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1992 | Book

Basic Anatomy and Physiology of the Heart Read chapter Read first chapter

Electrotherapy of the Heart

Technical Aspects in Cardiac Pacing

Author: Prof. Dr.-Ing. Max Schaldach

Publisher: Springer Berlin Heidelberg

Included in: Professional Book Archive

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About this book

Since 1958, when the first cardiac pacing system was implanted, the exemplary collaboration between medicine and engineering has developed into an extremely successful therapy. The book highlights many of the recent and most important technological advances and shows the multidisciplinary nature of the technical task of pacemaker development which is based on the diverse components of physiology, electronics, physics, electrochemistry and the material sciences.

Table of Contents

Frontmatter
Basic Anatomy and Physiology of the Heart
Abstract
To understand pacemaker technology, the knowledge of certain anatomical, physiological and pathophysiological facts is required. The aim of the following chapters is to briefly explain the basic functions of the heart and its control mechanisms such as the autonomic nervous system as a part of the central nervous system. However, the content should not be considered as a complete and detailed description of all anatomical and physiological details. It is recommended that other textbooks on physiology, electrophysiology, pathophysiology as well as cardiology be studied [1–4].
Max Schaldach
Physiology of the Heartbeat
Abstract
The spontaneous, rhythmic generation of APs in the intrinsic pacemaker centers of the heart is the basis of the autonomous heartbeat rhythm [6]. Normally, the heartbeat is initiated in the SA node. The AP generated propagates from one muscle cell to the next until it has spread over the entire atrial wall and reached the AV node at the boundary between the atrium and the ventricle. Because of the low conduction velocity in the AV node, there is a delay in transmission of the wave of excitation to the ventricles that allows the atrial contraction to be completed before contraction of the ventricles begins (Figure 10). From the AV node, the excitation passes along the His bundle through the atrioventricular boundary.
Max Schaldach
Monitoring the Electrical Activity of the Heart
Abstract
The standard method of observing the electrical events in the heart is electrocardiography (ECG) and is routinely used for pacing evaluation and patient follow-up [13]. All together, the individual APs in the heart produce an overall time-varying voltage difference between the excited and unexcited parts during a contraction phase. This voltage signal can be recorded with electrodes attached to the surface of the body, amplified and displayed with a multichannel recorder. The recording can be either bipolar, in which the potential difference between electrodes at two active sites is measured, or unipolar, in which case a single recording electrode measures the potential at one site with respect to a reference electrode. The most common bipolar arrangement is the combination of three limb leads known as Einthoven’s triangle. Unipolar recording usually employs Wilson’s precordial leads attached at various positions on the chest.
Max Schaldach
Therapy for Cardiac Rhythm Disturbances
Abstract
The most urgent goal in the treatment of rhythm disorders should be to ensure a coordinated contraction of the heart, corresponding to the physiological sequence of events during the heartbeat. In many cases, this goal is unattainable so that one must strive for at least partial success by achieving the following smaller goals:
1)
to ensure that the heart pumps effectively enough to keep the organism alive;
 
2)
to alleviate the symptoms that result from the disturbed rhythm;
 
3)
to protect the patient from complications that could develop from the existing disorder; and
 
4)
to provide a satisfactory quality of life (e.g., capacity for physical exertion).
 
