Interbeat interval variability in isolated working rat hearts at various dynamic conditions and temperatures.

This study quantifies the effect of afterload and preload changes and of temperature on interbeat interval variability of the intact isolated heart. Ventricular pressure pulse records were obtained from isolated working rat hearts. The variability of interbeat intervals (BIs) was quantified by C90, the central 90% range of the BIs during 10 min periods; predominant frequencies were searched for by power spectral analysis. At 37 degrees C the BI lengths oscillated pseudo-randomly with BI variability C90< or =4 ms. Alternating signs of consecutive BI differences were predominant, and no peaks. were seen in the power spectra. Changes in end-diastolic and aortic pressure had little effect. From 37 degrees C down to 27 degrees C the variability increased about sevenfold, run phase length became randomly distributed, and individual, time-variant peaks occurred in the power spectra. BI variability vanished during atrial pacing. We conclude that: (1) effective mutual synchronization with minimal fluctuation happens within the sino-atrial node of intact rat hearts at body temperature, and synchronization is not affected even by extreme changes in pre- and afterload, (2) the sino-atrial node is the sole source of BI variability in the intact isolated rat heart, (3) low temperature hampers this functional organization which can be reestablished by sinus node accelerating agents (isoprenaline, theophylline), (4) decreasing frequency by N6-Cyclopentyladenosine at normothermia also increases BI variability but less pronouncedly than hypothermia does.