Hyperventilation and circadian rhythm of the electrical stability of rat myocardium

Pavol Svorc,^1,2 Alexander Marossy,¹ Pavol Svorc Jr<sup>2

1</sup>Department of Physiology, Medical Faculty, Safarik University, Kosice, Slovak Republic; ²Department of Physiology, Medical Faculty, Ostrava University, Ostrava, Czech Republic

Objective: Respiratory alkalosis is an extremely common and complicated problem affecting virtually every organ system where the etiologies may be related to pulmonary or cardiovascular disorders. However, there are only few works describing daytime experiments or synchronization of animals to the external environmental periodicity. The aim of the study is to describe the circadian rhythm of the electrical stability of the heart under hyperventilatory conditions.
Methods: Circadian rhythms of the electrical stability of the heart, measured by ventricular arrhythmia threshold ([VAT] measurement in 3 hour intervals), were followed during normal artificial ventilation (40 breaths/minute, tidal volume = 1 mL/100 g; n = 17) and hyperventilation (80 breaths/minute, tidal volume = 2 mL/100 g; n = 7) in pentobarbital (40 mg/kg administered intraperitoneally) anesthetized female Wistar rats, after 4 week adaptation on the light/dark regime of 12 hour light/12 hour dark (40%–60% humidity, room temperature of 24°C in cages, two animals/cage with access to food and water ad libitum), with the dark period from 18.00h to 06.00h for 4 weeks.
Results: The 24 hour course of the VAT showed the highest susceptibility of the rat ventricular myocardium to arrhythmias between 12.00h and 15.00h, and highest resistance between 19.20h and 00.28h (acrophase −338° in time at 22.53h with confidence intervals −2,880° to −70°), under normoxic conditions. Mesor was 2.59 ± 0.53 mA and amplitude 0.33 ± 0.11 mA. Hyperventilation increased the VAT at each interval of the measurement, but did not change the character of its circadian rhythm. Acrophase was on −40° (02.40h), mesor was increased (2.91 mA), and amplitude was decreased (0.13 mA).
Conclusion: Although hyperventilation insignificantly increased the electrical stability of the heart compared to values of electrical stability of the heart during normal pulmonary ventilation during the whole 24 hour period, the results show that hyperventilation probably only modulates, but not disturbs, the circadian rhythm of the electrical stability of the heart in pentobarbital anaesthetized female Wistar rats.

Keywords: circadian rhythm, myocardial vulnerability, hyperventilation, rat