The use of single rectangular stimuli, applied directly to the heart surface, has been confirmed as a reliable method for measurement of the ventricular fibrillation threshold (VFT). The following factors which influence reproducibility were investigated: 1) spatial separation of bipolar electrodes, 2) interval between stimuli, and 3) anatomical position of electrodes on the heart. With bipolar electrodes placed 10–30 mm apart and an interval of 10–15 cycles between stimuli the VFT was remarkably constant for any single dog. An interelectrode separation of 2–4 mm or the placement of electrodes in a coagulated area of epicardium produced high, widely variable, and at times unobtainable thresholds believed due to short circuiting of the current delivered. Thresholds determined with stimuli 3–4 beats apart were significantly lower than when stimuli were delivered 10–15 beats apart. The VFT was significantly higher at the base than at the apex of the left ventricle. Investigations which measure the effect of a given influence on the VFT should control the factors studied here.
Cardiac contractility modulation increases action potential duration dispersion and decreases ventricular fibrillation threshold via β1-adrenoceptor activation in the crystalloid perfused normal rabbit heart
