Heart and breathing rate variability in the avian perinatal period: The chicken embryo as a model

Abstract

We used the chicken embryo at the internal pipping phase (just after the onset of pulmonary ventilation) as a model to quantify the changes in heart rate (fH), breathing frequency (fB) and their variabilities (heart rate variability and breathing rate variability) during air breathing (21% O₂) and successive 20-min periods of 15%, 10% and 5% O₂ and post-hypoxic recovery. For each condition, and for both fH and fB, variability was quantified by time-domain analysis with five standard criteria; these produced qualitatively similar results, which were combined into a single variability index. In normoxia, breathing rate variability was about five times higher than heart rate variability. With 10% O₂, the embryo’s oxygen consumption ( ${\stackrel{.}{V}}_{{\mathrm{O}}_2}$ ) and breathing rate variability decreased while heart rate variability increased. In normoxia, respiratory sinus arrhythmia was recognisable in a minority of embryos; its average value was low (~2%) and decreased further with hypoxia. With very severe hypoxia (5% O₂), in some cases, breathing stopped; when it did not, breathing rate variability was high. Within the 20-min post-hypoxia, all embryos recovered, and almost all parameters (fH, heart rate variability, fB, respiratory sinus arrhythmia and ${\stackrel{.}{V}}_{{\mathrm{O}}_2}$ ) were at the pre-hypoxic values; only breathing rate variability remained low. The possibility of simultaneous measurements of fB and fH makes the avian embryo, close to hatching, a suitable model for the investigations of heart rate variability and breathing rate variability in response to hypoxia during the transition from prenatal to postnatal life.