Continuous measurements of instantaneous heart rate and its fluctuations before and after hatching in chickens

There has been considerable interest in heart rate (fH) fluctuations in rel ation to cardiovascular control systems and foetal conditions during pregna ncy in mammals. prominent fluctuations in fH also occur in avian embryos, w hich are an important experimental model for studying developmental physiol ogy. The present study determined the instantaneous fH of seven chick embry os continuously from the last stage of prenatal development (day 18), throu ghout the pipping (perinatal) period (days 19-21) until hatching and, subse quently, of newly hatched chicks (up to day 2), The distinctive patterns of instantaneous fH fluctuations took the form of specific changes within a broad mean fH baseline. Cyclic oscillations (ultr adian rhythm) occurred until an early stage of the perinatal period, when t he fH baseline started rising, Subsequently, the baseline dropped and respi ratory arrhythmia began to appear concomitant with external pipping, During the final stage of external pipping, when the fH baseline rose again prior to hatching, three unique patterns of instantaneous fH fluctuations were e vident: relatively long-lasting cyclic small accelerations, irregular inter mittent large accelerations and short-term repeated large accelerations. Fu rthermore, repeated alternate occurrences of the latter two types of accele ration formed an additional oscillating pattern with a period of 10-15 min. During the early period after hatching, when the fH baseline reached its ma ximum, instantaneous fH changed relatively slowly accompanied by transient rapid decelerations, probably due to augmented vagal tone, Subsequently, th e mean fH baseline dropped to its minimum, and a circadian rhythm and three types of previously reported fH fluctuations (types I-III) appeared. Developmental patterns of mean fH and the appearance of distinctive pattern s of instantaneous fluctuations in fH and circadian rhythms were not influe nced by an ultimate failure of hatching after a normal development. The demonstration of complex, repeatable patterns of fH fluctuation that ch ange during development suggests that the avian embryo model should be usef ul in studying the phenomenon of fH fluctuation and its underlying causes.