A noninvasive study was conducted on intact awake humans to characterize the dynamic response of the heart to the vagus during slow-, comfortable-, and fast-paced respiration (8, 12, and 18 breaths/min), under both sitting and standing conditions. The respiration response curve (RRC) of respiration-associated vagal effects on the heart was estimated, and characteristics of entrainment and frequency dependence on respiration were demonstrated. It was shown that the degree of entrainment and magnitude of phase resetting decrease with increase of pacing rate from 8 to 18 breaths/min. Further, the RRC was examined by overlapping equivalent phase shifts in different respiration cycles. This examination of the RRC can help us not only to find the common pattern underlying the RRC during different respiration cycles but also to perceive its variation related to degree of entrainment.

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