In isolated perfused guinea-pig hearts the threshold for ventricular fibrillation was determined applying DC or sinusoidal 50-cycle AC pulses during the vulnerable period. The results were compared with the threshold for single responses during diastole.Using rectangular pulses from 0.5 to 40 msec the threshold for fibrillation as well as for single responses exhibited a nearly linear relationship between strength and duration of impulses when plotted in a log-log-scale. As compared with the threshold for single responses the fibrillation threshold was shifted in parallel to a higher intensity level.Vulnerability of the ventricles was independent on stimulus duration. When the hearts were beating in AV-rhythm maximal vulnerability appeared about 150 msec after the beginning of ventricular excitation.Alteration of the physical conditions by increasing the magnitude ~f the electrodes and changing their position resulted in an increase of the threshold for single diastolic responses accompanied by a decrease of the fibrillation threshold, both curves being shifted in parallel retainig their slope. Using the same electrode arrangement, the threshold for electrical defibrillation exhibited a similar characteristic as the fibrillation threshold, however on a higher intensity level.With sinusoidal 50-cycle AC pulses an increase of stimulus duration beyond 1 period caused no further change to the threshold for single responses, whereas the threshold for fibrillation declined continously until a total duration of about 6 periods was attained.The results are interpreted in terms of the re-entry mechanism of fibrillation.Die Wirksamkeit des elektrischen Stroms f(ir die Ausl6sung yon Vorhofbzw. Kammerflimmern des Herzens ist von zahlreichen physikalischen und biologischen Gegebenheiten abh~ingig (14, 24, 26, 29, 36, 41, 42). Als physikalische Bedingungen kommen haupts~ichlich in Betracht:1. die Stromdichte im Herzen: Sie ist abh/ingig yon der angelegten Spannung, der Gr61~e und Position der Elektroden und den Widerstands-verhMtnissen
MRI allows detecting abnormal elastic aortic wall properties already in the normal-sized aorta of adolescents with Marfan syndrome. Monitoring of these properties could be relevant for evaluating disease onset and progression. MRI has the potential value of compliance measurements for the follow-up and to guide therapy indications.
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