Long-term measurement of heart rate in chicken eggs

Taking advantage of acoustocardiogram (ACG), we measured the heart rate (HR ) of chick embryos continuously from day 12 until hatching and then investi gated the development of HR irregularities (HRI), HR variability (HRV), and the existence of a circadian rhythm in mean HR (MHR). HRI comprised transi ent bradycardia and tachycardia, which first developed on day 14 and 16 in most embryos, respectively. Transient bradycardia increased in frequency an d magnitude with embryonic development and occurred over periods of up to 3 0 min in some embryos. MHR was maximal on around days 14-15 and thereafter decreased to about 250-260 bpm on days 16-18. Baseline HRV, which is an osc illation of the MHR baseline, occurred as HR decreased from days 15-16 and became predominant on days 17-18. The magnitude of the baseline oscillation s reached up to 50 bpm in some embryos and the period ranged between about 40-90 min (ultradian rhythm). A circadian rhythm of MHR was not found in la te chick embryos. On days 18-19, embryonic activities were augmented and th en breathing movements began to occur, disturbing ACG signals and thus maki ng it difficult to measure the HR. Instead, the development of breathing ac tivities was recorded. Breathing frequency was irregular at first and then increased to a maximum of about 1.5 Hz prior to hatching. (C) 1999 Elsevier Science Inc. All rights reserved.