Max Schaldach
Pacemaker Technology
Abstract
To summarize the preceding chapters, the sequence of contractions in the heart is controlled by autonomous mechanisms that are subject to extracardiac regulative influences. Through sympathetic innervation, these mechanisms permit inotropic and chronotropic adaptation of cardiac output to physical workload. The automatic action begins at the SA node. If its impulse formation ceases, e.g., an SA blockage, or if there is a total AV block, the remaining parts of the conduction system are capable of spontaneous excitation. However, as the distance of the secondary site from the SA node increases, the spontaneous depolarization rate decreases and the workload-dependent variation of the heart rate is compromised so that the cardiac output is exclusively regulated through changes in stroke volume. This compensation mechanism presupposes a healthy ventricular myocardium, capable of adapting the stroke volume. In the event that this adaptation does not compensate for the absence of increase in heart rate, the bradycardia symptoms can be successfully treated by implanting a cardiac pacemaker [21–23]. Normally, heart rate makes the largest contribution to the workload-dependent adaptation of the cardiac output.
Max Schaldach
Control Aspects of Cardiac Output Adjustment
Abstract
It must be taken into account that even the detailed description of a physiological system and its behavior can only be a simplified model of the reality. Nevertheless, analysis and modelling are very efficient and powerful methods to a better understanding of complex physiological systems and their behavior, and are essential in the design of those pacing systems using a physiological control mechanism to maintain homeostasis.
Max Schaldach
Status of the Application of Corporeal Control Parameters
Abstract
Rate-adaptive pacemakers using different corporeal control signals have been developed. To achieve rate adaptation based on a sensor signal, a suitable signal transducer and a suitably robust algorithm must be available. Table IX summarizes the available sensor technology necessary to implement one of these adaptive strategies. The same high quality requirements imposed on the pacemaker also apply to the permanently implanted regulatory system. A stable and reliable measurement function must be maintained for many years. The sensor must fulfill the requirement of implantation and the same must apply to the power consumption. Advances in microelectronics permit the use of transducers in which the measurement and control variables are linked in a nonlinear fashion. As discussed below, there are numerous transducers that meet these requirements and may be incorporated into a rate-adaptive pacemaker [82].
Max Schaldach
Cardiac Control Parameters
Abstract
In comparison to corporeal control parameters, cardiac control parameters offer the general advantage that the sensed variable contains systemic control information [101]. A particularly useful control signal available via a cardiac parameter is the sympathetic tone, which is independent of the heart rate [53, 102]. This signal is an efferent control signal from the ANS to regulate cardiac output.
Max Schaldach
The Stimulating Electrode
Abstract
The principal method of artificial cardiac pacing involves the transvenous endocardiac placement of electrodes, usually unipolar, which transmit cathodal stimulus pulses to the myocardium as introduced by Furman in 1958 [119]. The response of an excitable cardiac muscle cell to an external stimulus depends on the electrical field strength—that is, on whether the field depolarizes the diastolic membrane potential sufficiently to generate spontaneous depolarization of the cell—an all-or-none AP which propagates excitation of the entire myocardium. Physiological experiments in which intracellular electrodes are used to excite the heart have shown that charge-carrier injection of 10-11As suffices to trigger the AP [120]. Artificial pacing with the electrodes currently in common use requires a greater charge, about 10-6 As.
Max Schaldach
Materials in Pacemaker Technology
Abstract
In the present state of the art, the usefulness of the implantable pacemaker is defined, not only by design constraints in achieving a shape appropriate to the physiological and biomedical requirements of the desired function, but also by the properties of the materials of which the device are made. The use of alloplastic materials in replacement surgery has a long history, and has mainly involved implants in the skeletomuscular system [151–153]. The implantation of artificial parts in the cardiovascular system have only recently been developed [154–157]. The major role played by implantable devices in modern medicine can be illustrated most simply by some statistics. Every year more than 1,500,000 people worldwide are provided with vascular prostheses; artificial heart valves are implanted in 100,000 patients, and about 220,000 receive an implantable cardiac pacemaker [158]. In addition to the electronic requirements discussed previously, the biocompatibility of the implantable pacemaker is of importance in the long-term success of arrhythmia treatment.
Max Schaldach
Pacemaker Power Sources
Abstract
Semiconductor microelectronics has played the same important role in the development of the implantable pacemaker as in the technology of electrochemical power sources. Well over five million patients have benefitted from the use of battery-powered implantable devices. The lithium battery has evolved into the principal power source since its introduction in 1972 [207]. The lithium-based primary cell has dominated pacemaker technology since the early 1980s, requiring power sources capable of delivering currents in the 10 to 100 microampere range. Compared to other electronic applications, the amount of energy required to operate a cardiac pacemaker is very small. In general, a stimulus pulse of 5–19 mA is delivered at 1–10 V for 0.25–1.0 ms with a rate from 30–150 bpm. For a typical application, a stimulus pulse of 10mA is delivered at 5 V for 0.5 msec at 70 bpm. Assuming a pacing rate of 70 pulses per minute, the average continuous energy drain is 30 µW.
Max Schaldach
Reestablishment of Physiological Regulation
A Challenge to Technology
Abstract
The exemplary collaboration between engineering sciences and medicine has, in recent decades, developed electropacing of the heart into an extremely successful therapy. Due to high technological standards, cardiac pacemakers have become therapeutic agents that reliably reestablish cardiovascular function and improve the quality of life for a large number of patients. Their superiority over previous forms of treatment arises from the utilization of still existing physiological regulatory processes of the cardiovascular system. An understanding of natural impulse formation, impulse conduction, and replacement by artificial processes substantiates the success of the aforementioned interdisciplinary efforts.
Max Schaldach
Backmatter
Metadata
Title
Electrotherapy of the Heart
Author
Prof. Dr.-Ing. Max Schaldach
Copyright Year
1992
Publisher
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-50209-5
Print ISBN
978-3-642-50211-8
DOI
https://doi.org/10.1007/978-3-642-50209-5