Effect of examination on the circadian structure of ECG parameters

Seasonal rhythms are an important tool for an organism to adapt to the environment. As part of the problem of adjustment to a load (“stress”), infradian (including seasonal) biorhythms deserve special attention, notably in a climate with a strong contrast between summer and winter. Evolutionary circannual rhythms contribute to the survival of individuals, species and populations of different animals in the face of seasonal changes in habitat. The changing of seasons imposes great demands on organisms, especially in the continental climate of Kazakhstan. This investigation examines any influence of seasons on the daily dynamics of some ECG parameters (notably endpoints of heart rate variability, HRV) in association with the load of an examination in this Central Asian region. All exams were conducted in the morning (at 09: 00). Circadian rhythm characteristics of ECG endpoints were compared between days with vs. without an exam, when exams were taken either during winter or summer. In summer, on days with an examination, 24-hour means of SDNNidx and rMSSD (HRV parameters) were decreased, more so at night, when the parameters SDNNidx, rMSSD, pNN50, pNN100, and pNN200 undergo statistically significant changes. Changes of lesser extent were observed in winter. In both seasons, the load of an examination was associated with a shift in the circadian acrophase (phase of maximum of cosine curve approximating the data) that was more pronounced in winter than in summer. On days with an examination, the harmonic content increased, components with a frequency higher than 1 cycle per day (ultradians) accounting for a larger proportion of the overall variance. Responses of blood pressure (BP) and heart rate (HR) to a load can vary greatly as a function of the circadian stage when the stimulus is applied. Recently, cycles other than circadian and circannual have also been reported, notably components with periods of about 5 and 16 months, detected in longitudinal BP and HR records as well as in mortality statistics from myocardial infarction and sudden cardiac death in different geographic locations. Whether the response to a load such as an examination is also characterized by such non-photic infradians (with a frequency lower than 1 cycle per day) deserves further investigation.