Cardiac rhythms in developing chicks

Instantaneous heart rate (IHR) of chicks was determined by electrocardiogra m measured non-invasively from the day of hatch to day 6 for continuity of investigation of HR fluctuations from embryos and for ascertainment of HR d iurnal rhythms. In Experiment I, IHR was determined for 1-h periods twice a day, in daytime and at night, to investigate development of heart rate flu ctuations (variability and irregularities). Chick IHR was substantially mor e arrhythmic than embryonic HR and spontaneous accelerations dominated HR f luctuations. Chick HR fluctuations were categorized into three types, [1] T ype I as a widespread baseline HR (20-50 bpm) due to respiratory arrhythmia , with a mean oscillatory frequency of 0.74 Hz (range 0.4-1.2 Hz); [2] Type II as low frequency oscillations of baseline HR, at a mean of 0.07 Hz (ran ge 0.04-0.10 Hz), and [3] Type III as non-cyclic irregularities, dominated by frequent transient accelerations. In Experiment II, continuous measureme nts of HR were made under conditions of a natural photoperiod, thermoneutra lity and with feed available throughout the first week after hatching and c ircadian rhythms of HR were ascertained. HR was very variable in the daytim e (250-500 bpm), due in part to feeding and activity, and decreased to a di urnal low (200-350 bpm) at night when mean HR was relatively stable. HR flu ctuations persisted throughout the diurnal cycle. (C) 1999 Elsevier Science Inc. All rights reserved.