CHARACTERISTICS AND MECHANISMS OF THE EFFECTS OF HEART-RATE HISTORY ON TRANSIENT AV NODAL RESPONSES

The characteristics and functional origin of the changes in transient atrioventricular (AV) nodal responses with heart rate history were stu died in isolated rabbit heart preparations. For this purpose, ramp sti mulation sequences were applied to the atrium from different initial c onditions. A ramp decrease and increase in the His-stimulus interval a nd a reverse sequence consisting of a ramp increase and decrease were performed starting from a control basic cycle length, after 5 min of f ast rate, or with 5 min of fast rate inserted between the two ramps. T he nodal conduction times (NCT) obtained during the ramp stimulations formed hysteresis loops, the direction, shape, and magnitude of which varied markedly with the nodal history. That is, the nodal response to a given ramp took a variety of forms, depending on the initial condit ion. The effects of the initial condition also depended on ramp direct ion and sequence. A paradoxical NCT-recovery relationship (decrease in NCT with shortening His-atrial interval) was consistently observed at the onset of any ramp decrease performed after 5 min of fast rate. Th ese effects also varied with the rate used to change the nodal history . The insertion of a control cycle at every 20th beat during repeated ramp protocols allowed the determination of the contribution of the no dal property of fatigue to these effects. Fatigue was found to account for all observed hysteresis patterns. In conclusion, heart rate histo ry can, by modulating beat-to-beat changes in fatigue, transform trans ient nodal responses and hysteresis observed during stimulation ramps. Interpretation of transient nodal responses thus requires exact knowl edge of previous nodal history.