[The implantable automatic cardioverter-defibrillator].
In addition to medical treatment for ventricular tachyarrhythmias which has not proven to be sufficient, nonmedical modes of treatment are available such as electrophysiologically-guided surgical measures and catheter ablation, both of which are restricted to only a relatively small patient population and require further technical refinement. In 1980, Mirowski introduced the automatic implantable defibrillator and, to date, world-wide, this device has been implanted in 8000 patients. CHARACTERISTICS AND IMPLANTATION OF THE AUTOMATIC IMPLANTABLE CARDIOVERTER/DEFIBRILLATOR (AICD): The AICD continuously monitors the electrical activity of the heart, recognizes the onset of threatening ventricular tachycardias and terminates these according to the respectively programmed mode by delivering direct current shocks or stimuli. The currently used defibrillators consist of an impulse generator with lithium batteries and an electrode system. The batteries can charge a capacitor with about 700 volts in five to eight seconds which produces a current with an energy up to 30 Joules on discharge. The current is delivered either by two plate electrodes on the right and left ventricles or a plate electrode on the left ventricle and a spiral electrode inserted in the superior vena cava. The electrodes also serve the purpose of tachycardia detection by means of an electrical signal, the probability density function (PDF), that is, a significant decrease in the potentials to isoelectric. With this, it is only possible to terminate ventricular fibrillation. Additional electrical detection criteria are obtained and analyzed by two adjacently positioned epicardial screw electrodes or a bipolar endocardial electrode, enable identification of ventricular tachycardia as well. If the tachycardia detection criteria are fulfilled, the capacitor is discharged according to its programmed shock energy. In 1988, programmable defibrillators were introduced. Current defibrillator treatment also incorporates the possibility for antitachycardia stimulation. Attempts to use, instead of the monophase, square-wave impulse, a biphasic defibrillation impulse, to achieve a sequential impulse and to make use of the bidirectional impulse extension have rendered improved reliability for tachycardia termination and energy savings. After median sternotomy, the plate electrodes are usually sutured to the epicardium and the spiral electrode for the bipolar ECG is positioned at the anterior aspect of the right ventricle. The generator is implanted on the left side para-umbilically in subcutaneous or subfascial tissue. With the subxyphoid approach to avoid sternotomy, the plate electrode is sutured extrapericardially over the left ventricle and the spiral electrode is positioned at the epicardium. Alternatively, for those in whom prior cardiac surgery has been carried out, a lateral thoracotomy can be used. The defibrillation threshold, that is the lowest possible energy for defibrillation of ventricular fibrillation or ventricular tachycardia, should be determined intraoperatively after stimulation of the arrhythmia. The energy required for termination of a stable ventricular tachycardia is usually less than that for termination of ventricular fibrillation and can be determined postoperatively. A margin of security should be taken into consideration which, for defibrillation thresholds of up to 10 Joules, is about twice the amount of the defibrillation threshold itself.(ABSTRACT TRUNCATED AT 400 WORDS)
Department Innere Medizin und Dermatologie, Medizinische Hochschule Hannover.
H
H
Klein
J
J
Tröster
H J
HJ
Trappe
I
I
Becht
F
F
Siclari
D004554:Electric Countershock / Q000295:instrumentation*; D004562:Electrocardiography; D004867:Equipment Design; D005500:Follow-Up Studies; D006352:Heart Ventricles; D006801:Humans; D013610:Tachycardia / Q000628:therapy*
Der implantierbare automatische Kardioverter-Defibrillator (AICD)]