Generally, it is assumed that heart-rhythm disorders during hypoxia result from the interplay between the autonomic nervous system (ANS) and the direct effect of hypoxia on cardiorespiratory structures of the central nervous system and on the myocardium. Circadian variability in the ANS may substantially influence the electrical stability of the myocardium, and thus it is associated with the preconditioning protective mechanism. We designed our study using anaesthetized Wistar rats (ketamine/xylazine 100 mg/15 mg/kg, i.m., open chest experiments) to evaluate the effect of preconditioning (PC) induced by 1 to 3 cycles (1 PC–3 PC) of asphyxia (5 min. of artificial hypoventilation, VT = 0.5 ml/100 g of b.w., 20 breaths/min.) and reoxygenation (5 min. of artificial ventilation, VT = 1 ml/100 g of b.w., 50 breaths/min.) on the heart rate (HR) during followed exposure 20 minutes of hypoventilation after adaptation to a light-dark (LD) cycle of 12 hours:12 hours. Hypoxic HR increases were only minimally prevented by 1 to 2 PC pre-treatment, particularly during the dark part of the day. A statistically significant HR increase required 3 PC and was seen only in the light part of the day. We concluded that possible ANS participation in asphyxic preconditioning depends not only on the number of preconditioned cycles but also on the LD cycle, when the ANS participation in preconditioning can be effective only in the light (nonactive) period.
 Meurling C.J., Waktare J.E.P., Holmqvist F., Hedman A., Camm A.J., Olsson S.B., Malik M., Diurnal variations of the dominant cycle length of chronic atrial fibrillation, Am. J. Physiol., 2001, 280, H401–H406
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