title: [Mechanism of spontaneous occurrence of tachycardia].
abstract: Tachycardias arise from an arrhythmogenic substrate and a trigger factor, an extrasystole, the two factors being under the influence of the autonomic nervous system. The study of the mechanisms of spontaneous initiation of arrhythmias must, therefore, take these three factors and their interactions into account. The frequency dependency of an arrhythmia and the sensitivity of the substrate to the adrenergic system varies with time in a given subject and from one patient to another according to the presence and type of cardiac disease. The mode of initiation of most ventricular tachycardias and the therapeutic consequences may be understood: in some forms of cardiac disease, such as arrhythmogenic right ventricular dysplasia, the increase in heart rate which usually precedes sustained ventricular arrhythmias is only perceptible in mild or recent forms, unlike the more chronic dysplasias. This suggests that the arrhythmogenic substrate becomes more sensitive to catecholamines with time, and therefore requires smaller changes in sympathetic tone in order to be expressed (adrenergic paradox). Heart rate changes accompany modifications of sinus variability. Holter monitoring has shown, and this has been confirmed by recordings obtained from patients with implanted automatic defibrillators, that global sinus variability decreases before the initiation of a ventricular arrhythmia. Studies of the dynamics of ventricular repolarisation should also confirm the changes of QT frequency-dependency. The analysis of the initiation of arrhythmias would only have an academic interest if this was limited to a purely descriptive exercise. It is one of the best means of understanding arrhythmias and their therapeutic implications. The development of computerised methods of analysis of Holter monitoring should lead to further progress in this field.
mesh_terms: D006332:Cardiomegaly / Q000150:complications* / Q000503:physiopathology; D016757:Death, Sudden, Cardiac / Q000209:etiology / Q000517:prevention & control; D006333:Heart Failure / Q000150:complications* / Q000503:physiopathology; D006801:Humans; D009203:Myocardial Infarction / Q000150:complications; D013610:Tachycardia / Q000209:etiology* / Q000503:physiopathology; D017180:Tachycardia, Ventricular / Q000209:etiology* / Q000401:mortality