Browsing by Subject "Pacing"

Now showing 1 - 4 of 4
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    Creation and In Vivo evaluation of a porous electrode for pacing in a coronary vein: an assessment of the potential for improved electrical performance and chronic stability of coronary venous pacing leads.
    (2009-05) Koop, Brendan Early
    In this work, a porous electrode was fabricated and evaluated in a chronic animal study on a coronary venous pacing lead in order to assess its potential for mitigating chronic lead dislodgements and reducing the characteristic rise in pacing thresholds after implant, both of which being important issues that impact safety and efficacy of implanted cardiac resynchronization therapy systems. Eight test leads were assembled with a porous tip electrode with an average pore size of approximately 30 micrometers, created via a novel fabrication method, and eight control leads were assembled with a standard solid tip electrode design. Both groups were created without steroid-eluting collars and without a capacitive coating on the tip electrodes in order to isolate the affects of electrode porosity. Leads were implanted in canines, and electrical data and x-rays of lead position were taken regularly throughout the 60-day study. Tissue histology was performed for each lead. Significantly lower (p<0.05) mean rise in pacing threshold after implant was observed at day 3 and day 21 for the test group leads (with porous electrodes) as compared to the control group leads. Despite the higher surface area of the porous tip electrodes, pacing impedance was not statistically different between the groups throughout the study, a result likely due to decreased chronic inflammatory response at the surface of porous electrodes. The test group had no lead retractions after day 3 as determined by inspection of x-ray radiographs, while 3-6 (of 8) control group leads retracted after day 3, a result likely due to anchoring of the lead tip due to observed tissue growth into porous electrodes. Mean fibrous capsule thickness at pre-defined measurement points on the tip electrode was not statistically different between the groups, which correlates with the nearly equal mean pacing thresholds for the groups at day 60. The lack of lead retractions for the test groups leads after day 3 is a promising result which should be investigated further, along with investigations of lead extraction force and further electrical data evaluations, using larger sample sizes and more challenging implant conditions.
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    Heart failure and associated structural and functional remodeling: assessment employing various magnetic resonance imaging methodologies.
    (2009-11) Eggen, Michael D.
    Cardiovascular magnetic resonance imaging (MRI), or cardiac MR, is currently considered the "gold" standard for noninvasively characterizing cardiac function and viability, having 3D capabilities and a high spatial and temporal resolution. More recently, the capabilities of MRI have been extended to study tissue microstrucure and fiber orientation in both the brain and the heart through specially designed pulse sequences which are sensitive to diffusion. In this specialized imaging method, known as diffusion tensor magnetic resonance imaging (DTMRI), myofiber orientation can be probed in high resolution and this technique has been successfully utilized to study the helical arrangement of muscle fibers within the myocardium. As such, the counter-wound helical structure of the myocardium is considered to be responsible for the torsional or wringing motion of the left ventricle and serves three main mechanical functions: (1) equalizing myofiber strain and workload; (2) optimizing the volume of blood ejected during systole (stroke volume); and/or (3) storing torsional energy in the intracellular and extracellular matrices and, when released, increasing ventricular filling during diastole. Therefore, cardiac fiber orientation can also be considered as a primary determinant of ventricular pump function, and is of great clinical interest in the study of structure and function within either the normal or diseased heart. To date, the primary focus of cardiac DTMRI has been to characterize myofiber orientation in healthy animal hearts, with little progress in the study of myofiber arrangement in the diseased heart. As such, due to the long scan times required for in vivo DTMRI, and the limited availability of freshly excised human hearts for ex vivo imaging, data are limited in the characterization of fiber orientation in both healthy and diseased human hearts. Therefore, in my thesis research, the primary objective was to investigate myofiber orientation in both healthy and diseased hearts using DTMRI. Specifically, changes in myofiber orientation were investigated in a high rate pacing model of dilated cardiomyopathy in swine, and also in excised healthy and diseased human hearts obtained from the Bequest Anatomy program at the University of Minnesota, and LifeSource (the Upper Midwest, a non-profit organ procurement organization). In addition, the mechanical activation due to cardiac pacing from the right ventricular apex was uniquely characterized in a case study of an isolated human heart using MRI, as cardiac pacing from the right ventricular apex is known to chronically result in deleterious changes in fiber orientation and cardiac function. My thesis was divided into several chapters, in the first it was considered paramount to gain a thorough understanding of cardiac MRI. As such, a review of cardiovascular MRI is provided in Chapter 1. The goal of this chapter was to familiarize the reader with cardiac MR and nomenclature, review current techniques to quantify cardiac function with MRI, and to introduce the reader to cardiac diffusion tensor magnetic resonance imaging (DTMRI), which is used in the present work to quantify fiber orientation in the heart. In Chapter 2, a literature review of cardiac fiber orientation and relevant changes resulting from disease is presented, and the measurement of fiber orientation using DTMRI is further discussed. The intent of this chapter is to familiarize the reader with diffusion imaging and the associated parameters used to characterize fiber orientation and tissue integrity. In addition, the methodologies and computational tools developed to measure fiber orientation using a 3 tesla Siemens MRI clinical scanner are described. Chapters 3-5 describe novel investigations in the assessment of fiber orientation using DTMRI. In chapter 3, the effects of decomposition on the diffusion properties of the myocardium were studied in freshly excised human hearts recovered at varying post mortem intervals. From this study, the time frame for the recovery of a human heart was determined to be 3 days, such that the tissue still remains viable for the measurement of fiber orientation using DTMRI. In Chapter 4, a swine model of dilated cardiomyaphy was used to assess in vivo functional and anatomical changes resulting from severe dilation of both the right and left ventricle. Following in vivo functional imaging, ex vivo DTMRI was used to investigate the resultant fiber orientation. Chapter 5 provides a preliminary comparison of fiber orientation in healthy and diseased human hearts, collected within a post-mortem interval of 3 days. Furthermore, in Chapter 6, the mechanical activation during pacing from the right ventricular apex was studied in an isolated human heart. Since pacing from the right ventricular apex is known to cause deleterious changes in fiber orientation, it was of great interest to characterize myocardial strain and motion during RVA pacing as part of my thesis work. In general, this research project has advanced our overall knowledge as to our understanding of ex vivo DTMRI, and the remodeling of the myocardial architecture in heart failure. This described work is not an exhaustive list of the changes in fiber orientation that occur in every type of cardiomyopathy, but provides novel insights into fiber reorganization which occurs in swine due congestive heart failure, and in human hearts excised from patients with a history of heart failure. Additionally, with the development of methodologies and computational tools presented here, and the study of post mortem intervals on the diffusion properties of the myocardium, the framework has been laid for the future analysis of fiber orientation in other cardiomyopathies presented in human cadaveric hearts.
