[Circadian rhythm of the ventricular fibrillation threshold in a model of hypoventilation-reoxygenation in female Wistar rats]. / Cirkadiánny rytmus prahu komorovej fibrilácie v hypoventilacno-reoxygenacnom modeli u samíc potkana kmena Wistar.

Hypoxia, similarly as myocardial ischemia, decreases the electric stability of the heart and thus produces the conditions for the genesis of ventricular arrhythmias within the 24 h period however a prompt restoration of oxygen supply or blood flow within myocardium causes serious ventricular arrhythmias. Therefore the aim of our study was to evaluate the reoxygenation impact on myocardium after hypoventilation in circadian dependence. The experiments were performed in female Wistar rats (pentobarbital anesthesia 40 mg/1 kg i.p., open chest experiments). The animals were adapted to the light regimen 12:12 hours, with the dark phase from 18.00 h to 06.00 h. Normal ventilation was used in the control group (n = 17) and in the second one (n = 4), 20 min reoxygenation followed 20 min of hypoventilation. The heart rate (HR) had been recorded just before the ventricular arrhythmias rose. Hypoventilation decreased significantly (p < 0.001) the VFT and HR when compared with the control group and changed the 24-hour rhythm of VFT to moderate bi-phase one. Reoxygenation counter changed this rhythm to inverse in the comparison with the control group. The lowest VFT values occurred when the top of VFT circadian rhythm was detected during normal ventilation. No dependence was detected between VFT and HR in both ventilation types. It is concluded that reoxygenation alternates the myocardial vulnerability to ventricular arrhythmias in dependence on alternation of light and dark and without the evident dependence on HR in the course of the whole 24-hour period. (Tab. 2, Fig. 2, Ref. 43.)

Relation of ventricular fibrillation threshold to heart rate during normal ventilation 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 ventilation in female Wistar rats from the circadian point of view. The experiments were performed under pentobarbital anesthesia (40 mg/kg i.p., adaptation to a light-dark cycle 12:12 h, open chest experiments) and the obtained results were averaged independently of the seasons. The VFT measurements were performed during normal ventilation (17 animals) and hypoventilation (10 animals). The HR was recorded immediately before the rise of ventricular arrhythmias. Results are expressed as arithmetic means -/+ S.D. and differences are considered significant when p<0.05. The basic periodic characteristics were calculated using single and population mean cosinor tests. The results 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 dark phases, and 2) hypoventilation changes the 24-h course of the VFT without a change in the 24-h rhythm of the HR. It is concluded that the HR and VFT behave as two independent functional systems without apparent significant circadian dependence during both types of ventilation.

24 h 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.

Circadian rhythm of the ventricular fibrillation threshold in female Wistar rats.

The circadian rhythm of ventricular fibrillation threshold (VFT) and its relation to the heart rate (HR) and the rectal temperature (RT) was studied in female Wistar rats. The animals were exposed to daily light-dark cycles of 12 h of light alternating with 12 h of darkness and were under pentobarbital anaesthesia (40 mg/kg i.p.). The experiments were performed on open chest animals and VFT was measured by direct stimulation of the myocardium. VFT in female rats showed a circadian rhythm with the acrophase -338 degrees (at 22.53 h), with the mesor 2.58 mA and the amplitude 0.33 mA. HR was not significantly changed during the experiments and no dependence was found between VFT and HR during the whole 24-hour period (r = 0.08). The acrophase of the circadian rhythm of HR (on -47 degrees, i.e. at 03.08 h) was shifted to the acrophase of VFT. The circadian rhythms of RT before the application of the anaesthetic agent and under general anaesthesia before the operative interventions had a very similar course with the nearly corresponding acrophases as the circadian rhythm of VFT. It is concluded that the electrical stability of the rat heart measured by VFT shows the significant circadian rhythm in a parallel with the circadian rhythm of RT and probably without dependence on the changes of HR.