The contribution of heart rate to the oxygen consumption of the chicken embryo during cold- or hypoxia-hypometabolism.

In embryos, cooling and hypoxia cause a decrease in oxygen consumption ( [Formula: see text] ); we asked what was the relative contribution of heart rate (HR) and of the 'not-HR' factor (the product of stroke volume and arterial-venous O2 difference) to the drop in [Formula: see text] . Data of HR (with subcutaneous electrodes) and [Formula: see text] (by an open-flow methodology) were collected simultaneously on chicken embryos close to end-incubation. Over the last four days of incubation (E16-E20) differences in HR contributed about 30% of the differences in resting [Formula: see text] among embryos. At E20, progressive cooling from 38 to 8°C decreased [Formula: see text] entirely because of the decrease in HR, with minimal compensation of the 'not-HR' component. The same pattern during cooling occurred in younger embryos (age E16), in E20 embryos simultaneously exposed to hypoxia (15% O2) and in E20 normoxic embryos which were incubated in hypoxia (15% O2). Differently, in E20 embryos in normothermia, progressive hypoxia (15%, 10% or 5% O2) lowered [Formula: see text] largely because of the reduction in the 'not-HR' component. We conclude that at end incubation during hypometabolism the changes in HR contribute very differently to the decrease in [Formula: see text] , from about the totality of it during cold to only about 10-20% during hypoxia, depending on its severity. It follows that during cold-hypometabolism, but not during hypoxic hypometabolism, the changes in HR are a good index of the changes in [Formula: see text] . The close relationship between [Formula: see text] and HR during cold-hypometabolism may permit estimates of the changes in [Formula: see text] from the changes in HR in infants undergoing therapeutic hypothermia. Copyright Â