Relation of ventricular fibrillation threshold to heart rate during normalventilation and hypoventilation in female Wistar rats: A chronophysiological study

The aim of our study was to verify the relationship between heart rate (HR) and ventricular fibrillation threshold (VFT) during different types of ven tilation in female Wistar rats from the circadian point of view. The experi ments were performed under pentobarbital anesthesia (40 mg/kg i.p., adaptat ion to a light-dark cycle 12:12 h, open chest experiments) and the obtained results were averaged independently of the seasons. The VFT measurements w ere performed during normal ventilation (17 animals) and hypoventilation (1 0 animals). The HR was recorded immediately before the rise of ventricular arrhythmias. Results are expressed as arithmetic means +/- S.D. and differe nces are considered significant when p<0.05, The basic periodic characteris tics were calculated using single and population mean cosinor tests. The re sults from our experiments have demonstrate that 1) the VFT and HR respond identically to hypoventilation by a decrease in the light and also in the d ark phases, and 2) hypoventilation changes the 24-h course of the VFT witho ut a change in the 24-h rhythm of the HR. It is concluded that the HR and V FT behave as two independent functional systems without apparent significan t circadian dependence during both types of ventilation.