Interpretation of intramyocardial electrograms

The recording and interpretation of intramyocardial electrograms (IEG) offers new chances for surveillance of patients with certain cardiac risks, e.g. patients after heart transplantation or cardiac infarction, with cardiomyopathies, on hemodialysis therapy or with beginning ischaemia. The main advantages of the analysis of IEGs, which are acquired with an implanted device as compared with the well-known surface ECG are the higher reproducibility in signal morphology, and the possibility of continuous life-long application. It has already been shown in previous studies that the morphology of IEGs represents information not only on the involved electrophysiological processes, but additionally also about the hemodynamic performance of the heart, e. g. stroke volume and enddiastolic volume, the beginning of hemodynamic insufficiency or rejection in transplanted hearts, and can be related with the NYHA classification. The interpretation of the individual signal morphology with regard to hemodynamics, however, is still difficult since it depends on the actual position of the recording electrode, the anatomical shape of the heart, the ingrowing of the recording electrode into the myocardial tissue including the actual size of the fibrotic capsule, the concentration and distribution of the most relevant electrolytes with their impact on the shape of the action potential of each single cell, disturbances on the spreading of the excitation wavefront, e. g. in the border zone to damaged tissue etc. The only way to gain a better understanding of these influences is to employ a model. Such a model, which is based on simple geometric forms and more recently on FEM and allows the coupling of electrophysiological processes with the mechanical contraction, has already been developed. A first evaluation with real signals and data about the anatomical shape of the heart in endsystolic and enddiastolic position has provided stimulating results. This model shall further be extended, evaluated and employed for systematic research on the morphology of IEGs, primarily obtained from paced hearts in order to reach better standardization with regard to heart rate (Ventricular Evoked Response VER). This research shall finally render possible the development of a worldwide system for the permanent surveillance of patients with cardiac high risks by using implanted devices with short-distance telemetric capabilities and the coupling with long-distance data transmission networks, e. g. the Internet. A comparable system has already successfully been evaluated by remote surveillance of heart transplant patients in different continents.

Failures of the heart are among the diseases with the highest rates of morbidity and mortality. The disease is associated with a high socio-economic burden. The incidence is still increasing in both industrialized and in non- industrialized countries, despite the availability of advanced methods for early diagnosis and effective therapy management. However, only few methodological approaches are available until now for continuous and permanent risk monitoring in patients with recognized cardiac risk. The hypothesis for this work supported by FWF project P16965-N04 was that intramyocardial electrograms which can be acquired directly from the heart have a promising potential for cardiac risk monitoring. The technology is available for implantable devices which transmit these signals to a patient-near relay station that forwards the signals worldwide to a data processing center in charge for risk surveillance and patient alarming. The bottleneck is the poor knowledge about the information represented in the morphology of these signals. Although the morphology is affected by risk factors, it additionally depends on individual properties and physiological features. The aim of the project has been to provide a better understanding of the signal morphology and thereby to facilitate its interpretation with particular regard to monitoring the trend of the identified risk. Typical examples of such risks are the augmented occurrence of ventricular premature beats, the disturbance of the balance between the sympathetic and the parasympathetic activity, or the impairment of the hemodynamic pumping efficiency of the heart. Furthermore, there is increasing evidence that intramyocardial electrograms can be utilized to support the management of certain therapies, particular of those therapies which make use of fusion beats, e.g. the resynchronization therapy by multisite stimulation, or the optimization of the atrio-ventricular delay in patients with hypertrophic obstructive cardiomyopathy. The scientific work in P16965- N04 has been focused on two models: (1) A bidomain slab model with rectangular volume that allowed the modification of all relevant impact factors like the shape of the cellular action potential including the repolarization phase, the fiber architecture, the electrode position, electrical impedance anisotropy, etc. (2) A biventricular heart model based on the 3D data set acquired from a patient with transplanted heart in endsystolic and enddiastolic state using the ultrafast X-ray CT technology. Special attention in this model was given to the impact of the specialized conduction system for excitations, particular in cases of artificial ventricular stimulation, in order to understand the morphology of fusion beats. Both models have been realized with the finite element method with a spatial resolution sufficient for adequate signal simulation. The results of the model simulation are in excellent agreement with clinical results and support the basic hypothesis that intramyocardial electrograms have a promising potential for cardiac risk monitoring and also for therapy management.