24H Rhythm of the Ventricular Fibrillation Threshold During Normal and Hypoventilation in Female Wistar Rats

A 24 h rhythm of the ventricular fibrillation threshold (VFT) was investigated in female Wistar rats under conditions of normal ventilation (NV) (17 animals) and hypoventilation (HV) (10 animals). The animals were adapted to a daily 12:12 h light-dark cycle with the dark period from 18:00 to 06:00 under constant temperature conditions. The experiments were performed in pentobarbital anesthesia (40 mg/kg ip, open chest experiments) during the whole year, and the obtained results were averaged independently of the seasons. During NV, the VFT in female rats showed a significant 24 h rhythm (p < 0.01) with the mesor 2.59 +/- 0.53 mA, amplitude 0.33 +/- 0.11 mA, and acrophase -338 degrees (at 22:53 h) and the confidence intervals from -288 degrees to -7 degrees (from 19:12 to 00:28 h) using the population mean cosinor test. The maximal values of the VFT were measured in the active phase between 24:00 and 03:00 h. During HV, the rhythmicity of the VFT showed a more pronounced biphasic character with a smaller peak between 15:00 h and 18:00 h hours and a higher peak between 24:00 h and 03:00 h of the daily regime. Hypoventilation significantly decreased the VFT (p < 0.001) at each interval of the measurement. It is concluded that the electrical stability of the heart measured by the VFT shows a significant 24 h rhythm in female Wistar rats and that HV decreased the VFT during the whole 24 h period.