse problems with applications to body, endocardial and intramyocardial mapping, the impedance-based determination of cardiac geometry, modeling the microstructure of cardiac tissue and its response to electrical stimulation and defibrillation, and the design of the algorithm for the optimum estimation of the defibrillation threshold. While the topics are mathematically diverse. they all relate closely to studies carried out in the experimental part of the Bioelectric Stimulation Unit.