The effect of left ventricular intracavitary volume on the unipolar electrogram

In order to examine the effects of ventricular distention on the unipolar electrogram (UEG), an isolated rabbit heart modified Langendorff preparation was utilized. Left ventricular (LV) volume was adjusted using ionically permeable (PB = 9 hearts) or ionically impermeable balloons (IB = 4 hearts). LV UEGs, LV end-diastolic pressure (EDP), and LV minor axis dimension (MAD), as measured by ultrasonic transducers, were recorded. Three hundred twenty-five electrograms were digitized and analyzed with custom-designed software. In the PB group, a significant inverse linear relationship was found between UEG amplitude and changes in MAD (P < 0.0001). For each animal, this relationship had an R value > 0.8 and a P value < 0.0001. There was also a significant inverse linear relationship between UEG slope and changes in MAD (P < 0.01). UEG amplitude and slope also exhibited a significant inverse relationship to changes in LV EDP, which were best described by a third order polynomial function. In the IB group, no significant relationship was found between either UEG amplitude or slope and MAD or EDP. In this study, intracavitary volume exerted a profound and significant influence on UEG amplitude and slope. This effect was due to increases in conductive intraventricular volume and not to myocardial stretch.