BIOLOGICAL EFFECTS OF SHOCK-WAVES - INDUCTION OF ARRHYTHMIA IN PIGLETHEARTS

During lithotripsy by electrohydraulic or electromagnetic lithotripter s, the application of extracorporeal shock waves has to be synchronize d with the electrocardiogram to reduce the induction of arrhythmias. T he relation between the refractory period of the cardiac cycle and arr hythmia induction by shock waves, and the underlying mechanism have so far not been examined. In this experiment, the cardiac response to sh ock waves administered at 20 kV by an electrohydraulic lithotripter wa s assessed in nine piglets. The focus was positioned 5, 10 and 15 cm c audal to the apex of the left ventricle, and in some piglets also at t he apex. The interval following the R-wave was determined during which the heart was refractory to shock wave stimulation by either single d ischarges or shock-wave bursts, i.e., groups of discharges separated b y 10 ms intervals. This mechanical refractory period was compared to i ts electrical counterpart, which was determined by transvenous intraca rdiac atrial stimulation. As a result, mechanical refractory periods f ollowing the R-wave were at 5 cm distance 60 ms for single discharges and 70 ms for bursts (medians; range 10-180 ms); both stimulation mode s were highly correlated (r = 0.88). While a similar result was obtain ed with the focus positioned directly at the cardiac apex, at 10 cm di stance from it, bursts elicited a cardiac response significantly more often (in nine vs. two piglets). At 15 cm distance, no response was ob tained at all. Both mechanical and electrical atrial refractory period s were in a similar range. The more effective cardiac stimulation by s hock-wave bursts at 10 cm distance is compatible with the induction of arrhythmia by a cavitation-based instead of a direct shock wave effec t.