Objectives: To determine the effect of the light-dark cycle (LD) and type of anesthetic agent used on changes in the electrical stability of the heart in disorders of pulmonary ventilation.
Methods: The ventricular arrhythmia threshold (VAT) and heart rate (HR) were measured in anesthetized female Wistar rats adapted to an LD regimen of 12 h:12 h (pentobarbital sodium 30 mg/kg, intraperitoneally, or ketamine/xylazine 100 mg/15 mg/kg, intramuscularly; open chest experiments) under artificial normoventilation (tidal volume 1 mL/100 g, respiratory rate 40 breaths/min) and during 20 min of hypoventilation (tidal volume 0.5 mL/100 g, respiratory rate 20 breaths/min) followed by 20 min of reoxygenation. Control and experimental groups were set according to LD cycle adaptation and type of anesthesia used: light control pentobarbital sodium group, n=67; dark control pentobarbital sodium group n=50; light hypoventilation-reoxygenation (hypo-reoxy) pentobarbital sodium group, n=26; dark hypo-reoxy pentobarbital sodium group, n=16; light control ketamine/xylazine group, n=90; dark control ketamine/xylazine group, n=57; light hypo-reoxy ketamine/xylazine group, n=13; and dark hypo-reoxy ketamine/xylayine group, n=18.
Results: In the control groups, the LD cycle only had an effect on animals under pentobarbital sodium anesthesia (P<0.001). During hypoventilation, the VAT was significantly lower in animals under both types of anesthesia and during the light period of the LD cycle than in the control groups. During the light part of the LD cycle, the period of reoxygenation increased VAT values (P<0.001). However, during the dark period of the LD cycle the VAT values decreased. Heart rate was significantly lower in the ketamine/xylazine groups than in the pentobarbital sodium groups during the light and dark periods of the LD cycle, respectively (P<0.001).
Conclusions: The mechanisms mediating the effects of pentobarbital sodium anesthesia on myocardial vulnerability likely depend on the LD cycle, in contrast with the mechanisms that mediate the effects of ketamine/xylazine anesthesia, in female Wistar rats.
Keywords: Anesthesia; Electrical stability of the heart; Heart rate; Light-dark cycle; Rat.