Heart rate responses to altered ambient oxygen in early (days 3-9) chick embryos in the intact egg

Normal heart rate (HR), and the HR responses to hypoxia and hyperoxia durin g early heart development in chick embyros have not been studied in detail, particularly in undisturbed embryos within the intact egg. HR was measured in day 3-9 chick embryos at 38 degrees C using relatively noninvasive impe dance cardiography. Embryos were exposed to air (control) and to hypoxic (1 0% O-2) or hyperoxic (100% O-2) gas for a 2-h or 4-h period, during which H R was continually monitored. Control (normoxic) HR increased from about 150 beats per min (bpm) on day 3 to about 240 bpm on days 7-9. HR in very earl y embryos showed a variety of moderate responses to hypoxia (all survived), but as development progressed beyond day 6, hypoxic exposure induced a pro found bradycardia that frequently terminated in death before the end of the measurement period. In contrast to the marked developmental changes in hyp oxic sensitivity, HR showed little response to hyperoxia throughout develop ment, suggesting no "hypoxic drive" to HR. We speculate that hypoxia has li ttle effect early in development because of the embryo's small absolute O-2 demand, but as the embryo grows, hypoxia represents a progressively more s evere perturbation. Although general trends were identified, there was cons iderable variation in both HR and HR responses to ambient O-2 changes betwe en individuals of the same developmental stage.