[Changes in electric stability and ultrastructure of the myocardium in heart rate-decreasing paired stimulation]

Experimental rabbits were subjected to a ventricular rate-decreasing paired pacing over an hour. Ventricular rates showed a 1.4-1.9-fold decrease, whereas left ventricular pressure evolution became higher, the periods of ejection and myocardial isovolumetric relation, longer, aortic diastolic pressure, lower than the baseline values. The induction thresholds of ventricular fibrillation were decreased from 16.5 +/- 1.6 mA to 7.2 +/- 3.1 mA (p less than less than 0.001). The examination of left ventricular myocardial ultrastructure indicated that the paired stimulation regimen resulted in focal contractural changes in myocardiocytes to form overcontraction zones.