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    The implications and effects of coronary venous anatomy on clinical interventions
    (2013-12) Spencer, Julianne
    Background: The anatomy of the coronary venous system has important implications for clinical interventions including cardiac resynchronization therapy, ablation therapy, defibrillation, perfusion therapy, and mitral valve annuloplasty. Method: The effect of coronary venous anatomy on cardiac therapies was assessed by characterizing the venous anatomy, the clinically relevant adjacent anatomy, and the anatomical effect on pacing thresholds. Coronary venous anatomy was evaluated using perfusion-fixed human cardiac specimens with direct measurements using a microscribe digitizer and with contrast-computed tomography. Contrast-computed tomography and magnetic resonance imaging were used to investigate the coronary venous anatomical relationship with the phrenic nerve, coronary arteries, and mitral isthmus. Finally, electrostatic field simulations were performed to evaluate how cardiac anatomy and pacing electrode location affect clinical pacing thresholds and phrenic nerve stimulation. Results/Discussion: In short, the human coronary venous system is a highly variable physiological system that is essential for several current cardiac therapies. The consistent observation of high anatomical variation throughout all of the performed studies stress the clinical importance of understanding a given patient's individual anatomy when performing a planned intervention within the coronary veins. Importantly, the database of coronary venous anatomical parameters and pacing thresholds presented here can be incorporated into the development and implementation of therapies that utilize these vessels.
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    Respiratory Exchange Ratio is Not Associated with Slowing in the Marathon
    (2016-08) McGuirk, Erin
    Background: Previous research has shown that males slow more throughout the course of a marathon than females. Proposed reasons for differences in slowing include the fact that females oxidize proportionately more lipids and fewer carbohydrates during exercise when compared to males, and possible differences in thermoregulation. Respiratory exchange ratio (RER) can be used to estimate the ratio of fat to carbohydrates being metabolized. Purpose: To compare the degree of slowing (time in the first vs. second half of a marathon) between men and women, and determine if steady-state RER or ambient temperature differences predict the rate of slowing in male and female novice marathon runners. Methods: Chip times for 123 female and 44 male recreational marathon runners (21.0 ± 1.7yrs) were used to determine change in pace observed in the second half of the marathon compared to the first half. A two-mile time trial (2MI) was used to assess baseline fitness and pace for steady-state measurements. A submaximal 6-minute treadmill run at 75% of 2MI velocity was completed 1-3 weeks before the marathon. RER was collected using a metabolic cart (Medical Graphics Diagnostics, St. Paul, MN). Baseline measures and outcomes (RER and percent slowing) were analyzed using independent samples t-tests to detect differences between the groups (men vs. women and by year 2014, cool weather vs. 2015, warm weather). Univariate ANOVA tests were run to analyze the differences in percent slowing (%slowing) and RER by year and sex. Pearson’s Product Moment Correlation Coefficient (r) was used to determine the strength of the relationship between RER and %slowing as well as the relationship between %slowing and percent body fat (%BF), weight, height, body surface area (BSA), and BSA to mass ratio (BSA/M). Results: The mean %slowing for the total sample for 2014 and the total sample in 2015 was 14.1± 12.0% and 22.0 ± 16.5%, respectively (p<0.05). The mean %slowing for the combined group from 2014 and 2015 males and females was 20.6± 14.8% and 17.02 ± 14.8%, respectively (p <0.05). Females had a significantly lower RER during steady-state exercise in comparison with males (Female = 0.87 ± 0.05, Male = 0.89 ± 0.05, p<0.05). Sex and year were predictors of %slowing. There was no significant relationship between RER, temperature of marathon, weight, %BF, BSA, or BSA/M and %slowing in the total group, but RER and height were significantly related (p<0.05). Conclusion: Consistent with previous research, males slow more than females from the first to second half of the marathon. However, RER was not associated with slowing during the marathon. Temperatures of the race did affect the rate of slowing, but men and women were not affected differently. This suggests that pace maintenance is not due to substrate metabolism.