Time course of diurnal rhythm disturbances in autonomic function of rats with myocardial infarction

The diurnal rhythm of the autonomic function is known to be blunted in heart failure, but the timing of this blunting is not well understood. We examined the time course of the alterations in autonomic function in rats with myocardial infarction (MI) by analyzing heart rate variability (HRV). MI was induced by coronary artery ligation, and HRV was analyzed at 2, 4, 6, and 8 weeks post-MI. Diurnal rhythm in heart rate (HR) was maintained over the study period. However, diurnal rhythm in the standard deviation of averages of normal R-R intervals (SDANN) and the ratio between low and high frequency band powers (LF/HF ratio) were disrupted in MI rats at 2 weeks, which persisted up to 8 weeks, with the exception of 4 weeks. The dark-light differences in the LF/HF ratio changed from negative to positive values between weeks 2 and 4 in the MI rats. We also found decreases in HR, SDANN, and the LF/HF ratio in the dark phase at weeks 6 and 8 and an increase in plasma norepinephrine (NE) level at week 8. Collectively, the results indicate that the timing of the disturbance of diurnal rhythm in SDANN and the LF/HF ratio is different from those in HR and in plasma NE level, suggesting that the mechanisms underlying these changes are different. In addition, there is a transition from the compensatory to the decompensatory phase between 4 and 6 weeks post-MI. These findings may help to understand the progression and pathophysiology of heart failure.