Grundlagen der elektrischen Stimulation des Herzens

Abstract

An essential part of a pacemaker system is the lead with a stimulation and sensing electrode. Both funtions, which are completely different processes, are performed by the same electrode. Requirements for a pacemaker lead are: outstanding bio-compatibility, low pacing threshold, reliable sensing, high pacing impedance, low polarisation and corresponding longterm stability. This article summarizes the up-to-date knowledge of the effects of cardiac pacing on the cell and the electrode-myocardium-inferface and its consequences on the design of modern pacemaker leads. The underlying mechanisms for the electrical stimulation of the myocardial cell are determined by the membrane. The myocardial answer of the cell on an adequate stimulus is the generation of a self-regenerating depolarisation wave called excitability. The stimulation impulse generates an electrical field, which causes a decrease in the baseline membrane potential (hypopolarisation) by an ionic current. As a consequence further sodium channels are opened until the threshold potential is reached and the depolarisation of the cell is initiated. The minimal amount of energy required for this process is called stimulation threshold, which depends on the electrode surface, the stimulation impedance, the alignment of the myocardial fibers in the electrical field, the distance to excitable tissue and the polarisation current. The relationship between pacing current and impulse duration is described by rheobase and chronaxy. Implantable pacemaker devices work with biphasic stimulation impulses, which allow a recharge of the delivered impulse energy from the heart to the stimulator and therefore avoid an electrolysis. The improved understanding of the basic principles of cardiac pacing led to the development of modern stimulation electrodes. Steroid eluting leads avoid an increase in pacing threshold in the initial period. The geometrical surface of electrodes was reduced with the effect of lower pacing thresholds. The introduction of new electrodes with a porous or fractal surface led to a reduction in polarisation and therefore to an improved stimulation with regard to energy delivery. The new pacemaker generation allows a monitoring or analysis of stimulation. With the help of certain algorithms the efficacy of every single ventricular stimulus is monitored (autocapture function) and the pacing amplitude automatically adjusted to the actual situation. This prolongs the pacemaker's life and simplifies follow-up.