DEVELOPMENT OF HEART-RATE IRREGULARITIES IN CHICK-EMBRYOS

Heart rate (HR) irregularities in chick embryos were defined as large fluctuations (>10 beats/min) comprising irregular, brief deceleration and/or acceleration of instantaneous HR (IHR). IHR was determined dire ctly from the arterial blood pressure while adequate gas exchange was maintained through an eggshell and chorioallantoic membrane. Five embr yos were examined on each day from day II to day 19 of incubation. Bas eline HR was stable until day 12-13, and on around day 13-14 transient , rapid deceleration of HR (termed V pattern) began to appear, with a subsequent increase in its frequency and magnitude. The acceleration p atterns (lambda, avian omega, and periodic patterns) appeared later, a nd the IHR became increasingly irregular, with additional, spontaneous deceleration and acceleration patterns toward hatching. Additional ex periments with intravenous administration of autonomic drugs clearly s howed that rapid deceleration of HR was mediated by parasympathetic ne rvous function but did not always show clear relations of sympathomime tic and sympathetic blocking agents to the acceleration patterns.