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Abstract
1. Among three types of fluctuations of instantaneous heart rate (IHR) found previously in newly hatched chicks, a high frequency oscillation with a mean frequency of about 0·7 Hz (Type I) appeared to be concurrent with breathing (Moriya et al., Comparative Biochemistry and Physiology, 124A: 461–468, 1999).
2. In order to confirm that Type I HR fluctuation is respiratory sinus arrhythmia (RSA), breathing activity was measured by a condenser microphone, simultaneously with IHR.
3. The microphone detected pressure changes caused by breathing (acoustorespirogram, ARG) and also unexpectedly movement, probably twitch, of hatchlings.
4. Simultaneous measurements of IHR and ARG demonstrated that oscillatory frequency of Type I HR fluctuation coincided with breathing frequency and IHR increased with inspiration, confirming that Type I HR oscillation is RSA.
5. In addition, large transient HR accelerations (Type III HR fluctuation) simultaneously occurred with movement or twitch of the hatchlings, suggesting that Type III HR fluctuation and movement of hatchlings have the same origin, probably sympathetic nerve function.
Acknowledgements
K.M. was grateful to the Japan Society for Promotion of Science for a Grant-in-Aid for Scientific Research (No. 14750355).