CHANGES IN MEAN CHORIOALLANTOIC ARTERY BLOOD-FLOW AND HEART-RATE PRODUCED BY HYPOXIA IN THE DEVELOPING CHICK-EMBRYO

Hypoxia in the mammalian fetus produces cardiovascular changes, such a s bradycardia, systemic hypertension, and changes in heart rate variab ility. This response was studied in 140 chick embryos ranging from sta ge 34 to stage 42 (d 9-16 of the 21-d incubation), by measuring the ch anges in mean chorioallantoic artery blood flow (CABF) and heart rate for 5 min in two levels of hypoxia (group 1; n = 90; 100% N-2) or (gro up 2; n = 50; 5% O-2). Eggs were opened at the air cell and placed in a small plexiglass holder, which had a continuous gas flow of an O-2/N -2 mixture (5 L/min), at 38 degrees C and 60% humidity. The chorioalla ntoic artery was placed in the lumen of a flow probe to measure mean C ABF, heart rate, peak flow, and blood flow acceleration. After baselin e measurements, the gas mixture was changed to 100% N, or 5% O-2 in N- 2 for 5 min. Mean CABF and heart rate decreased significantly in both groups (Wilcoxon paired sample test, p < 0.05). This response was more pronounced with the development of the chick embryo. Chorioallantoic artery peak flow (mL/min) and CABF acceleration (mL/s(2)) increased wi th incubation time and decreased during periods of hypoxia. During rec overy, heart rate returned to baseline levels, whereas mean CABF showe d an overshoot. The initial decrease in mean CABF and heart rate was s imilar in both groups. The cardiovascular response to hypoxia in the c hick embryo is similar to the response in the mammalian fetus. The mor e pronounced response in the more developed chick embryo may represent a maturation of cardiovascular control.